Motoneurons

A vertebrate motoneuron is a good example of a unipolar neuron. Its cell body lies in the spinal cord. Dendrites (branching, treelike processes) arise from the soma and serve as points for synaptic inputs from presynaptic excitatory and inhibitory neurons. Analog epsp and ipsp transients are summed in space and time on the dendrites to determine a generator potential at the point where the axon joins the soma. This region is also called the spike generator locus (SGL) because it is where the all-or-none nerve action potential (spike) originates if the generator potential exceeds the firing threshold for the neuron. The spike propagates along the axon, away from the soma (orthodro-mically), toward the muscle motor unit the motorneuron activates. The end of the axon branches to end in a number of highly specialized structures called motor end plates, which are in intimate contact with the muscle membrane. The arrival of the spike causes the excitatory neurotransmitter ACh to be released from the motor end plates; ACh diffuses to receptors on the muscle membrane where it binds, causing the muscle membrane to depolarize, triggering the contraction process.

Mammalian motoneurons generally have a myelin sheath. The myelin sheath is composed of specialized, flattened glial cells called Schwann cells, which wrap themselves around the axon the way that one would wrap a thin pancake around a pencil. That is, many turns of each Schwann cell cover the axon. Between each Schwann cell, along the axon, is a small gap called a node of Ranvier. Each Schwann cell forms a cylindrical, myelin "bead" of about 1 to 3 mm in length; the dense, multiturn wrapping around the axon is about 0.3 |im thick, and each nodal gap is about 2 to 3 |im wide. The myelin coating has several important functions: One is to increase the spike conduction velocity on the axon. Another is mechanical and biochemical support of the axon. Still another is to provide electrical isolation of the axon in a large bundle of nerve fibers, so that its action currents during spike propagation do not cause any "cross talk" with other fibers.

A Unipolar cell y y

Dendrite

B Pseudo-unipolar cell

Cell body

—Central axon

Single J process

Peripheral axon

Invertebrate neuron

I-to skin and muscle

Dorsal root ganglion cell

C Bipolar cell ^•tx/titiJC—Dendrites

-Axon

Retinal bipolar cell

Neuron Unipolar

FIGURE 1.1-1 A sampler of neuron anatomy. Neurons are broadly classified as unipolar (A and B), bipolar (C), or multipolar (D). Unipolar neurons are found in invertebrate nervous systems. Pseudo-bipolar cells are typical of vertebrate sensory neurons. Their cell bodies are found in the dorsal root ganglia of the spinal cord. Many different types of sensory endings exist. Spinal motor neurons cell bodies lie in the spinal cord; their axons travel to the muscles they innervate. Very complex multipolar architectures are found in CNS interneurons. The dendritic field of the cerebellar Purkinje cells is amazingly complex. (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

FIGURE 1.1-1 A sampler of neuron anatomy. Neurons are broadly classified as unipolar (A and B), bipolar (C), or multipolar (D). Unipolar neurons are found in invertebrate nervous systems. Pseudo-bipolar cells are typical of vertebrate sensory neurons. Their cell bodies are found in the dorsal root ganglia of the spinal cord. Many different types of sensory endings exist. Spinal motor neurons cell bodies lie in the spinal cord; their axons travel to the muscles they innervate. Very complex multipolar architectures are found in CNS interneurons. The dendritic field of the cerebellar Purkinje cells is amazingly complex. (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

Was this article helpful?

0 0
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment