Chapter-21 Neural Control and Coordination
As you know, the functions of the organs/organ systems in our body must be coordinated to maintain homeostasis. Coordination is the process through which two or more organs interact and complement the functions of one another. For example, when we do physical exercises, the energy demand is increased for maintaining an increased muscular activity. The supply of oxygen is also increased. The increased supply of oxygen necessitates an increase in the rate of respiration, heart beat and increased blood flow via blood vessels. When physical exercise is stopped, the activities of nerves, lungs, heart and kidney gradually return to their normal conditions. Thus, the functions of muscles, lungs, heart, blood vessels, kidney and other organs are coordinated while performing physical exercises. In our body the neural system and the endocrine system jointly coordinate and integrate all the activities of the organs so that they function in a synchronised fashion.
The neural system provides an organised network of point-to-point connections for a quick coordination. The endocrine system provides chemical integration through hormones. In this chapter, you will learn about the neural system of human, mechanisms of neural coordination like transmission of nerve impulse, impulse conduction across a synapse and the physiology of reflex action.
21.1 NEURAL SYSTEM
The neural system of all animals is composed of highly specialised cells called neurons which can detect, receive and transmit different kinds of stimuli.
The neural organisation is very simple in lower invertebrates. For example, in Hydra it is composed of a network of neurons. The neural system is better organised in insects, where a brain is present along with a number of ganglia and neural tissues. The vertebrates have a more developed neural system.
21.2 HUMAN NEURAL SYSTEM
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)
The CNS includes the brain and the spinal cord and is the site of information processing and control. The PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord). The nerve fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres
The afferent nerve fibres transmit impulses from tissues/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system and autonomic neural system. The somatic neural system relays impulses from the CNS to skeletal muscles while the autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system. Visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system.
21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF NEURAL SYSTEM
A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon (Figure 21.1). The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. These fibres transmit impulses towards the cell body. The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction. Based on the number of axon and dendrites, the neurons are divided into three types, i.e., multipolar (with one axon and two or more dendrites; found in the cerebral cortex), bipolar (with one axon and one dendrite, found in the retina of eye) and unipolar (cell body with one axon only; found usually in the embryonic stage). There are two types of axons, namely, myelinated and nonmyelinated. The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomous and the somatic neural systems.
Figure 21.1 Structure of a neuron
21.3.1 Generation and Conduction of Nerve Impulse
Neurons are excitable cells because their membranes are in a polarised state. Do you know why the membrane of a neuron is polarised? Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K ) and nearly impermeable to sodium ions (Na + ). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration of K + and negatively charged proteins and low concentration of Na+. In contrast, the fluid outside the axon contains a low concentration of K +, a high concentration of Na+ and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na outwards for 2 K into the cell. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised. The electrical potential difference across the resting plasma membrane is called as the resting potential.
Figure 21.2 Diagrammatic representation of impulse conduction through an axon (at points A and B)
You might be curious to know about the mechanisms of generation of nerve impulse and its conduction along an axon. When a stimulus is applied at a site (Figure 21.2 e.g., point A) on the polarised membrane, the membrane at the site A becomes freely permeable to Na+. This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site A is thus reversed and hence depolarised. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse. At sites immediately ahead, the axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B. On the outer surface current flows from site B to site A (Figure 21.2) to complete the circuit of current flow. Hence, the polarity at the site is reversed, and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B. The sequence is repeated along the length of the axon and consequently the impulse is conducted. The rise in the stimulus-induced permeability to Na+ is extremely shortlived. It is quickly ollowed by a rise in permeability to K+. Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.
21.3.2 Transmission of Impulses
A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft. There are two types of synapses, namely, electrical synapses and chemical synapses. At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse. Electrical synapses are rare in our system.
At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft (Figure 21.3). Do you know how the pre-synaptic neuron transmits an impulse (action potential) across the synaptic cleft to the post-synaptic neuron? Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.
Figure 21.3 Diagram showing axon terminal and synapse
21.4 CENTRAL NEURAL SYSTEM
The brain is the central information processing organ of our body, and acts as the ‘command and control system’. It controls the voluntary movements, balance of the body, functioning of vital involuntary organs (e.g., lungs, heart, kidneys, etc.), thermoregulation, hunger and thirst, circadian (24-hour) rhythms of our body, activities of several endocrine glands and human behaviour. It is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. The brain can be divided into three major parts: (i) forebrain, (ii) midbrain, and (iii) hindbrain (Figure 21.4).
Figure 21.4 Diagram showing sagital section of the human brain
21.4.1 Forebrain
The forebrain consists of cerebrum, thalamus and hypothalamus (Figure 21.4). Cerebrum forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum. The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The neuron cell bodies are concentrated here giving the colour. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication. Fibres of the tracts are covered with the myelin sheath, which constitute the inner part of cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter. The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling. Another very important part of the brain called hypothalamus lies at the base of the thalamus. The hypothalamus contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones. The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus, it is involved in the regulation of sexual behaviour, expression of emotional reactions (e.g., excitement, pleasure, rage and fear), and motivation.
21.4.2 Midbrain
The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passess through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina.
21.4.3 Hindbrain
The hindbrain comprises pons, cerebellum and medulla (also called the medulla oblongata). Pons consists of fibre tracts that interconnect different regions of the brain. Cerebellum has very convoluted surface in order to provide the additional space for many more neurons. The medulla of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions. Three major regions make up the brain stem; mid brain, pons and medulla oblongata. Brain stem forms the connections between the brain and spinal cord.
Summary
The neural system coordinates and integrates functions as well as metabolic and homeostatic activities of all the organs. Neurons, the functional units of neural system are excitable cells due to a differential concentration gradient of ions across the membrane. The electrical potential difference across the resting neural membrane is called the ‘resting potential’. The nerve impulse is conducted along the axon membrane in the form of a wave of depolarisation and repolarisation. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron which may or may not be separated by a gap called synaptic cleft. Chemicals involved in the transmission of impulses at chemical synapses are called neurotransmitters.
Human neural system consists of two parts : (i) central neural system (CNS) and (ii) the peripheral neural system. The CNS consists of the brain and spiral cord. The brain can be divided into three major parts : (i) forebrain, (ii) midbrain and (iii) hindbrain. The forebrain consists of cerebrum, thalamus and hypothalamus. The cerebrum is longitudinally divided into two halves that are connected by the corpus callosum. A very important part of the forebrain called hypothalamus controls the body temperature, eating and drinking.Inner parts of cerebral hemispheres and a group of associated deep structures form a complex structure called limbic system which is concerned with olfaction, autonomic responses, regulation of sexual behaviour, expression of emotional reactions, and motivation.
The midbrain receives and integrates visual, tactile and auditory inputs. The hindbrain comprises pons, cerebellum and medulla.The cerebellum integrates information received from the semicircular canals of the ear and the auditory system. The medulla contains centres, which control respiration, cardiovascular reflexes, and gastric secretions. Pons consist of fibre tracts that interconnect different regions of the brain. The entire process of involuntary response to a peripheral nervous stimulation is called reflex action.
Exercises
1. Briefly describe the structure of the following:
(a) Brain
(b) Eye
(c) Ear
Show Answer
Answer
(A) Brain: Brain is the main coordinating centre of the body. It is a part of nervous system that controls and monitors every organ of the body. It is well protected by cranial meninges that are made up of an outer layer called dura mater, a thin middle layer called arachnoid, and an inner layer called pia mater.
It is divided into three regions - forebrain, midbrain, and hindbrain.
