Neurons and Spinal Cord Review

Neuron Figure from the Lab Manual

Recall there are two types of cells in the brain and spinal cord, which together are called the central nervous system:

The figure to the right reviews the parts of a typical neuron.

Divisions of the Nervous System

The nervous system is divided into the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of everything outside the CNS.

Note that the CNS is surrounded by connective tissue coverings known as meninges. The outermost meningeal layer, called the dura mater, is tough and thick, and is the one layer that is visible in gross specimens.

Afferent and Efferent Divisions

divisions schematicA spinal nerve projects to a particular body part, such as the fingertip, and contains two types of axons. Afferent neurons convey sensory information from the periphery into the CNS (“afferent” means “toward”). Efferent neurons convey information from the CNS to control targets in the periphery.

Efferent neurons are of two types. Somatic efferent neurons innervate skeletal muscles cells, while autonomic efferent neurons innervate everything else, such as smooth muscle, cardiac muscle, or endocrine glands.

Afferent Neurons

An afferent neuron has a somewhat different structure from most neurons. It is activated at its sensory dendrites. (In a Meissner's corpuscle, for example, the sensory dendrites are found in the connective tissue structure that you saw on your slides.) The sensory dendrites connect to the axon, which runs in the spinal nerve, and then enters the spinal cord at the dorsal side via the dorsal root. Afferent neuron cell bodies are located in a dorsal root ganglion. (A ganglion is a cluster of neuronal cell bodies in the peripheral nervous system; a dorsal root ganglion is a cluster of afferent neuron cell bodies.) Within the spinal cord, the afferent neuron axon branches, with some branches connecting with neurons in the spinal cord and others with neurons in the brain.

Somatic Efferent Neurons

The large cell bodies of somatic efferent neurons are located in the ventral horn of the gray matter in the spinal cord. Their axons exit the spinal cord via the ventral root.