5: Efferent NS and autonomic NS

• Autonomic neurons primarily act on “involuntary” target tissues. Somatic motor neurons act on “voluntary” target tissues. Both are types of efferent neurons.
• Parasympathetic neurons are autonomic neurons that primarily release the neurotransmitter acetylcholine into "involuntary" tissue.
• Sympathetic neurons release epinephrine or norepinephrine into "involuntary" target tissues.
• "Involuntary" target issues, which are innervated by sympathetic neurons, include the heart, smooth muscle, and glands.
• Parasympathetic NS slows heart rate (ACh release) while sympathetic NS increases heart rate (norepinephrine and epinephrine release).
• Sensory receptors for afferent neurons are NOT cell bodies (which are instead located closer to the CNS on the axon); rather, they are specialized sensory endings that encode stimulus energy.
• Sensory receptors can sometimes be located on receptor cells. Here, a graded potential generated by stimulus energy is directly transduced to a NT release; no action potentials are generated.
• Descending pathways can often modify afferent and ascending pathway transmission; for example, they can inhibit the release of neurotransmitter from an afferent neuron into an ascending pathway.