Forebrain: It is the main thinking part of the brain. It consists of cerebrum, thalamus, and hypothalamus.
(a) Cerebrum:
Cerebrum is the largest part of the brain and constitutes about four-fifth of its weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibre known as corpus callosum. The cerebral hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has sensory regions known as association areas that receive sensory impulses from various receptors as well as from motor regions that control the movement of various muscles. The innermost part of cerebrum gives an opaque white appearance to the layer and is known as the white matter.
(b) Thalamus:
Thalamus is the main centre of coordination for sensory and motor signalling. It is wrapped by cerebrum.
(c) Hypothalamus:
It lies at the base of thalamus and contains a number of centres that regulate body temperature and the urge for eating and drinking. Some regions of cerebrum, along with hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as excitement, pleasure, fear, etc.
Midbrain:
It is located between the thalamus region of the forebrain and pons region of hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four round lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the midbrain. Midbrain is concerned with the sense of sight and hearing.
Hindbrain:
It consists of three regions - pons, cerebellum, and medulla oblongata.
(a) Pons is a band of nerve fibre that lies between medulla oblongata and midbrain. It connects the lateral parts of cerebellar hemisphere together.
(b) Cerebellum is a large and well developed part of hindbrain. It is located below the posterior sides of cerebral hemispheres and above medulla oblongata. It is responsible for maintaining posture and equilibrium of the body.
(c) Medulla oblongata is the posterior and simplest part of the brain. It is located beneath the cerebellum. Its lower end extends in the form of spinal cord and leaves the skull through foramen magnum.
(B) Eye: Eyes are spherical structures that consist of three layers.
(a) The outer layer is composed of sclera and cornea.
(i) Sclera is an opaque tissue that is usually known as white of the eye. It is composed of a dense connective tissue.
(ii) Cornea is a transparent anterior portion of eye that lacks blood vessels and is nourished by lymph from the nearby area. It is slightly bulged forward and helps in focusing light rays with the help of lens.
(b) The middle layer of eye is vascular in nature and contains choroid, ciliary body, and iris.
(i) Choroid lies next to the sclera and contains numerous blood vessels that provide nutrients and oxygen to the retina and other tissues.
(ii) Ciliary body: The choroid layer is thin over posterior region and gets thickened in the anterior portion to form ciliary body. It contains blood vessels, ciliary muscles, and ciliary processes.
(iii) Iris: At the junction of sclera and cornea, the ciliary body continues forward to form thin coloured partition called iris. It is the visible coloured portion of eye.
The eye contains a transparent, biconvex, and elastic structure just behind the iris. It is known as lens. The lens is held in position by suspensory ligaments attached to the ciliary body. The lens divides the eye ball into two chambers - an anterior aqueous and posterior vitreous chamber.
(c) The innermost nervous coat of eye contains retina. Retina is the innermost layer. It contains three layers of cells inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. The receptor cells present in the retina are of two types - rod cells and cone cells.
(a) Rod cells -The rods contain the rhodopsin pigment (visual purple) that is highly sensitive to dim light. It is responsible for twilight vision.
(b) Cone cells -The cones contain the iodopsin pigment (visual violet) and are highly sensitive to high intensity light. They are responsible for daylight and colour visions.
The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain.
(C) Ear: Ear is the sense organ for hearing and equilibrium. It consists of three portions - external ear, middle ear, and internal ear.
1. External ear:
It consists of pinna, external auditory meatus, and a tympanic membrane.
(a) Pinna is a sensitive structure that collects and directs the vibrations into the ear to produce sound.
(b) External auditory meatus is a tubular passage supported by cartilage in external ear.
(c) Tympanic membrane is a thin membrane that lies close to the auditory canal. It separates the middle ear from external ear.
2. Middle ear:
It is an air-filled tympanic cavity that is connected with pharynx through eustachian tube. Eustachian tube helps to equalize air pressure in both sides of tympanic membrane. The middle ear contains a flexible chain of three middle bones called ear ossicles. The three ear ossicles are malleus, incus, and stapes that are attached to each other.
3. Internal ear:
It is also known as labyrinth. Labyrinth is divided into bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. Membranous labyrinth is divided into 2 parts.
(a) Vestibular apparatus
Vestibular apparatus is a central sac-like part that is divided into utriculus and sacculus. A special group of sensory cells called macula are present in sacculus and utriculus.
Vestibular apparatus also contains three semi-circular canals. The lower end of each semi-circular canal contains a projecting ridge called crista ampularis. Each ampulla has a group of sensory cells called crista. Crista and macula are responsible for maintaining the balance of body and posture.
(b) Cochlea:
Cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists of three membranes. The organ of corti, a hearing organ, is located on the basilar membrane that has hair cells.
2. Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
(c) Choroid and retina
Show Answer
Answer
(a) Central neural system (CNS) and Peripheral neural system (PNS)
| Central neural system | Peripheral neural system |
|---|---|
| 1. It is the main coordinating centre of the body. | 1. It is not the main coordinating centre of the body. |
| 2. It includes brain and spinal cord. | 2. It includes cranial and spinal nerves that connect central nervous system to different parts of the body. |
(b) Resting potential and action potential
| Resting potential | Action potential |
|---|---|
| 1. It is the potential difference across the nerve fibre when there is no conduction of nerve impulse. | 1. It is the potential difference across nerve fibre when there is conduction of nerve impulse. |
| 2. The membrane is more permeable to $K^{+}$ions than to $Na^{+}$ions. | 2. The membrane is more permeable to $Na^{+}$ions than to $K^{+}$ions. |
(c) Choroid and retina
| Choroid | Retina |
|---|---|
| 1. Choroid is the middle vascular layer of eye. | 1. Retina is the innermost nervous coat of eye. |
| 2. It contains numerous blood vessels that provide nutrients and oxygen to retina and other tissues. | 2. It contains photoreceptor cells, rods and cones that are associated with twilight and colour vision respectively. |
3. Explain the following processes:
(a) Polarisation of the membrane of a nerve fibre
(b) Depolarisation of the membrane of a nerve fibre
(c) Conduction of a nerve impulse along a nerve fibre
(d) Transmission of a nerve impulse across a chemical synapse
Show Answer
Answer
(a) Polarisation of the membrane of a nerve fibre
During resting condition, the concentration of $K^{+}$ions is more inside the axoplasm while the concentration of $Na^{+}$ions is more outside the axoplasm. As a result, the potassium ions move faster from inside to outside as compared to sodium ions. Therefore, the membrane becomes positively charged outside and negatively charged inside. This is known as polarization of membrane or polarized nerve.
(b) Depolarisation of the membrane of a nerve fibre
When an electrical stimulus is given to a nerve fibre, an action potential is generated. The membrane becomes permeable to sodium ions than to potassium ions. This results into positive charge inside and negative charge outside the nerve fibre. Hence, the membrane is said to be depolarized.
(c) Conduction of a nerve impulse along a nerve fibre
There are two types of nerve fibres - myelinated and non-myelinated. In myelinated nerve fibre, the action potential is conducted from node to node in jumping manner. This is because the myelinated nerve fibre is coated with myelin sheath. The myelin sheath is impermeable to ions. As a result, the ionic exchange and depolarisation of nerve fibre is not possible along the whole length of nerve fibre. It takes place only at some point, known as nodes of Ranvier, whereas in non-myelinated nerve fibre, the ionic exchange and depolarization of nerve fibre takes place along the whole length of the nerve fibre. Because of this ionic exchange, the depolarized area becomes repolarised and the next polarized area becomes depolarized.
