CHAPTER - 19 NEURAL CONTROL AND COORDINATION

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1 CHAPTER - 19 NEURAL CONTROL AND COORDINATION 1. Action potential: A sudden change in the electrical charges in the plasma membrane of a nerve fibre. 2. Aqueous humour: The thin watery fluid that occupy space between lens and cornea in eye. 3. Blind spot: A spot on retina which is free from rods and cones and lack the ability for vision. 4. Cerebrospinal fluid: An alkaline fluid present in between inner two layer of meninges and also in the cavities (ventricles) found Inside brain and spinal cord. 5. Cerebellum: A part of hind brain that controls the balance and posture of the body. 6. Cochlea: A spirally coiled part of internal ear which is responsible for hearing. 7. Corpus callosum: A curved thick bundle of nerve fibres that joins two cerebral hemisphere. 8. Depolarisation: A condition when polarity of the plasma membrane of nerve fibre is reversed. 9. Endolymph: The fluid filled within membranous labyrinth. 10. Ecustachian tube: A tube which connect ear cavity with the pharynx. 11. Fovea: An area of highest vision on the retina which contain only cones. 12. Meninges: Three sheets of covering of connective tissue wrapping the brain and spinal cord. 13. Grey Matter: This shows many convolutions which increase the amount of vital nerve tissue. 126

2 14. Medulla oblongata: Posterior most part of the brain which is continuous with spinal cord and control respiration, heart rate, swallowing, vomiting. 15. Pons: Thick bundles of fibres on the ventral side of brain below cerebellum. 16. Foramen magnum: A big aperture in the skull posteriorly through which spinal cord emerges out. 17. Spinal cord: A tubular structure connected with medulla oblongata of brain and situated in the neural canal of the vertebral column, covered by meninges. 18. White matter: It lies inner to the grey matter in the brain and outer the grey matter in the spinal cord. This white matter is made of medullated (myelinated) axons of millions of nerve cells (neurons). 19. Ventricles: The hollow spaces found in side the brain and spinal cord. These ventricles are filled with cerebro spinal fluid. 20. Synaptic cleft: A narrow fluid filled space which separates two membranes of the two neurons at the synapse. 21. Synaptic vesicles: These are membrane bound vesicles in the axoplasm of the axon terminal and these store neurotransmitters substances like acetylcholine, norepinephrine. 22. Neurotransmitter: These are chemicals stored in synaptic vesicles, diffuse to reach the membrane of next neuron for its stimulation. 23. Synapse: A physiological junction between axon of one neuron and dendrite of next neuron. CNS Central Neural system PNS Peripheral Neural system ANS Autonomic Neural system 127

3 Neural Sytem Central neural Sytem Peripheral neural System Brain Spinal cord Cranial nerve Spinal nerve Nerve fibres of PNS 12 pairs 31 pairs Afferent fibres Transmit impulse from Tissue/organ to CNS Efferent fibres Transmit impulse from CNS to Peripheral tissue/organ Division of PNS Somatic neural System Relays impulse from CNS to Skeletal muscle Autonomic neural system Transmit impulse from CNS to involuntary Organ and smooth muscles Cell body (cyton) = Cytoplasm with nucleus, Parts of Neuron Cell organelles and Nissle s granules Dendrites = Short fibres which branch rapidly And project out of cell Axon = Single, long fibre, branched at distal end Axon Multi polar = One axon and two or more dendrites Found is cerebral cortex Bipolar = One axon and one dendrite Found in retina of eye Unipolar = Cell body with axon only Found usually in the embryonic stage Conduction of nerve impulse along axon 128

4 Polarized membrane/resting Potential In resting phase when neuron is not conducting an impulse, the axonal membrane is called polarized. This is due to difference in concentration of ions across the axonal membrane. At Rest: Axoplasm inside the axon contain high conc. of K + and low conc. of Na + etc., resulting in the gross electrical charge Negative. The fluid outside the axon contain low conc. of K + and high conc. of Na + etc., resulting in the gross electrical charge Positive. As a result the outer surface of axonal membrane is positively charged and inner surface is negatively charged. The electric potential difference across the resting plasma membrane is called resting potential. Action Potential: When a nerve fibre is stimulated, the permeability of membrane to Na + is greatly increased at the point of stimulus (rapid influx of Na + ) and hence polarity of membrane is reversed and now membrane is said to be depolarized. The electric potential difference across the plasma membrane at that site is called action potential, which infact termed as nerve impulse. Depolarization is very rapid, so that conduction of nerve impulse along the entire length of axon occurs in fractions of second. Transmission of Impulses at Synapse (i) At electrical synapses: Here the membrane of pre and post-synaptic neuron are in very close proximity. Electric current can flow directly from one neuron into other across these synapses, like impulse conduction along a single axon. (ii) At chemical synapses: Here the membrane of pre and post-synaptic neuron are separated by fluid filled space called synaptic cleft. Neurotransmitter are involved here. When an impulse arrives at the axon terminal, it stimulates the 129

