Cranial Nerves and Taste Buds

Taste (Gustation)

• The glossopharyngeal nerve connects to taste buds in the posterior two-thirds of the tongue.
• The vagus nerve connects to taste buds in the extreme posterior of the tongue, which are more sensitive to noxious stimuli like bitterness.
• Axons from the three cranial nerves carrying taste information travel to the medulla.
• The primary gustatory cortex, located near the inferior margin of the post-central gyrus, is responsible for our sensations of taste.

Smell (Olfaction)

• Olfactory receptor neurons are incorporated into a limited region of the nasal epithelium in the superior nasal cavity.
• The olfactory epithelium contains bipolar sensory neurons with dendrites extending from the apical surface of the epithelium into the mucus lining the nasal cavity.
• Odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites.
• The odorant-protein complex binds to a receptor protein on the membrane of the olfactory cell.
• Each cilia may have as many as 40 specific receptor membrane proteins for interaction with different odorant molecules.
• Rapid adaptation and removal of the odorants permit continued recognition and discrimination of new aromas.
• Action potentials generated in the axon terminals of activated neurons are propagated into the glomeruli within the olfactory bulb.
• The axons of the olfactory neurons extend into the olfactory bulb located on the ventral surface of the frontal lobe.
• Smell is the one sensory modality that does not require a synaptic connection in the thalamus before connecting to the cerebral cortex.

Divisions of the Autonomic Nervous System

• The efferent nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system.
• The major differences between the two systems are evident in the responses that each produces.
• The somatic nervous system causes contraction of skeletal muscles.

Vision

• Light reduces cGMP, causing the Na channels to close, and the membrane potential becomes more negative.
• The hyperpolarizing response to light is initiated by the absorption of electromagnetic radiation by the photopigment in the membrane of the stacked disks in the rod outer segments.
• Rhodopsin can be thought of as a receptor protein with a prebound chemical agonist.
• The bleaching of rhodopsin stimulates a G-protein called transducin in the disk membrane, which in turn activates the effector enzyme phosphodiesterase (PDE), which breaks down the cGMP that is normally present.