(d) Transmission of a nerve impulse across a chemical synapse
Synapse is a small gap that occurs between the last portion of the axon of one neuron and the dendrite of next neuron. When an impulse reaches at the end plate of axon, vesicles consisting of chemical substance or neurotransmitter, such as acetylcholine, fuse with the plasma membrane. This chemical moves across the cleft and attaches to chemo-receptors present on the membrane of the dendrite of next neuron. This binding of chemical with chemo-receptors leads to the depolarization of membrane and generates a nerve impulse across nerve fibre.
The chemical, acetylcholine, is inactivated by enzyme acetylcholinestrase. The enzyme is present in the post synaptic membrane of the dendrite.
It hydrolyses acetylcholine and this allows the membrane to repolarise.
4. Draw labelled diagrams of the following:
(a) Neuron
(b) Brain
(c) Eye
(d) Ear
Show Answer
Answer
(a) Neuron
(b) Brain
(c) Eye
(d) Ear
5. Write short notes on the following:
(a) Neural coordination
(b) Forebrain
(c) Midbrain
(d) Hindbrain
(e) Retina
(f) Ear ossicles
(g) Cochlea
(h) Organ of Corti
(i) Synapse
Show Answer
Answer
(a) Neural coordination
The neural system provides rapid coordination among the organs of the body. This coordination is in the form of electric impulses and is quick and short lived. All the physiological processes in the body are closed linked and dependent upon each other. For example, during exercise, our body requires more oxygen and food. Hence, the breathing rate increases automatically and the heart beats faster. This leads to a faster supply of oxygenated blood to the muscles. Moreover, the cellular functions require regulation continuously. These functions are carried out by the hormones. Hence, the neural system along with the endocrine system control and coordinate the physiological processes.
(b) Forebrain
It is the main thinking part of the brain. It consists of cerebrum, thalamus, and hypothalamus.
(i) Cerebrum:
Cerebrum is the largest part of the brain and constitutes about four-fifth of its weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibres known as corpus callosum. The cerebral hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has sensory regions known as association areas that receive sensory impulses from various receptors as well as from motor regions that control the movement of various muscles. The innermost part of cerebrum gives an opaque white appearance to the layer and is known as the white matter.
(ii) Thalamus:
Thalamus is the main centre of coordination for sensory and motor signalling. It is wrapped by cerebrum.
(iii) Hypothalamus:
It lies at the base of thalamus and contains a number of centres that regulate body temperature and the urge for eating and drinking. Some regions of cerebrum, along with hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as excitement, pleasure, fear, etc.
(c) Midbrain
It is located between the thalamus region of the forebrain and pons region of hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four round lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the midbrain. Midbrain is concerned with the sense of sight and hearing.
(d) Hindbrain
It consists of three regions - pons, cerebellum, and medulla oblongata.
(i) Pons is a band of nerve fibres that lies between medulla oblongata and midbrain. It connects the lateral parts of cerebellar hemisphere together.
(ii) Cerebellum is a large and well developed part of hindbrain. It is located below the posterior sides of cerebral hemispheres and above the medulla oblongata. It is responsible for maintaining posture and equilibrium of the body.
(iii) Medulla oblongata is the posterior and simplest part of the brain. It is located beneath the cerebellum. Its lower end extends in the form of spinal cord and leaves the skull through foramen magnum.
(e) Retina
Retina is the innermost layer. It contains three layers of cells - inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. The receptor cells present in the retina are of two types - rod cells and cone cells.
(i) Rod cells -The rods contain rhodopsin pigment (visual purple), which is highly sensitive to dim light. It is responsible for twilight vision.
(ii) Cone cells - The cones contain iodopsin pigment (visual violet) and are highly sensitive to high intensity light. They are responsible for daylight and colour visions.
The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain. In this region, the photoreceptor cells are absent. Hence, it is known as the blind spot. At the posterior part, lateral to blind spot, there is a pigmented spot called macula lutea. This spot has a shallow depression at its middle known as fovea. Fovea has only cone cells. They are devoid of rod cells. Hence, it is the place of most distinct vision.
(f) Ear ossicles
The middle ear contains a flexible chain of three middle bones called ear ossicles. The three ear ossicles are as follows.
(i) Malleus
(ii) Incus
(iii) Stapes
The malleus is attached to tympanic membrane on one side and to incus on the other side. The incus is connected with stapes. Stapes, in turn, are attached with an oval membrane, fenestra ovalis, of internal ear. The ear ossicles act as a lever that transmits sound waves from external ear to internal ear.
(g) Cochlea
Cochlea is a long, coiled outgrowth of sacculus. It is the main hearing organ. The cochlea forms three chambers.
(i) Upper - scala vestibule
(ii) Middle - scala media
(iii) Lower - scale tympani
The floor of the scala media is basilar membrane while its roof is Reissner’s membrane. Reissner’s membrane gives out a projection called tectorial membrane. The organ of corti, a hearing organ, is located on the basilar membrane. Organ of corti contains receptor hair cells. The upper scala vestibule and lower scala tympani contain perilymph.
(h) Organ of corti
Organ of corti is the hearing organ. It is located on the basilar membrane that contains hair cells. Hair cells act as auditory receptors. They are present on the internal side of organ of corti.
(i) Synapse
Synapse is a junction between the axon terminal of one neuron and the dendrite of next neuron. It is separated by a small gap known as synaptic cleft.
There are two types of synapses.
(a) Electrical synapse
(b) Chemical synapse
In electrical synapses, the pre and post synaptic neurons lie in close proximity to each other. Hence, the impulse can move directly from one neuron to another across the synapse. This represents a faster method of impulse transmission.
In chemical synapses, the pre and post synaptic neurons are not in close proximity. They are separated by a synaptic cleft. The transmission of nerve impulses is carried out by chemicals such as neurotransmitters.
6. Give a brief account of:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing
Show Answer
Answer
(a) Mechanism of synaptic transmission
Synapse is a junction between two neurons. It is present between the axon terminal of one neuron and the dendrite of next neuron separated by a cleft.
There are two ways of synaptic transmission.
(1) Chemical transmission
(2) Electrical transmission
-
Chemical transmission - When a nerve impulse reaches the end plate of axon, it releases a neurotransmitter (acetylcholine) across the synaptic cleft. This chemical is synthesized in cell body of the neuron and is transported to the axon terminal. The acetylcholine diffuses across the cleft and binds to the receptors present on the membrane of next neuron. This causes depolarization of membrane and initiates an action potential.
-
Electrical transmission - In this type of transmission, an electric current is formed in the neuron. This electric current generates an action potential and leads to transmission of nerve impulse across the nerve fibre. This represents a faster method of nerve conduction than the chemical method of transmission.
(b) Mechanism of vision
Retina is the innermost layer of eye. It contains three layers of cells - inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. A photoreceptor cell is composed of a protein called opsin and an aldehyde of vitamin A called retinal. When light rays are focused on the retina through cornea, it leads to the dissociation of retinal from opsin protein. This changes the structure of opsin. As the structure of opsin changes, the permeability of membrane changes, generating a potential difference in the cells. This generates an action potential in the ganglionic cells and is transmitted to the visual cortex of the brain via optic nerves. In the cortex region of brain, the impulses are analysed and image is formed on the retina.
