Auditory System| Visual System | Olfactory and Limbic System | General Sensation

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Auditory System

Auditory Animation

• 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.

• The major sense organ of the olfactory system is the nose. Within the nasal mucosa are located bipolar receptor neurons. When these neurons are stimulated they send impulses via the olfactory bulb and tract to the anterior perforated substance, the amygdaloid nucleus, hippocampus, fornix and mamillary bodies.
  
  
• The olfactory system is closely related to the limbic system. This linkage is largely responsible for the complex memories and emotions which are associated with certain odors.

• Pain and temperature sensation are extremely important to our survival. Therefore there are very complex connections which are even now poorly understood. Pain and temperature receptors are located in the dermis and epidermis of the skin and in the viscera. These receptors send synaptic projections to the dorsal root ganglion (DRG) which in turn projects via the dorsal horn of spinal cord, crosses to the lateral column of the spinal cord and ascends via the lateral spinothalamic tract to the ventral posterior lateral (VPL) nucleus of the thalamus. This thalamic projection accounts for much of the emotional response to chronic pain.
  
  
• Finally, fibers project to the post central gyrus which enables us to localize the pain sensation to a particular part of the body.
  
  
• A reflex arc exists in the spinal cord level, so that we withdraw our hand from a hot object even before we are aware that we have touched something hot.
  
  
• Pressure and Crude touch sensation is a relatively simple pathway. Receptors are located in the dermis. They project to the dorsal root ganglion (DRG) and travel via the dorsal root to the ventral spinal cord where they ascend to the thalamus via the ventral spinothalamic tract.
  
  
• Proprioception, fine touch and vibration is also a relatively straightforward pathway. Receptors are located in muscles, joints and tendons and project to the dorsal root ganglion (DRG). Fibers then ascend via the ipsilateral dorsal white column. The lumbosacral fasciculus ascends to nucleus gracilis and the thoracocervical fasciculus ascends to nucleus cuneatus. Both of these nuclei are loctaed in the lower medulla. Fibers then cross as the medial lemniscus and finally ascend to the ventral posterior lateral (VPL) nucleus of the thalamus.
  
  
• General sensation from the face is mediated by the trigeminal nerve (cranial nerve V). It has three branches, The Ophthalmic (V1), Maxillary (V2) and Mandibular (V3) which project to the trigeminal ganglion in the pons and the trigeminal nucleus in the medulla. Fibers cross and ascend to the ventral posterior medial (VPM) nucleus in the thalamus.
  
  
• Fibers for pressure and crude touch synapse in the pons and also ascend to the VPM.

last update Jan 21, 2002