Riku is a 19-year-old college student. One morning, after a long night of studying, Riku woke up and made himself a hot cup of coffee and toast

Question

Much to his surprise, when he brought the cup to his mouth to drink, the coffee spilt onto the table. Riku went to the bathroom mirror and noticed the left side of his face seemed to droop. He quickly got dressed and ran to the medical clinic on the college campus. As he ran, his left eye began to feel scratchy and dry, but he could not blink in response. The physician at the clinic listened to Riku's story and then did a careful cranial nerve examination. She concluded that Riku had Bell palsy, an inflammatory condition of the facial nerve most likely caused by a virus.

What are an afferent neuron and efferent neuron? What are efferent components of the facial nerve and their actions?

Under certain circumstances, axons in the peripheral nervous system can regenerate after sustaining damage. Why is axonal regeneration in the central nervous system much less likely?

At a healthy myoneural junction, acetylcholine is responsible for stimulating muscle activity. What mechanisms are in place to prevent the continuous stimulation of a muscle fiber after the neurotransmitter is released from the presynaptic membrane?

Answer 1

An afferent neuron passes sensory input to the central nervous system and enters the central nervous system by the dorsal root. An efferent neuron is responsible for carrying motor output away from the central nervous system and is located in the ventral root. The efferent output supplied by the facial nerve consists of general visceral and pharyngeal components. The general visceral component is involved with the reflexive activity of the nasopharynx. It is also involved with mucous secretion in the nasopharynx, lacrimation, and salivation in the sublingual and submandibular glands. The pharyngeal efferent path supplies motor innervation to the facial muscles and to the stapedius muscle.

The axons of peripheral nerves are surrounded by myelin and covered by endoneurium. This loose connective tissue creates a directional tube to the target organ. If the endoneurium is intact after axonal injury, the axonal "bud" is able to grow within the endoneurial tube to its former destination. In the central nervous system, endoneurial tissue is absent. Axonal regeneration is therefore limited in this part of the nervous system.

The actions of a chemical synapse are controlled, in part, by the efficient removal of neurotransmitter after it has been released from the synaptic terminals. Three methods to inactivate neurotransmitters are in place. One involves the reuptake of the chemical into the presynaptic membrane for reuse. Another process involves the deactivation of the neurotransmitter by enzymes in the synaptic cleft. (In this situation, the enzyme is acetylcholinesterase.) The third method involves the simple diffusion of the neurotransmitter into the surrounding intercellular fluid so that it becomes dilute and ineffective on postsynaptic membranes.