(c) Mechanism of hearing
The pinna of the external region collects the sound waves and directs it towards ear drum or external auditory canal. These waves strike the tympanic membrane and vibrations are created. Then, these vibrations are transmitted to the oval window, fenestra ovalis, through three ear ossicles, named as malleus, incus, and stapes. These ear ossicles act as lever and transmit the sound waves to internal ear. These vibrations from fenestra ovalis are transmitted into cochlear fluid. This generates sound waves in the lymph. The formation of waves generates a ripple in the basilar membrane. This movement bends the sensory hair cells present on the organ of corti against tectorial membrane. As a result of this, sound waves are converted into nerve impulses. These impulses are then carried to auditory cortex of brain via auditory nerves. In cerebral cortex of brain, the impulses are analysed and sound is recognized.
7. Answer briefly:
(a) How do you perceive the colour of an object?
(b) Which part of our body helps us in maintaining the body balance?
(c) How does the eye regulate the amount of light that falls on the retina.
Show Answer
Answer
(a) Photoreceptors are cells that are sensitive to light. They are of two types - rods and cones. These are present in the retina. Cones help in distinguishing colours. There are three types of cone cells - those responding to green light, those responding to blue light, and those responding to red light. These cells are stimulated by different lights, from different sources. The combinations of the signals generated help us see the different colours.
(b) Vestibular apparatus is located in the internal ear, above the cochlea and helps in maintaining body balance. Crista and macula are the sensory spots of the vestibular apparatus controlling dynamic equilibrium.
(c) Pupil is the small aperture in the iris that regulates the amount of light entering the eye. Cornea, aqueous humour, lens, and vitreous humour act together and refract light rays, focussing them onto the photoreceptor cells of the retina.
8. Explain the following:
(a) Role of $Na^{+}$in the generation of action potential.
(b) Mechanism of generation of light-induced impulse in the retina.
(c) Mechanism through which a sound produces a nerve impulse in the inner ear.
Show Answer
Answer
(a) Sodium ions play an important role in the generation of action potential. When a nerve fibre is stimulated, the membrane potential decreases. The membrane becomes more permeable to $Na^{+}$ions than to $K^{+}$ions. As a result, $Na^{+}$diffuses from the outside to the inside of the membrane. This causes the inside of the membrane to become positively-charged, while the outer membrane gains a negatively charge. This reversal of polarity across the membrane is known as depolarisation. The rapid inflow of $Na^{+}$ions causes the membrane potential to increase, thereby generating an action potential.
Resting nerve fibre
(b) Retina is the innermost layer of the eye. It contains three layers of cells - inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. Photoreceptor cells are composed of a protein called opsin and an aldehyde of vitamin A called retinal. When light rays are focused on the retina through the cornea, retinal gets dissociated from opsin. As a result, the structure of opsin gets changed. This in turn causes the permeability of the membrane to change, thereby generating a potential difference in the cells. Consequently, an action potential is generated in the ganglion cells and is transmitted to the visual cortex of the brain via the optic nerves. In the cortex region of the brain, the impulses are analysed and the image is formed on the retina.
(c) The pinna of the external ear collects the sound waves and directs them to the tympanic membrane (ear drum) via the external auditory canal. The ear drum then vibrates the sound waves and conducts them to the internal ear through the ear ossicles. The ear ossicles increase the intensity of the sound waves. These vibrating sound waves are conducted through the oval window to the fluid in the cochlea. Consequently, a movement is created in the lymph. This movement produces vibrations in the basilar membrane, which in turn stimulate the auditory hair cells. These cells generate a nerve impulse, conducting it to the auditory cortex of the brain via afferent fibres. The auditory cortex region interprets the nerve impulse and sound is recognised.
9. Differentiate between:
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Rods and cones
(d) Thalamus and Hypothalamus
(e) Cerebrum and Cerebellum
Show Answer
Answer
(a) Myelinated and non-myelinated axons
| Myelinated axons | Non-myelinated axons |
|---|---|
| 1. Transmission of nerve impulse is faster | 1. |
| 2. Myelinated axon has a myelin sheath. | 2. Myelin sheath is absent |
| 3. Node of Ranvier is present between adjacent myelin sheaths. | 3. Node of Ranvier is absent |
| 4. Found in the brain, the spinal cord, the cranial and spinal nerves | 4. Found in autonomous and somatic neural systems |
| 5. Schwann cells are observed inside the myelin sheath | 5. Schwann cells are not observed inside the myelin sheath |
(b) Dendrites and axons
| Dendrites | Axons |
|---|---|
| 1. Dendrite is a small projection arising from the neuron. It conducts the nerve impulse toward the cell body. | 1. Axon is a single, long projection that conducts the nerve impulse away from cell body to the next neuron. |
| 2. Nissl’s granules are present in dendrites. | 2. Nissl’s granules are absent from axons. |
| 3. Dendrites are always non-myelinated. | 3. Axons can be myelinated or non-myelinated. |
(c) Rods and cones
| Rods | Cones |
|---|---|
| 1. Rods help in twilight vision. | 1. Cones help in colour vision. |
| 2. They have visual purple pigment called rhodopsin. | 2. They have visual violet pigment called iodopsin. |
| 3. Rods are the photoreceptor cells of the retina that are sensitive to dim light. | 3. Cones are the photoreceptor cells of the retina that are sensitive to bright light. |
(d) Thalamus and Hypothalamus
| Thalamus | Hypothalamus |
|---|---|
| Thalamus is the part of the forebrain that receives nerve impulses of pain, temperature, touch, etc., and conducts them to the cerebral hemisphere. | Hypothalamus is the part of the forebrain that controls involuntary functions such as hunger, thirst, sweating, sleep, fatigue, sexual desire, temperature regulation, etc. |
(e) Cerebrum and Cerebellum
| Cerebrum | Cerebellum |
|---|---|
| It is the part of the forebrain that controls voluntary functions. It is the place where intelligence, will power, memory, etc., reside. | It is the part of the hindbrain that controls voluntary functions and controls the equilibrium. |
10. Answer the following:
(a) Which part of the ear determines the pitch of a sound?
(b) Which part of the human brain is the most developed?
(c) Which part of our central neural system acts as a master clock?
Show Answer
Answer
(a) Cochlea determines the pitch of a sound.
(b) Forebrain is largest and the most developed part of the human brain.
(c) Hypothalamus acts as a master clock in the human body.
11. The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the
(a) fovea
(b) iris
(c) blind spot
(d) optic chaisma
Show Answer
Answer
(c) Blind spot
Blind spot is the part where the optic nerve passes out of the retina. Photoreceptors are absent from this region.
12. Distinguish between:
(a) afferent neurons and efferent neurons
(b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre
(c) aqueous humor and vitreous humor
(d) blind spot and yellow spot
(f) cranial nerves and spinal nerves.
