The Mammalian Nervous System Has Two Major Subdivisions: the Central Nervous System and the Peripheral Nervous System

The central nervous system (CNS) is divided into the brain and spinal cord (Box 3-1). A series of protective bones sur­round the entire CNS. The brain is surrounded by the skull, and the spinal cord is surrounded by a series of cervical, thoracic, and lumbar vertebrae and ligaments.

The peripheral nervous system (PNS) is composed of the spinal and cranial nerves that carry electrical signals, called

Box 3-1

Organization of the Nervous System

Central nervous system (CNS)

Brain

Spinal cord

Peripheral nervous system (PNS)

Efferent (motor)

Somatic —to skeletal muscle

Visceral —to cardiac muscle

—to smooth muscle

—to exocrine glands

Afferent (sensory)

Somatic—from skin

—from retina

—from membranous labyrinth

Visceral —from thoracic and abdominal organs —from olfactory epithelium —from taste buds action potentials, away from or toward the CNS. These nerves are bundles of PNS axons. The axons carrying action potentials toward the CNS are called afferents, and those carrying such signals away are efferents. One way to group the elements of the PNS functionally is into sensory and motor subsystems. The elements of spinal and cranial nerves that serve a motor function are (1) axons of somatic efferent neurons, which carry action potential commands from the CNS to junctions, called synapses, at skeletal muscles, and (2) axons of visceral efferent neurons, which carry action potentials toward synapses at smooth muscle, cardiac muscle, and some exocrine glands. PNS components serving a sensory function are axons of afferent neurons that bring action potential messages to the CNS from peripheral sensory receptors. These receptors are directly or indirectly responsible for transducing energy from the body’s external or internal environment into action potentials that travel to the CNS. The intensity of this energy’s stimulation of the receptor is encoded by changing the frequency of action potentials as the intensity of stimulation changes.

Spinal and cranial nerve sensory components are axons of (1) somatic afferent neurons and (2) visceral afferent neurons. Somatic afferent axons carry action potentials resulting from Stimulation of receptors such as the photoreceptors of the eye, auditory receptors of the ear, and stretch receptors of the skeletal muscle. Action potentials generated by stretch receptors or chemoreceptors (e.g., O₂, CO>) located within visceral organs of the chest and abdomen are carried to the CNS along visceral afferent axons. Visceral efferent and affer­ent axons are part of the autonomic nervous system; the por­tions of the PNS and CNS responsible for involuntary control of smooth muscle, cardiac muscle, some endocrine glands, and many physiological life support functions (e.g., heart rate, blood pressure, digestion).

Peripheral nerve axons converge to form a single spinal nerve at each of the intervertebral foramina. Within the spinal canal, afferent sensory and efferent motor axons are separated; afferent sensory axons enter the spinal cord through the dorsal roots, whereas the efferent motor axons exit the spinal cord through the ventral roots (Figure 3-1).

The PNS and CNS differ in the regenerative ability of their neural axons following physical injury. Peripheral nerve axons can slowly regrow and reconnect to their peripheral targets. Damaged CNS axons do not effectively regenerate, possibly resulting from inhibitory features of their local environment. Experimental manipulations of this environment have been shown to improve CZNS axonal regrowth.