Question

explain signal transduction and the sensory motor pathways of animals using the examples of the vertebrate ear and eye

Answer 1

signal yransduction is the process by which a chemical or physical signal is transmitted through the cell as a series of molecular events ,most commonly protein phosphorylation which ultimate result in a response .each component of signaling pathwayis classified according to the role it playswith respect to the initial stimulus.legand termed as first massenger.whilr the receptor are the signal transducers whichthen activate primary effectors.

Audition or hearing is importent to humans and other animals for many different intractions.auditory stimuli are sound waves which are mechanical pressure waves that move througha medium such as air or water.In mammals sound waves are collected by the external cartilaginous part of the ear called Pinna then travels through the auditory canel and cause vibration of the thin diaphagrm called Tympanum or eardrum the innermost part of outer ear.interior to tympanum is middle ear .the middle ear has 3 bones called ossicles.which transfer energy from the moving tympanum to inner ear.the three bones are malleus,incus,stapes.this bone play role in hearing.sound travels through the outer earto the middle earcwhich is bounded on its exterior by the tympanic membrane.the organ of corti which is the organ of sound transductionslies inside the cochlea.the vibrations enter the cochlea via round window and exit via the round window.as they pass through the endolumph of the scala vestibuli and tympani sound waves cause the basilar membrane to vibrate .this is the key to the auditory function.the basilar membrane is stiff at its base and loose at its apex .just like guiter string.hair cells transduce vibrations into depolarization.this in turn leads to vesicular release that excites aditory afferant fibres and cause them to discharge.hair cells are frimly attouched to the basilar membrane and therefore move upvand down with it as it vibrates.the 'hair, or cilia of these cells are attouched to a tectorial membrane .this menbrane is fixed it does not vibrate in response tovsound .when the basilar membrane move upward the cilia will be bent .this is the first step in the transduction process.when cilia bent to one direction it causes the hair cells to hyperpolarize,bending in the opposite direction cause depolarization. The depolarization cause ca²⁺ entry and the fusion of vesicals and release glutamate from hair cells .tjis cause excitation and spiking of auditory afferent fibres.

vertebrate eye: the eye of human is a typical vertebrate eye.the whiteof the eye is scala ,formed of tough connective tissue .light enters the eye through atransparent cornea,which begins to focus the light.this occurs because light is refracted (bent) when it travels into a medium of different density.the coloured part of the eye is iris.the concentration of iris muscles in bright light decreases the size of its opening ,the pupil ,the light passes through the pupil to the lens .lens is a transparent structure that completes the focusing of the light onto the retina at the back of the eye.the lens is attouched to ciliary muscles.vertebrate retina contains two kinds of photoreceptors called rods and cones . Rods are responsible for black and white vision.when the illumination is dim,while the cons are responsible for high visual acuity and colour vision.humans have 100million rods and3 million cons .the transduction of light energy into nerve impulses foll9wsxa sequencecthat is inverse of the usual way that sensory stimuli are detected .this is because in the darkvthe photoreceptors release an inhibitory neurotransmitters thathyperpolarize the bipolar neurons .thus inhibited,the bipolar neurons do not release excitatory neurotransmitter and by this means stimulates the bipolarccellscand thus ganglion cells,which transmit action potential to thecbrain. A rod or cone contains Na ionchannels in the plasma membrane of its outer segment and in the dark,many of these channels are open . As a consequences ,Na ioncontinously diffuses into the outer segment and accross the narrowvstalkvto the inner segment.this flow of Na ion that occursin the absense of light is called dark current and it causes themenbrane of a photoreceptor to be some what depolarized in dark.in the light,the Na ion channelvin the outer segmentrapidly close ,reducing the dark currentvand causing the photoreceptor to hyperpolarize.cyclic guanosine monophosphate (cGMP) is required to keep Na ion channels open,that the channel will close if the cGMP is converted into GMP.how does light cause this conversion and cinsequent closing of Na ion channels? When a photoreceptor absorb light, cis retinal isomerizes and dissociates from opsin in what is known as the bleaching reaction.as a result of this dissociation, the opsin protein changes its shape .each opsin is associated with iver a hundred regulatary G proteins .when the opsin changes shape,the G protein dissociate,releasing subunits that activayes hundreds of molecule of the enzyme phospho diesterase .this enzyme convert cGMP to GMP , thus closing the Na ion channels at a rate of about 1000 per second and inhibiting the dark current. The absoroption of a signal photon of light can block the entry of more then a million Na ions,thereby causing the photoreceptor to hyperpolarize and release neurotransmittres.