Show Answer
Answer
(a) Afferent neurons and efferent neurons
| Afferent neurons | Efferent neurons |
|---|---|
| Afferent neuron conducts nerve impulses toward the brain or the spinal cord. | Efferent neuron conducts nerve impulses from the brain or spinal cord to the effector organs such as muscles or glands. |
(b) Impulse conduction in a myelinated nerve fibre and an unmyelinated nerve fibre
| Impulse conduction in a myelinated nerve fibre | Impulse conduction in an unmyelinated nerve fibre |
|---|---|
| 1. In a myelinated nerve fibre, the action potential is conducted from one node to another. | 1. In an unmyelinated nerve fibre, the action potential is not conducted from node to node. It is carried along the whole length of the nerve fibre. |
| 2. The conduction of impulses is faster. | 2. The conduction of impulses is slower. |
(c) Aqueous humour and vitreous humour
| Aqueous humour | Vitreous humour |
|---|---|
| It is a thin, watery fluid present between the cornea and the lens. | It is a transparent gel present between the lens and the retina. |
(d) Blind spot and yellow spot
| Blind spot | Yellow spot |
|---|---|
| 1. Blind spot is a spot on the retina present at the point of origin of the optic nerve. | 1. Yellow spot is a small area on the retina present at the posterior pole of the eye, lateral to the blind spot. |
| 2. Photoreceptor cells are absent from this region. | 2. Only cones are present in this region. |
| 3. They are insensitive to light as both rods and cones are absent. | 3. They are sensitive to bright light as cones are present. |
(f) Cranial nerves and spinal nerves
| Cranial nerves | Spinal nerves | ||
|---|---|---|---|
| 1. | Cranial nerves arise from the brain. | 1. | Spinal nerves arise from the spinal cord. |
| 2. | There are 12 pairs of cranial nerves. | 2. | There are 31 pairs of spinal nerves. |
जैसा कि तुम जानते हो मानव शरीर में बहुत से अंग एवं अंग तंत्र पाए जाते हैं जो कि स्वतंत्र रूप से कार्य करने में असमर्थ होते हैं। जैव स्थिरता (समअवस्था) बनने हेतु इन अंगों के कार्यों में समन्वय अत्यधिक आवश्यक है। समन्वयता एक ऐसी क्रियाविधि है, जिसके द्वारा दो या अधिक अंगों में क्रियाशीलता बढ़ती है व एक-दूसरे अंगों के कार्यों में मदद मिलती है। उदाहरणार्थ, जब हम शारीरिक व्यायाम करते हैं तो पेशियों के संचालन हेतु ऊर्जा की आवश्यकता भी बढ़ जाती है। ऑक्सीजन की आवश्यकता में भी वृद्धि हो जाती है। ऑक्सीजन की अधिक आपूर्ति के लिए श्वसन दर, हृदय स्पंदन, दर एवं वृक्क वाहिनियों में रक्त प्रवाह की दर बढ़ना स्वाभाविक हो जाता है। जब शारीरिक व्यायाम बंद कर देते हैं तो तंत्रिकीय क्रियाएं, फुप्फुस, हृदय रुधिर वाहिनियों, वृक्क व अन्य अंगों के कार्यों में समन्वय स्थापित हो जाता है। हमारे शरीर में तंत्रिका तंत्र एवं अंतःस्रावी तंत्र सम्मिलित रूप से अन्य अंगों की क्रियाओं में समन्वय करते हैं तथा उन्हें एकीकृत करते हैं, जिससे सभी क्रियाएं एक साथ संचालित होती रहती हैं।
तंत्रिकीय तंत्र ऐसे व्यवस्थित जाल तंत्र गठित करता है, जो त्वरित समन्वय हेतु बिंदु दर बिंदु जुड़ा रहता है। अंतःस्रावी तंत्र हार्मोन द्वारा रासायनिक समन्वय बनाता है। इस अध्याय में आप मनुष्य के तंत्रिकीय तंत्र एवं तंत्रिकीय समन्वय की क्रियाविधि जैसे तंत्रिकीय आवेग का संचरण तथा आवेगों का सिनेप्स से संचरण का अध्ययन करेंगे।
18.1 तंत्रिकीय तंत्र
सभी प्राणियों का तंत्रिका तंत्र अति विशिष्ट प्रकार की कोशिकाओं से बनता है, जिन्हें तंत्रिकोशिका कहते हैं। ये विभिन्न उद्दीपनों को पहचान कर ग्रहण करती हैं तथा इनका संचरण करती हैं।
निम्न अकशेरुकी प्राणियों में तंत्रिकीय संगठन बहुत ही सरल प्रकार का होता है। उदाहरणार्थ हाइड्रा में यह तंत्रिकीय जाल के रूप में होता है। कीटों का तंत्रिका तंत्र अधिक व्यवस्थित होता है। यह मस्तिष्क अनेक गुच्छिकाओं एवं तंत्रिकीय ऊतकों का बना होता है। कशेरुकी प्राणियों में अधिक विकसित तंत्रिका तंत्र पाया जाता है।
18.2 मानव का तंत्रिकीय तंत्र
मानव का तंत्रिका तंत्र दो भागों में विभाजित होता है:
(i) केंद्रीय तंत्रिका तंत्र
(ii) परिधीय तंत्रिका तंत्र।
केंद्रीय तंत्रिका तंत्र में मस्तिष्क तथा मेरूरज्जु सम्मिलित है, जहाँ सूचनाओं का संसाधन एवं नियंत्रण होता है। मस्तिष्क एवं परिधीय तंत्रिका तंत्र सभी तंत्रिकाओं से मिलकर बनता है, जो केंद्रीय तंत्रिका तंत्र (मस्तिष्क व मेरूरज्जू) से जुड़ी होती हैं। परिधीय तंत्रिका तंत्र में दो प्रकार की तंत्रिकाएं होती हैं :
(अ) संवेदी या अभिवाही एवं
(ब) चालक/प्रेरक या अपवाही।
संवेदी या अभिवाही तंत्रिकाएं उद्दीपनों को ऊतकोंअंगों से केंद्रीय तंत्रिका तंत्र तक तथा चालक/अभिवाही तंत्रिकाएं नियामक उद्दीपनों को केंद्रीय तंत्रिका तंत्र से संबंधित परिधीय ऊतक/अंगों तक पहुँचाती हैं।
