Sympathetic nervous system motor pathways

Sympathetic nervous system motor pathways


In this video, I’m going to talk about
the sympathetic nervous system, which is a division of the autonomic nervous
system. We’re going to talk about the motor pathways as well as the
neurotransmitters and hormones involved. In this system the motor pathways
consists of two neurons end to end, the so-called pre ganglionic neuron and then
the post ganglionic neuron. The pre ganglionic neuron cell bodies are
located within the CNS, more specifically within the spinal cord between T1 thoracic region number one through lumbar region number two. The system can also be
referred to as the thoracocolumbar division. The preganglionic neuron
extends out an axon that will synapse with a cell body of the post ganglionic
neuron. The postganglionic neuron will synapse with the target tissue. And
of course these target tissues can either be smooth muscle, cardiac muscle
or glands. The region where the pre and post ganglionic neurons synapse is
termed a ganglia. There are two types of ganglia within the sympathetic nervous
system. The sympathetic chain ganglia, and another type of
ganglia called the collateral ganglia. The difference between these two
pathways is the location of the ganglia and the types of target tissues. In those pathways that run through the
sympathetic chain ganglia and synapse their, target tissues include the eye, the
heart, the airways to the lungs, and also the saliva glands. In pathways that run
through the collateral ganglia and synapse here, the target tissues include
the GI tract as well as accessory organs to the digestive system such as the
liver and the pancreas. As well as the bladder and genitals. Other than that, the
neurotransmitters that are involved in these two pathways are exactly the same.
The neurotransmitter released from the preganglionic neuron is termed
acetylcholine and the neurotransmitter released from the postganglionic neuron
is termed norepinephrine. The receptors for the acetylcholine are located on the
cell body and dendrites of the postganglionic neuron, and these are
called nicotinic cholinergic receptors. The receptors that are located on the
target tissues are all members of the same super family of receptors called
the adrenergic receptors. Before we leave the sympathetic nervous system I need to
tell you about one more type of motor pathway. In this motor pathway we still
have a pre ganglionic neuron, it extends an axon out into the peripheral nervous
system and the cell body still located within the
spinal cord. It is called the pre ganglionic neuron,
however instead of synapsing with another neuron, this pre ganglionic
neuron synapses with the adrenal medulla. now, if we take a cross-sectional look through the adrenal gland, we’ll find it consists of two areas, a
central medulla, the adrenal medulla, and the adrenal cortex. These synapses occur
with Chromaffin cells located within the adrenal medulla. Upon stimulation of this
gland the cells will secrete two hormones, epinephrine and norepinephrine. Because this is an endocrine gland, the
two substances that are secreted will move into the blood flow and be
transported around the body. When these hormones reach the target tissues, which
will still be either a smooth muscle a cardiac muscle for a gland, they will
bind to the same family of receptors, the so called adrenergic receptors. So we
have a pre ganglionic neuron synapsing with the adrenal medulla. Stimulation of
the adrenal medulla causes release of the hormones epinephrine and
norepinephrine which will move through the blood until they reach their target
tissues, and bind to and activate adrenergic receptors. In terms of the
neurotransmitter, the same neurotransmitter is used here, the
acetylcholine, and that will still bind to the nicotinic cholinergic receptors. Of course in this pathway there is no
ganglia, the synapse between a neuron and an endocrine gland is called a neuroglandular junction. The other point I want to make
is that because norepinephrine is released from the adrenal medulla into
the blood supply it does act as a hormone, but in the previous two diagrams where norepinephrine was released from the axon terminal of postganglionic
cells here it is functioning as a neurotransmitter

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