Page 349 - Atlas of Histology with Functional Correlations

Extending from the neurons are dendrites that divide in a treelike fashion

allowing the dendrites to connect with and receive stimuli from axon
terminals of other neurons. The surface of the dendrites is covered by
dendritic spines that connect (synapse) with axon terminals from other
neurons. The surface membranes of the neurons and the dendrites are
specialized to receive and to integrate information from other dendrites,

neurons, or axons. The axons, in turn, conduct the received information from
the neuron to an interneuron, another neuron, or to an effector organ, such as
a muscle or gland.

Axons arise from the funnel-shaped region of the cell body called the

axon hillock. The initial segment of the axon is located between the axon
hillock and where myelination starts. It is at the initial segment that the
stimuli, whether inhibitory or stimulatory, are summated and nerve stimuli
are generated. The rate of conduction of the stimulus is dependent on the size

of the axon and myelination. Myelinated axons conduct at a much faster rate
(velocity) than the unmyelinated axons of the same size. To initiate a nerve
impulse, neurotransmitters are released at different synapses.

In addition to impulse conduction, axons also exhibit a bidirectional

transport of chemical substances, organelles, or membrane-bound
neurotransmitters between the neuron and the axon terminals. Materials that
are synthesized in the neurons are transported in tiny microtubules to the
region where the axon terminates or synapses with other dendrites, a cell
body, or other axons. This movement in axons is called anterograde

transport. Similarly, material carried from the axon terminals and dendrites
toward the neurons is called retrograde transport. Transport by
microtubules in either direction requires energy, which is used by

microtubule-associated motor proteins. The mechanism for anterograde
transport involves kinesin, a microtubule-associated motor protein that
moves substances along the axonal microtubules away from the neuron. The
retrograde transport in axons toward the neurons is mediated by microtubule-
associated motor protein called dynein.

In addition, microtubules and microfilaments serve a role in the growth
of axons during development and their regeneration following an injury.

FIGURE 9.9 | Neurofibrils and Motor Neurons in

Gray Matter of Anterior Horn of Spinal Cord

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