परिधीय तंत्रिका तंत्र दो भागों में विभाजित होता है कायिक तंत्रिका तंत्र तथा स्वायत्त तंत्रिका तंत्र। कायिक तंत्रिका तंत्र उद्दीपनों को केंद्रीय तंत्रिका तंत्र से शरीर के अनैच्छिक अंगों व चिकनी पेशियों में पहुँचाता है। स्वायत्त तंत्रिका तंत्र पुनः दो भागों (अ) अनुकंपी तंत्रिका तंत्र व (ब) परानुकंपी तंत्रिका तंत्र में वर्गीकृत किया गया है। अंतरंग तंत्रिका तंत्र परिधीय तंत्रिका तंत्र का एक भाग है। इसके अंतर्गत वे सभी तंत्रिकाएँ, तंत्रिका तंतु, गुच्छिकाएँ एवं जालिकाएँ सम्मिलित हैं जिनके द्वारा केंद्रीय तंत्रिका तंत्र से आवेग, अंतरंगों तक तथा अंतरंगों से केंद्रीय तंत्रिका तंत्र तक संचरित होते हैं।
18.3 तंत्रिकोशिका (न्यूरॉन) तंत्रिका तंत्र की संरचनात्मक एवं क्रियात्मक इकाई
न्यूरॉन एक सूक्ष्मदर्शीय संरचना है जो तीन भागों से मिलकर बनती है - कोशिका काय, दुम्राक्ष्य व तंत्रिकाक्ष (चित्र 18.1)। कोशिका काय में कोशिका द्रव्य व प्रारूपिक कोशिकांग व विशेष दानेदार अंगक निसेल ग्रेन्यूल पाए जाते हैं। छोटे तंतु जो कोशिका काय से प्रवर्धित होकर लगातार विभाजित होते हैं तथा जिनमें निसेल ग्रेन्यूल भी पाए जाते हैं, दुम्राक्ष्य कहलाते हैं। ये तंतु उद्दीपनों को कोशिका काय की ओर भेजते हैं। एक तंत्रिकोशिका में एक तंत्रिकाक्ष निकलता है। इसका दूरस्थ भाग शाखित व प्रत्येक शाखित भाग का अंतिम छोर लड़ीनुमा संरचना सिनेप्टिक नोब जिसमें सिनेप्टी पुटिकाएं होती हैं, इसमें रसायन न्यूरोट्रांसमीटर्स पाए जाते हैं। तंत्रिकाक्ष तांत्रिकीय आवेगों को कोशिका काय से दूर सिनेप्स पर अथवा तांत्रिकीयपेशी संधि पर पहुँचाते हैं। तंत्रिकाक्ष तथा दुम्राक्ष की संख्या के आधार पर न्यूरोंस को तीन समूहों में बाँटते हैं। जैसे बहुध्रुवीय (एक तंत्रिकाक्ष व दो या अधिक दुम्राक्ष्य युक्त जो प्रमस्तिष्क वल्कुट में पाए जाते हैं।) तथा द्विध्रुवीय (एक तंत्रिकाक्ष एवं एक द्रुमाक्ष्य जो दृष्टि पटल में पाए जाते हैं।) तंत्रिकाक्ष दो प्रकार के होते हैं: आच्छदी व आच्छदहीन। आच्छदी तंत्रिका तंतु श्वान कोशिका से ढके रहते हैं, जो तंत्रिकाक्ष के चारो ओर माइलिन आवरण बनाती है। माइलिन आवरणों के बीच अंतराल पाए जाते हैं, जिन्हें रेनवीयर के नोड कहते हैं। आच्छदी तंत्रिका तंतु मेरू व कपाल तंत्रिकाओं में पाए जाते हैं। आच्छदहीन तंत्रिका तंतु भी श्वान कोशिका से घिरे रहते हैं; लेकिन वे ऐक्सोन के चारों ओर माइलीन आवरण नहीं बनाते हैं। सामान्यतया स्वायत्त तथा कायिक तंत्रिका तंत्र में मिलते हैं।
चित्र 18.1 तंत्रिकोशिका की संरचना
18.3.1 तंत्रिका आवेगों की उत्पत्ति व संचरण
तंत्रिकोशिकाएं (न्यूरोंस)उद्दीपनशील कोशिकाएं हैं; क्योंकि उनकी झिल्ली ध्रुवीय अवस्था में रहती है। क्या आप जानते हैं, यह झिल्ली ध्रुवीय अवस्था में क्यों रहती है? विभिन्न प्रकार के आयन पथ (चैनल) तंत्रिका झिल्ली पर पाए जाते हैं। ये आयन पथ विभिन्न आयनों के लिए चयनात्मक पारगम्य हैं। जब कोई न्यूरॉन आवेगों का संचरण नहीं करते हैं जैसे कि विराम अवस्था में तंत्रिकाक्ष झिल्ली सोडियम आयंस की तुलना में पोटैसियम आयंस तथा क्लोराइड आयंस के लिए अधिक पारगम्य होती है। इसी प्रकार से झिल्ली, तंत्रिकाक्ष द्रव्य में उपस्थित ऋण आवेषित प्रोटिकाल में भी अपारगम्य होती है। धीरे-धीरे तंत्रिकाक्ष के तंत्रिका द्रव्य में $\mathrm{K}^{+}$तथा ऋणात्मक आवेषित प्रोटीन की उच्च सांद्रता तथा $\mathrm{Na}^{+}$की निम्न सांद्रता होती है। इस भिन्नता के कारण सांन्द्रता प्रवणता बनती है। झिल्ली पर पाई जाने वाली इस आयनिक प्रवणता को सोडियम पोटैसियम पंप द्वारा नियमित किया जाता है। इस पंप द्वारा प्रतिचक्र $3 \mathrm{Na}^{+}$बाहर की ओर व $2 \mathrm{~K}^{+}$कोशिका में प्रवेश करते हैं। परिणामस्वरूप तंत्रिकाक्ष झिल्ली की बाहरी सतह धन आवेशित; जबकि आंतरिक सतह ऋण आवेशित हो जाती है; इसलिए यह ध्रुवित हो जाती है। विराम स्थिति में प्लाज्मा झिल्ली पर इस विभवांतर को विरामकला विभव कहते हैं।
चित्र 18.2 एक तंत्रिकाक्ष में तंत्रिका आवेग का संचरण प्रदर्शित करते हुए आरेख
आप यह जानने के लिए उत्सुक होंगे कि तंत्रिकाक्ष पर तंत्रिका आवेग की उत्पत्ति एवं उसका संचरण किस प्रकार होता है? जब किसी एक स्थान पर ध्रुवित झिल्ली पर आवेग होता है (चित्र 18.2 का उदाहरण) तब $\mathrm{A}$ स्थल की ओर स्थित झिल्ली $\mathrm{Na}^{+}$के लिए मुक्त पारगमी हो जाती है। जिसके फलस्वरूप $\mathrm{Na}^{+}$तीव्र गति से अंदर जाते हैं और एक सतह पर विपरीत ध्रुवता हो जाती है अर्थात् झिल्ली की बाहरी सतह ऋणात्मक आवेशित तथा आंतरिक सतह धनात्मक आवेशित हो जाती है। $A$ स्थल पर झिल्ली की विपरीत ध्रुवता होने से विध्रुवीकरण हो जाता है। $A$ झिल्ली की सतह पर विद्युत विभवांतर क्रियात्मक विभव कहलाता है, जिसे तथ्यात्मक रूप से तंत्रिका आवेग कहा जाता है। तंत्रिकाक्ष से कुछ आगे (जैसे स्थान $\mathrm{B}$ ) झिल्ली की बाहरी सतह पर धनात्मक आवेश तथा आंतरिक सतह पर ऋणात्मक आवेश होता है। परिणामस्वरूप $A$ स्थल से $B$ स्थल की ओर झिल्ली की आंतरिक सतह पर आवेग विभव का संचरण होता है। अतः स्थान $\mathrm{A}$ पर आवेग क्रियात्मक विभव उत्पन्न होता है। तंत्रिकाक्ष की लंबाई के समांतर क्रम का पुनरावर्तन होता है और आवेग का संचरण होता है। उद्दीपन द्वारा प्रेरित $\mathrm{Na}^{+}$ के लिए बढ़ी पारगम्यता क्षणिक होती है उसके तुरंत पश्चात $\mathrm{K}^{+}$की प्रति पारगम्यता बढ़ जाती है। कुछ ही क्षणों के भीतर $\mathrm{K}^{+}$झिल्ली के बाहरी ओर परासरित होता है और उद्दीपन के स्थान पर (विराम विभव का) पुनः संग्रह करता है तथा तंतु आगे के उद्दीपनों के लिए एक बार फिर उत्तरदायी हो जाते हैं।