5 movement of the synaptic vesicles towards membrane and they fuse with the plasma membrane and release their neurotransmitter in the synaptic cleft. These chemicals bind to specific receptors, present on the post-synaptic membrane. Their binding opens up ion channels and allow the entry of ion which generate new potential in post synaptic neuron. Parts of Brain Fore brain Mid brain Hind brain (a) Cerebrum (b) Thalamus (c) Hypothalamus (a) Cerebellum (b) Pons (c) Medulla oblongata Functions of parts of brain: Cerebrum: Centre of intelligence, memory and imagination, reasoning, judgement, expression of will power. Thalamus: Acts as relay centre to receive and transmit general sensation of pain, touch and temperature. Hypothalamus: Centre for regulation of body temperature, urge for eating and drinking (Homeostasis). Mid brain: Responsible for coordinating visual reflexes and auditory reflexes -- corpora quadrigemine. Cerebellum: Maintains posture and equilibrium of the body as well as coordinates and regulates voluntary movement. Pons varoli: Relays impulses between medulla oblongata and cerebral hemisphere and between the hemisphere of cerebrum and cerebellum. Medulla oblongata: Centre that control heart beat, breathing, swallowing, salivation, sneezing, vomiting and coughing gives rise to cranial nerves (12 pairs). 130

6 Organ of Sight Eye Layer Component Function 1. External layer Sclera Protects and maintain shape of the eye ball. Cornea Absorb O2 from the air, helps to focus light rays. 2. Middle layer Choroid Absorb light and prevent light from being reflected within the eye ball. Ciliary body Holds lens, regulate shape of the lens. Iris Control amount of light entering. 3. Inner layer Retina Vision in dim light, colour vision, vision in bright light. Organ of Hearing Ear Portion of the ear Component Function 1. External ear Pinna Collect sound waves. External auditory canal 2. Middle ear Tympanic membrane Ear ossicles Eustachian tube Direct sound waves towards ear drum, ear wax prevents the entry of foreign bodies. Acts as resonator that reproduces the vibration of sound. Transmit sound waves to internal ear. Helps in equalizing the pressure of either side of ear drum. 131

7 3. Internal ear Cochlea Hearing. Vestibular Balancing of body. apparatus Note: For Diagrams and Figures see the NCERT Standard XI Biology Text Book 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. Information regarding changes in the environment is received by the CNS through the sensory organs which are processed and analysed. Signals are then sent for necessary adjustments. The wall of the human eye ball is composed of three layers. The external layer is composed of cornea and sclera. Inside sclera is the middle layer, which is called the choroid. Retina, the innermost layer, contains two types of photoreceptor cells, namely rods and cones. The daylight (photopic) vision and colour vision are functions of cones and twilight (scotopic) vision is the function of the rods. The light enters through cornea, the lens and the images of objects are formed on the retina. The ear can be divided into the outer ear, the middle ear and the inner ear. The middle ear contains three ossicles called malleus, incus and stapes. The fluid filled inner ear is called the labyrinth, and the coiled portion of the labyrinth is called cochlea. The organ of corti is a structure which contains hair cells that act as auditory receptors and is located on the basilar membrane. The vibrations produced in the ear drum are transmitted through the ear ossicles and oval window to the fluid-filled inner ear. Nerve impulses are generated and transmitted by the 132

8 afferent fibres to the auditory cortex of the brain. The inner ear also contains a complex system located above the cochlea called vestibular apparatus. It is influenced by gravity and movements, and helps us in maintaining balance of the body and posture. EXERCISE QUESTIONS: Very Short Answer Questions (1 mark each) 1. Name the fluid present in membranous labyrinth. 2. Name the area of retina where only cones are densely packed. 3. Name the inner most of the meninges of the brain. 4. With which part of the brain communication and memory are associated? 5. Name the bundle of fibres that connect two cerebral hemisphere in human being. 6. Name the photosensitive pigment present in the rod cells. 7. Why can impulses flow only in one direction? 8. Where is hypothalamus located in the brain? 133