Autonomic Nervous System
Autonomic Sensory Neurons : These neurons are mostly associated with Interoceptors which are sensory receptors located in blood vessels, visceral organs, smooth muscles. Examples of interoceptors are chemoreceptors, mechanoreceptors. Autonomic sensory sensations tend not to reach the consciousness. Autonomic Motor Neurons : regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) activities in effectors tissues (cardiac muscle, smooth muscle and glands). Examples include pupil diameter changes, dilation and constriction of blood vessels, and adjustment of the rate and force of your heartbeat. The output motor part of the ANS has 2 divisions: Sympathetic and Parasympathetic Divisions . Most organs have dual innervation receiving impulses from both divisions. In most cases, one division stimulates the organ to increase activity, while the other decreases organ activity. Most ANS motor pathways consist of 2 motor neurons in series (one following the other). The first neuron has its cell body in the CNS; its myelinated axon extends from the CNS to an Autonomic Ganglion . The cell body of the second neuron is also in the ganglion and its unmyelinated axon extends directly from the ganglion to the effector. In some ANS pathways, the first motor neuron extends to a specialized cell rather than a ganglion. Preganglionic Neuron : is the first of the two motor neurons in any ANS motor pathway. Its cell body is in the brain or spinal cord, and the axon exits the CNS as part of cranial or spinal nerve (with somatic neurons). This axon is a small-diameter, myelinated type B fibre and usually extends to an autonomic ganglion where it synapses with a postganglionic neuron. Postganglionic Neuron : is entirely outside of the CNS. Its cell body and dendrites are located in an autonomic ganglion, where it forms synapses with one or more preganglionic axons. The postganglionic axon is a small-diameter, unmyelinated type C fibre that terminates at a visceral effector. In the sympathetic division, preganglionic neurons have cell bodies in the lateral horns of the gray matter in the 18 thoracic segments and the first 2 lumbar segments of the spinal cord. For this reason, this sympathetic division is called the Thoracolumbar Division . In the parasympathetic division, preganglionic neurons have their cell bodies in the nuclei of cranial nerves in the brain stem (III, VII, IX, X) and in the lateral gray matter of the 2 nd to 4 th sacral segments of the spine. Therefore, the parasympathetic division of the ANS is also known as the Craniosacral Division . Lastly, all somatic motor neurons release only ACh but the ANS can release either ACh or NE (norepinephrine).
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