18.3.2 आवेगों का संचरण
तंत्रिका आवेगों का एक न्यूरॉन से दूसरे न्यूरॉन तक संचरण सिनेप्सिस द्वारा होता है। एक सिनेप्स का निर्माण पूर्व सिनैप्टिक न्यूरॉन तथा पश्च सिनेप्टिन न्यूरॉन की झिल्ली द्वारा होता है, जो कि सिनेप्टिक दरार द्वारा विभक्त हो भी सकती है या नहीं भी। सिनेप्स दो प्रकार के होते हैं, विद्युत सिनेप्स एवं रासायनिक सिनेप्स। विद्युत सिनेप्सिस पर, पूर्व और पश्च सिनेप्टिक न्यूरॉन की झिल्लियाँ एक दूसरे के समीप होती है। एक न्यूरॉन से दूसरे न्यूरॉन तक विद्युत धारा का प्रवाह सिनेप्सिस से होता है। विद्युतीय सिनेप्सिस से आवेग का संचरण, एक तंत्रिकाक्ष से आवेग के संचरण के समान होता है। विद्युतीय-सिनेप्सिस से आवेग का संचरण, रासायनिक सिनेप्सिस से संचरण की तुलना में अधिक तीव्र होता है। हमारे तंत्र में विद्युतीय सिनेप्सिस बहुत कम होते हैं।
रासायनिक सिनेप्स पर, पूर्व एवं पश्च सिनेप्टिक न्यूरोंस की झिल्लियाँ द्रव से भरे अवकाश द्वारा पृथक होती है जिसे सिनेप्टिक दरार कहते हैं (चित्र 18.3)। क्या आप जानते हैं किस प्रकार पूर्व सिनेप्टिक आवेग (सक्रिय विभव) का संचरण सिनेप्टिक दरार से पश्च सिनेप्टिक न्यूरॉन तक करते हैं? सिनेप्सिस द्वारा आवेगों के संचरण में न्यूरोट्रॉसमीटर (तंत्रिका संचारी) कहलाने वाले रसायन सम्मिलित होते हैं। तंत्रिकाक्ष के छोर पर स्थित (आश्रय पुटिकाऐं) तंत्रिका संचारी अणुओं से भरी होती हैं। जब तक आवेग तंत्रिकाक्ष के छोर तक पहुँचता है। यह सिनेप्टिक पुटिका की गति को झिल्ली की ओर उत्तेजित करता है, जहाँ वे प्लाज्मा झिल्ली के साथ जुड़कर तंत्रिका संचारी अणुओं को सिनेप्टिक दरार में मुक्त कर देते हैं। मुक्त किए गए तंत्रिका संचारी अणु पश्च सिनेप्टिक झिल्ली पर स्थित विशिष्ट ग्राहियों से जुड़ जाते हैं। इस जुड़ाव के फलस्वरूप आयन चैनल खुल जाते हैं और उसमें आयनों के आगमन से पश्च सिनेप्टिक झिल्ली पर नया विभव उत्पन्न हो जाता है। उत्पन्न हुआ नया विभव उत्तेजक या अवरोधक हो सकता है।
चित्र 18.3 तंत्रिकाक्ष सिरा एवं सिनेप्स को प्रदर्शित करते हुए
18.4 केंद्रीय तंत्रिका तंत्र - मानव मस्तिष्क
मस्तिष्क हमारे शरीर का केंद्रीय सूचना प्रसारण अंग है और यह ‘आदेश व नियंत्रण तंत्र’ की तरह कार्य करता है। यह ऐच्छिक गमन शरीर के संतुलन, प्रमुख अनेच्छिक अंगों के कार्य (जैसे फेफड़े, हद्य, वृक्क आदि), तापमान नियंत्रण, भूख एवं प्यास, परिवहन, लय, अनेकों अंतःस्रावी ग्रंथियों की क्रियाएं और मानव व्यवहार का नियंत्रण करता है। यह देखने, सुनने, बोलने की प्रक्रिया, याददाश्त, कुशाग्रता, भावनाओं और विचारों का भी स्थल है।
मानव मस्तिष्क खोपड़ी के द्वारा अच्छी तरह सुरक्षित रहता है। खोपड़ी के भीतर कपालीय मेनिंजेज से घिरा होता है, जिसकी बाहरी परत ड्यूरा मैटर, बहुत पतली मध्य परत एरेक्नॉइड और एक आंतरिक परत पाया मैटर (जो कि मस्तिष्क ऊतकों के संपर्क में होती है) कहलाती है। मस्तिष्क को 3 मुख्य भागों में विभक्त किया जा सकता है: (i) अग्र मस्तिष्क, (ii) मध्य मस्तिष्क, और (iii) पश्च मस्तिष्क (चित्र 18.4)।
चित्र 18.4 मानव मस्तिष्क का सममितार्धी (सेजीटल) काट
18.4.1 अग्र मस्तिष्क
अग्र मस्तिष्क सेरीब्रम, थेलेमस और हाइपोथेलेमस का बना होता हैं सेरीब्रम (प्रमस्तिष्क) मानव मस्तिष्क का एक बड़ा भाग बनाता है। एक गहरी लंबवत विदर प्रमस्तिष्क को दो भागों, दाएं व बाएं प्रमस्तिष्क गोलार्द्धो में विभक्त करती है। ये गोलार्द्ध तंत्रिका तंतुओं की पट्टी कार्पस कैलोसम द्वारा जुड़े होते हैं (चित्र 18.4)। प्रमस्तिष्क गोलार्द्ध को कोशिकाओं की एक परत आवरित करती है, जिसे प्रमस्तिष्क वल्कुट कहते हैं तथा यह निश्चित गर्तो में बदल जाती है। प्रमस्तिष्क वल्कुट को इसके धूसर रंग के कारण धूसर द्रव्य कहा जाता है। तंत्रिका कोशिका काय सांद्रित होकर इसे रंग प्रदान करती है। प्रमस्तिष्क वल्कुट में प्रेरक क्षेत्र, संवेदी भाग और बड़े भाग होते हैं, जो स्पष्टतया न तो प्रेरक क्षेत्र होते हैं न ही संवेदी। ये भाग सहभागी क्षेत्र कहलाते हैं तथा जटिल क्रियाओं जैसे अंतर संवेदी सहभागिता, स्मरण, संपर्क सूत्र आदि के लिए उत्तरदायी होते हैं। इस पथ के रेशे माइलिन आच्छद से आवरित रहते हैं जो कि प्रमस्तिष्क गोलार्द्ध का आंतरिक भाग बनाते हैं। ये इस परत को सफेद अपारदर्शी रूप प्रदान करते हैं, जिसे श्वेत द्रव्य कहते हैं। प्रमस्तिष्क थेलेमस नामक संरचना के चारों ओर लिपटा होता है, जो कि संवेदी और प्रेरक संकेतों का मुख्य संपर्क स्थल है। थेलेमस के आधार पर स्थित मस्तिष्क का दूसरा मुख्य भाग हाइपोथेलेमस स्थित होता है। हाइपोथेलेमस में कई केंद्र होते हैं, जो शरीर के तापमान, खाने और पीने का नियंत्रण करते हैं। इसमें कई तंत्रिका स्रावी कोशिकाएं भी होती हैं जो हाइपोथेलेमिक हार्मोन का स्रवण करती हैं। प्रमस्तिष्क गोलार्द्ध का आंतरिक भाग और अंदरूनी अंगों जैसे एमिगडाला, हिप्पोकैपस आदि का समूह मिलकर एक जटिल संरचना का निर्माण करता है, जिसे लिंबिकलोब या लिबिंक तंत्र कहते हैं। यह हाइपोथेलेमस के साथ मिलकर लैंगिक व्यवहार, मनोभावनाओं की अभिव्यक्ति (जैसे उत्तेजना, खुशी, गुस्सा और भय) आदि का नियंत्रण करता है।
18.4.2 मध्य मस्तिष्क
मध्य मस्तिष्क अग्र मस्तिष्क के थेलेमस/हाइपोथेलेमस तथा पश्च मस्तिष्क के पोंस के बीच स्थित होता है। एक नाल प्रमस्तिष्क तरल नलिका मध्य मस्तिष्क से गुजरती है। मध्य मस्तिष्क का ऊपरी भाग चार लोबनुमा उभारों का बना होता है जिन्हें कॉर्पोरा क्वाड्रीजेमीन कहते हैं।
