Question 6 Give a brief account of:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing
Class - XI - Biology Chapter Neural Control and Coordination Page No. 329
Answers
At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity so 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 and transmission is always faster than that across a chemical synapse however it is not common in human body.
At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.
(b) The light rays in visible wavelength focussed on the retina through the cornea and lens generate impulses in rods and cones. The photosensitive compounds in the human eyes is composed of opsin and retinal. Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin. This causes membrane permeability changes therefore potential differences are generated in the photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells. These impulses are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed and the image formed on the retina is recognised based on earlier memory and experience.
(c) The external ear receives sound waves and directs them to the ear drum.The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles to the oval window. The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane therefore nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognised.
From MrMysterious
Answer:
Mechanism of synaptic transmission:
Synapses are the junctions where the nerve impulses are transmitted from one neuron to another. These are formed by the membranes of a pre-synaptic and a post-synaptic neuron, that may or may not be segregated by a gap known as the synaptic cleft. These are the two types of chemical synapses and electrical synapses.
The membranes of a pre-synaptic and a post-synaptic neuron at the electrical synapses are in close proximity so that electrical current can directly flow from one neuron to the other across these particular synapses. The transmission of an impulse across an electrical synapses is similar to conduction of an impulse along a single axon where the transmission is always quicker than that across a chemical synapse which is not commonly observed in the human body.
The membranes of a pre-synaptic and a post-synaptic neuron at the chemical synapses are segregated by a synaptic cleft (fluid-filled space). The chemicals known as neurotransmitters are involved in the impulse-transmission at these particular synapses.
Mechanism of vision:
The passage of light rays is as follows – pupil, lens, aqueous humour, vitreous humour and finally retina. This light causes the dissociation of the photo-pigment rhodopsin to retinal and opsin. The structure of opsin is subjected to changes due to the dissociation of opsin from the retinal which generates an action potential in the cones and rods of the retina. Furthermore, the action potential is transmitted to the ganglion cells via the bipolar neurons and ultimately transmitted to the visual cortex of the brain through the optic nerve. Analysis of impulses take place at the visual cortex, responses are sent back in order to form images on the retina.
Mechanism of hearing:
The pinna of the external ear collects sound waves which pass through the external auditory meatus all the way to the eardrum. This causes the eardrum to vibrate. These vibrations are passed from the eardrum to the malleus, incuse and stapes of the middle ear which causes an increase in the frequency of the vibrations. The vibrations furthermore are passed to the cochlea of the inner ear through the oval window. These vibrations in the endolymph of the cochlea cause vibrations to be induced in the basilar membrane, which inturn cause sensory hair of thr organ of corti to vibrate.
The receptor hair cells force themseleves against the tectorial membrane converting sound energy to a nerve impulse or action potential. This nerve impulse is transmitted to the auditory cortex of the brain, where the impulse is evaluated and analyzed causing the sound to be recognized.