Q. 53.5( 6 Votes )

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.

Answer :

(a) Neural Coordination. The overall functions performed by the nervous system to coordinate and control all the activities of the body are called as Neural Coordination. The brain is the centre of performing physical activities as well as the seat of emotions, will, feelings, intelligence etc. The nerves perform the task of transmitting nerve impulses from the sense organs to the brain and also from the brain to the sense organs. The Nervous system carried out the function of interaction of two or more organs and complementing the functions of each other. (b) Forebrain. It consists of cerebrum, thalamus and hypothalamus. Cerebrum is made up two cerebral hemispheres connected by corpus callosum. Cerebrum is responsible for functions like memory and communication. The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signalling. Hypothalamus is present at the base of thalamus. It controls body temperature, eating and drinking. It also has neurosecretory cells, which secrete hormones (c) Midbrain. It consists of cerebral peduncles (hands of nerve fibres) and this lie in the floor of mid brain. These connect the various parts of the fore and hindbrain. It consists of two pairs of round elevations, the corpora quadrigemina, and act as centres for auditory and visual reflexes. Functions of mid brain nuclei: (i) Mid brain nuclei control muscle tone. (ii) These centres modify motor activities initiated by cerebral cortex. (d) Hindbrain. It consists of cerebellum dorsally pons varolie and medulla oblongata. Cerebellum. It lies at the base or posterior part of the brain. The outer cerebellar cortex is made of grey matter. Centrally it has white matter which appears branched. Functions of Pons. It co- ordinates the movements of two sides of body. Medulla oblongata. It lies between the pons and the spinal cord. It is roughly triangular. The medulla contains such vital centres as cardiac, respiratory and arterial pressure. Functions of Medulla oblongata. (i) It controls the activities of internal organs such as heart, lung and digestive tract, vasomotor centre. (ii) It also carries nerve tracts between the spinal cord and higher brain centre. Retina. 1. Retina is the inner sensory coat of eyeball and is formed of following four layers and cells bearing protoplasmic process on inner side. 2. Sensory layer. It lies next to pigmented layer and composed of rods and cons. Rods contain visual pigment rhodopsin and function during dim light. The cones contain pigment iodopsin and function in bright light. Scattered amongst the rods and cones are supporting cells. Layer of bipolar nerve cells. It is a single layer of neuron. Its dendrites synapse with sensory layer and axon with dendrites of ganglionated layer. Ganglionated Layer. It is the innermost layer of the nerve cells. The axons of which converge to form optic nerve. e. Ear Ossicles. There are three tiny bones in the middle ear. The middle ear bones called malleus (hammer- shaped), incus (anvil shpaed) and stapes (stirrup-shaped) form a chain across the middle ear. They extended from the inner surface of ear drum to a similar membrane present on the fenestra ovalis (opening of internal ear). Function. The ear ossicles transmit the vibrations from tympanic membrane to internal ear after increasing the force. (f) Cochlea. Cochlea is a spirally coiled duct, it arises from sacculus of vestibule. It is an organ which aids hearing. If a section of cochlea is cut through, it reveals three tubes as follows: Structure of CochleaScala vestibuli, scala media and scala tympani. The scala vestibuli is continuous with the vestibuli. The scale tympani is continuous with middle ear at fenestra rotunda. The scala tympani communicate with the scala vestibuli by helicotrema. The roof of the scale mdia is called the Reissner’s membrane while the floor is known as basilar membrane. In this chamber the endolymph, is present. On the basilar membrane is present the organ of Corti. It is made up of sensory cells bearing sensory hairs. These hair are embedded in the tectorial membrane arising from basilar membrane. Organ of Corti is the sound perceiving apparatus. Nerve impulse thus set up carry the information to the brain (g) Organ of Corti. The organ of corti is enclosed at the tip of the cochlean duct in the internal ear. It is made up of five longitudial rows of cells (phonoreceptors) held in position by supporting cells. The supporting cells are of two types – long pillar cells and short phalangeal cells. Each auditory or hair cell is a neurosensory cell which has a number of auditory hair on free side and a nerve fibre on other side. The nerve fibres join to form cochlear branch of auditory nerve. The auditory hair is embedded in a gelatinous ribbon like tectorial membrane. (h) Synapse. A synapse is a close proximity of end knob of axon of one neuron and dendron or cell body of the next neuron. The former cell is called the Presynaptic cell while the latter is called the Postsynaptic cell. The presynaptic cell has numerous mitochondria and synaptic vesicles that contain neurotransmitters. The space between the two cells is called synaptic cleft. There is a release of neurotransmitter such as Acetylcholine from the pre synaptic cell into the cleft by exocytosis. The acteylcholine binds to receptor present over the postsynaptic cell. Binding of acetylcholine to its receptor causes depolarisation of the next neuron. To cause inactivation of the process, acetylcholine is inactivated by enzyme acerylcholines – terase into acetic acid and choline.

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