18.4.3 पश्च मस्तिष्क
पश्च मस्तिष्क पोंस, अनुमस्तिष्क और मध्यांश (मेड्यूला ओबलोगेंटा) का बना होता है। पोंस रेशेनुमा पथ का बना होता है जो कि मस्तिष्क के विभिन्न भागों को आपस में जोड़ते हैं। अनुमस्तिष्क की सतह विलगित होती है जो न्यूरोंस को अतिरिक्त स्थान प्रदान करती है। मस्तिष्क का मध्यांश मेरूरज्जु से जुड़ा होता है। मध्यांश में श्वसन, हृदय परिसंचारी प्रतिवर्तन और पाचक रसों के स्राव के नियंत्रण केंद्र होते हैं। मध्य मस्तिष्क, पोंस और मेडुला ओबलोगेटा मस्तिष्क स्तंभ के तीन प्रमुख क्षेत्र हैं। मस्तिष्क स्तंभ, मस्तिष्क और मेरू रज्जू के बीच संयोजन स्थापित करता है।
सारांश
तंत्रिका तंत्र समन्वयी तथा एकीकृत क्रियाओं के साथ ही अंगों की उपापचयी और समस्थैनिक क्रियाओं का नियंत्रण करता है। तंत्रिका तंत्र की क्रियात्मक इकाई न्यूरोंस, झिल्ली के दोनों ओर सांद्रता प्रवणता अंतराल के कारण उत्तेजक कोशिकाएं होती हैं। स्थिर तंत्रिकीय झिल्ली के दोनों ओर का विद्युत विभवांतर विरामकला विभव कहलाता है। तंत्रिकांक्ष झिल्ली पर विद्युत विभावांतर प्रेरित उद्दीपन द्वारा संचारित होता है। इसे सक्रिय विभव कहते हैं। तंत्रिकाक्ष झिल्ली की सतह पर आवेगों का संचरण विध्रुवीकरण और पुनध्रुवीकरण के रूप में होता है। पूर्व सिनेप्टिक न्यूरॉन और पश्च सिनेप्टिक न्यूरॉन की झिल्लियाँ सिनेप्स का निर्माण करती है, जो कि सिनैप्टिक विदर द्वारा पृथक हो सकती है या नहीं होती है। सिनेप्स दो प्रकार के होते हैं - विद्युत सिनैप्स और रासायनिक सिनेप्स। रासायनिक सिनैप्स पर आवेगों के संचरण में भाग लेने वाले रसायन न्यूरोट्रांसमीटर कहलाते हैं।
मानव तंत्रिका तंत्र दो भागों का बना होता है : (i) केंद्रीय तंत्रिका तंत्र और (ii) परीधीय तांत्रिका तंत्र। सी एन एस मस्तिष्क और मेरूरज्जु का बना होता है। मस्तिष्क को तीन मुख्य भागों में विभाजित किया जा सकता है। (i) अग्र मस्तिष्क (ii) मध्य मस्तिष्क (iii) पश्च मस्तिष्क। अग्र मस्तिष्क प्रमस्तिष्क, थेलेमस और हाइपोथेलेमस से बना होता है। प्रमस्तिष्क लंबवत् दो अर्धगोलार्धों में विभक्त होता है, जो कॉर्पस कैलोसम से जुड़े रहते हैं। अग्र मस्तिष्क का महत्वपूर्ण भाग हाइपोथेलेमस शरीर के तापक्रम, खाने और पीने आदि क्रियाओं का नियंत्रण करता है। प्रमस्तिष्क गोलार्द्धो का आंतरिक भाग और संगठित गहराई में स्थित संरचनाएं मिलकर एक जटिल संरचना बनाते हैं, जिसे लिम्बिक तंत्र कहते हैं और यह सूंघने, प्रतिवर्त्ती क्रियाओं, लैंगिक व्यवहार के नियंत्रण, मनोभावों की अभिव्यक्ति और अभिप्रेरण से संबंधित होता है। मध्य मस्तिष्क ग्राही व एकीकरण तथा एकीकृत दृष्टि तंतु तथा श्रवण अंतर क्रियाओं से संबंधित है।
पश्च मस्तिष्क पोंस, अनु मस्तिष्क और मेड्यूला का बना होता है। अनु मस्तिष्क कर्ण की अर्द्धचंद्राकार नलिकाओं तथा श्रवण तंत्र से प्राप्त होने वाली सूचनाओं को एकीकृत करता है। मध्यांश (मैड्यूला) में श्वसन, हृदय परिसंचयी, प्रतिवर्तित और जठर स्रावों के नियंत्रण केंद्र होते हैं। पोंस रेशेनुमा पथ का बना होता है, जो मस्तिष्क के विभिन्न भागों को आपस में जोड़ता है। परिधीय तंत्रिका तंत्र को प्राप्त उद्दीपनों के लिए अनैच्छिक प्रतिक्रियाओं को प्रतिवर्ती क्रियाएं कहा जाता है।
अभ्यास
1. निम्नलिखित संरचना का संक्षेप में वर्णन कीजिए-
(अ) मस्तिष्क
#missing
#missing
Show Answer
#Content Missing2. निम्नलिखित की तुलना कीजिए-
(अ) केंद्रीय तंत्रिका तंत्र और परिधीय तंत्रिका तंत्र
(ब) स्थिर विभव और सक्रिय विभव
#missing
Show Answer
#Content Missing3. निम्नलिखित प्रक्रियाओं का वर्णन कीजिए-
(अ) तंत्रिका तंतु की झिल्ली का ध्रुवीकरण
(ब) तंत्रिका तंतु की झिल्ली का विध्रुवीकरण
(स) रासायनिक सिनेप्स द्वारा तंत्रिका आवेगों का संवहन
#missing
Show Answer
#Content Missing4. निम्नलिखित का नामांकित चित्र बनाइए-
(अ) न्यूरॉन
(ब) मस्तिष्क
#missing
#missing
Show Answer
#Content Missing5. निम्नलिखित पर संक्षिप्त टिप्पणी लिखिए-
(अ) तंत्रीय समन्वयन
(ब) अग्र मस्तिष्क
(स) मध्य मस्तिष्क
(द) पश्च मस्तिष्क
#missing
#missing
#missing
#missing
#missing
Show Answer
#Content Missing6. निम्न पर संक्षिप्त टिप्पणी दीजिए-
(अ) सिनेप्टिक संचरण की क्रियाविधि
#missing
#missing
Show Answer
#Content Missing7. (अ) सक्रिय विभव उत्पन्न करने में $\mathrm{Na}^{+}$की भूमिका का वर्णन कीजिए।
(ब) सिनेप्स पर न्यूरोट्रांसमीटर मुक्त करने में $\mathrm{Ca}^{++}$की भूमिका का वर्णन कीजिए।
#missing
#missing
Show Answer
#Content Missing8. निम्न के बीच में अंतर बताइए-
(अ) आच्छादित और अनाच्छादित तंत्रिकाक्ष
(ब) दुम्राक्ष्य और तंत्रिकाक्ष
(स) थेलेमस और हाइपोथेलेमस
Show Answer
#Content Missing9. (अ) मानव मस्तिष्क का सर्वाधिक विकसित भाग कौनसा है?
(ब) केंद्रीय तंत्रिका तंत्र का कौनसा भाग मास्टर क्लॉक की तरह कार्य करता है?
#missing
#missing
#missing
#missing
Show Answer
#Content Missing10. निम्न में भेद स्पष्ट कीजिए-
(अ) संवेदी तंत्रिका एवं प्रेरक तंत्रिका
(ब) आच्छादित एवं अनाच्छादित तंत्रिका तंतु में आवेग संचरण
(स) कपालीय तंत्रिकाएं एवं मेरू तंत्रिकाए
Show Answer
#Content Missing
#missing #missing #missing
