Peripheral Nervous System

   The spinal cord and spinal nerves

Together the CNS and the PNS provide three general functions: sensory, integrative, and motor. The sensory function consists of receptors that monitor the body both externally and internally. The sensory receptors convert their information into nerve impulses, which are then transmitted via the PNS to the CNS, and the signals are integrated. They are brought together, creating sensations and helping to produce thoughts and perceptions. As a result, we make decisions and use motor functions to act on them. The PNS includes the cranial and spinal nerves as well as the ANS.

Cranial Nerves

Twelve pairs of cranial nerves come from the brain. They are named both by Roman numeral and name. The Roman numeral partially identifies the cranial nerves’ location in the brain. Refer to Table for a summary of the cranial nerves and their function.

Table. Cranial Nerves

NUMBER AND NAME FUNCTION(S)
I Olfactory • Sense of smell
II Optic • Sense of sight
III Oculomotor • Movement of the eyeball• Constriction of pupil in bright light or for near vision
IV Trochlear • Movement of eyeball
V Trigeminal • Sensation in face, scalp, and teeth• Contraction of chewing muscles
VI Abducens • Movement in the eyeball
VII Facial • Sense of taste• Contraction of facial muscles

• Secretion of saliva

VIII Acoustic (vestibulocochlear) • Sense of hearing• Sense of equilibrium
IX Glossopharyngeal • Sense of taste• Sensory for cardiac, respiratory, and blood pressure reflexes

• Contraction of pharynx

• Secretion of saliva

X Vagus • Sensory in cardiac, respiratory, and blood pressure reflexes• Sensory and motor to larynx (speaking)

• Decreases heart rate

• Contraction of alimentary tube (peristalsis)

• Increases digestive secretions

XI Accessory • Contraction of neck and shoulder muscles• Motor to larynx (speaking)
XII Hypoglossal • Movement of the tongue

Spinal Nerves

Thirty-one pairs of spinal nerves branch from the spinal cord. There are 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves. Each set of nerves has a specialized task.

The spinal cord and spinal nerves

FIGURE. The spinal cord and spinal nerves. The distribution of spinal nerves is shown only on the left side. The nerve plexuses are labeled on the right side. A nerve plexus is a network of neurons from several segments of the spinal cord that combine to form nerves to specific parts of the body. For example, the radial and ulnar nerves to the arm emerge from the brachial plexus. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 173, with permission.)