Auditory System| Visual System | Olfactory and Limbic System | General Sensation
- The sensory organ of the auditory system is the ear. The outer and middle ear is designed to amplify and transmit sound waves to the inner ear where hair cells within the cochlea are stimulated by sound waves to generate nerve impulses which are transmitted via the acoustovestibular nerve (cranial nerve VIII) to the dorsal and ventral cochlear nuclei in the medulla, near the floor of the fourth ventricle. These nuclei then send projections via the lateral lemniscus to the inferior colliculus which is located superior to the aqueduct in the midbrain. The inferior colliculus in turn projects to the medial geniculate nucleus, which some authors consider to represent part of the thalamus, and finally to the primary auditory cortex which is located in the superior temporal cortex, also known as Brodmann area 41 and 42.
- The sensory organ of the visual system is the eye which detects and focuses stimuli. The cornea is the clear window to the eye. It covers the iris which is the colored part or the eye. The principle function of the iris is not to make us attractive but rather to control the amount of light which enters the eye via the pupil. You will have probably noticed that in bright light the pupil is small and that in dim light it is large. Light then travels through the lens, which focuses light, to the retina which covers the back of the eye. The retina is a complex mutilayered organ. Within the retina are the primary visual sensory cells, the rods and cones. When the rods and cones are stimulated they send impulses via the optic nerves to the optic chiasm. At the optic chiasm, half of the axons cross to the other side. The axons that cross are those from the medial retina which represent the temporal visual field. Axons from the medial or nasal retina (temporal visual field) on one side and the temporal retina (nasal field) on other, then join to form the optic tract. The optic tract projects to the lateral geniculate nucleus.
- The lateral geniculate nucleus consists of six cell layers. The two most ventral laminae are composed of large cells and thus called magnocellular layers. The dorsal four layers are composed of small cells and called parvocellular layers. Large retinal ganglion cells send axons to the magnocellular layers and small retinal ganglion cells send axons to the parvocellular layers.
- After the neurons of the lateral geniculate are stimulated they send axonal projections via the optic radiations to the primary visual cortex around the calcarine fissure in the occipital lobe, which is also known as Brodmann area 17.
- There are several important terms used to describe difficulties with the visual system. Anopsia means blindness. Hemianopsia means loss of vision in one half of the visual field. This is caused by a lesion in one eye or one optic nerve. Bitemporal hemianopsia means that a person has tunnel vision. This is caused by a lesion in the optic chiasm, most commonly a pituitary tumor. Homonymous hemianopsia means that the person is blind on either the right or the left side. This is caused by injury to the optic tract, optic radiations or visual cortex.
last update Jan 21, 2002