Human Anatomy Lesson 4
The brain and spinal cord make up the central nervous system (CNS), and the nerves that come off the brain and spinal cord to innervate structures in the body wall and limbs make up the peripheral nervous system (PNS). Study of the CNS is the province of neuroanatomy and histology, so we will not worry further about details of the CNS except, later, when we talk about gross anatomy of the brain, and in this lesson, where we talk about the spinal cord. We will, however, investigate the PNS in more detail.
The main anatomical division of the nervous system is the one we've already discussed, the division into CNS and PNS. There are several other ways to partition the nervous system along functional lines. The first such division is between afferent and efferent. Any nerve cell that carries instructions from the CNS outward is an efferent fiber (from the Latin for "away from") and, along with its cell body, makes up an efferent neuron. Most efferent neurons innervate glands, muscle, and other motor tissues of the body and therefore comprise the motor portion of the PNS.
In the motor portion of the PNS, nervous tissue related to striated voluntary muscle is under somatic motor control, whereas smooth muscle, cardiac muscle, and glands are under visceral motor control. Most of the body wall is under somatic motor control, with the exception of vasculature, arrector pili muscles, and sweat and sebaceous glands, which are under visceral motor control. All the internal organs of the body are under visceral motor control.
Any nerve fiber that carries information back to the CNS from the body is known as an afferent fiber (from the Latin for "toward"), and, along with its cell body, makes up an afferent neuron. Most afferent information comes into the CNS as conscious sensation such as temperature, pressure, vibration, stretch, and pain, and forms the sensory part of the PNS.
So, somatic motor fibers stimulate striated muscle tissue; visceral motor fibers innervate smooth muscle, cardiac muscle, and glands; somatic sensory fibers transmit conscious sensation from striated muscle tissue; and visceral sensory fibers transmit unconscious sensation from visceral structures.
Visceral motor fibers make up the autonomic division of the PNS, which can be further divided into a sympathetic part, a parasympathetic part, and an enteric part. The sympathetic part of the autonomic division is associated with energy expenditure during moments of great effort or exertion (i.e., fight or flight), and innervates visceral motor tissues in the body wall and internal organs; the parasympathetic part is associated with energy conservation in the internal organs; and the enteric part is concerned largely with digestion, and occurs without CNS input. We will cover the sympathetic part of the autonomic division more fully in lesson 5, and come back to the parasympathetic part when we cover the thorax, head, and neck.