(Transcribed from Dr. Glasser’s lecture, 17 Feb 2000 by Brian Buschman)
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Neurons have a cell body called a soma, receptor processes called dendrites and a transmission process called an axon.
Nervous tissue also has supporting cells that are neuroglia in the central nervous system (CNS) and satellite cells in the peripheral nervous system (PNS).
The CNS is composed of the brain and spinal cord and is derived from the neural tube.
The PNS is derived in part from neural crest cells and composes all of the nervous material in the body not in the CNS. It is divided into:
1) Somatic (voluntary) nervous system that innervates skeletal muscle that mostly derived from the somatic mesoderm.
2) Autonomic (involuntary or visceral) nervous system. It innervates the viscera and controls basic body functions that do not reach levels of consciousness. It is composed of two parts:
a) Sympathetic Nervous System that controls “fight of flight” responses.
b) Parasympathetic nervous system that regulated the homeostatic bodily functions.
Neurons are excitable, generate and propagate electrical responses. Their parts are:
1) Soma (cell body) that has a nucleus, prominent nucleolus, RER, and free ribosomes (nissel bodies).
2) Dendrites that have nissel bodies.
3) Axons that contain lots of mitochondria, microtubules, neurofilaments. They do NOT have RER (nissel). The first part of the axon is the INITIAL SEGMENT that is the first place an action potential (AP) is fired. There is only one axon but it can have several branches near the terminal end.
a) Axolemma is the plasma membrane of an axon.
b) Axoplasm is the cytoplasm of an axon.
The axon hillock is part of the soma because it does not fire an action potential but it also lacks RER.
There are three types of neurons usually found in the human:
1) Bipolar neurons have one axon and one dendrite attached to its soma.
2) Multipolar neurons have one axon and multiple dendrites attached to its soma. There are two classes of multipolar neurons:
a) Golgi Type I – Neurons with long axons.
b) Golgi type II – Neurons with short axons.
3) Pseudounipolar neurons have one process that is connected by a side branch to the soma. The majority of the process is considered to be axon as it is myelinated and fires an action potential. Only the very first part that has the receptor and is not myelinated is called the dendrite.
There are three functional classes of neurons in the human body:
1) Sensory neurons are pseudounipolar and receive sensation from all over the body. They are afferent as they carry signals towards the CNS. Not all sensory pathways reach levels of conscious thought (travel to the cerebellum).
2) Motor Fibers are multipolar neurons that carry sensation from the CNS to the body. They include sympathetic, parasympathetic and autonomic fibers. They are efferent since they carry away from the CNS.
3) Interneurons compose the vast majority of neurons in the body (about 1012) and connect between two other neurons.
Myelin is an insulator made of lipid surrounding many of our neurons. Neurons in the body are myelinated by one of two types of cells:
1) Neurons of the CNS are myelinated by oligodendrocytes that myelinate multiple axons of multiple neurons at one spot on each.
2) Neurons of the PNS are myelinated by schwann cells that myelinate a single axon at a single spot.
Depending on the level of myelination neurons come in three sizes and have varying conduction velocities:
1) Large diameter neurons have a thick myelin coat and a rapid conduction velocity.
2) Small diameter neurons have a thin myelin coat and a slow conduction velocity.
3) Very small diameter neurons have no myelin sheath and a very slow conduction velocity.
a) In CNS they are non-myelinated because they truly have no myelin.
b) In PNS they are called unmyelinated because they have no myelin sheath but they are enclosed in myelin. Multiple unmyelinated axons are stuck in the middle of a section of myelin (of a schwann cell). Each axon has a canal within the myelin that contains extracellular fluid so they function as if they had no myelin yet they do not cause interference with each other.
(Extra info – Multiple sclerosis involves the loss of myelin in the CNS and will never be repaired. The similar situation in the PNS will be repaired in 18 months of less.)
Internodes (nodes of Ranvier) are the gaps between sections of myelin sheathes where the AP is actually fired. The conduction velocity is increased because they do not have to fire an AP at every point along the path and this form of conduction is known as salutatory conduction.
Tracts are collections of axons running together in the CNS. In the PNS we call then nerves.
Nuclei are collections of cell bodies with a similar function in the CNS. In the PNS they are called ganglia.
Gray matter is a collection of cell bodies and dendrites and so named because the are not myelinated.
White mater is a collection of axons and so called because of the white appearance from the lipid of the myelin.
In the CNS they are neuroglia (glia) that include oligodendrocytes, astrocytes, ependymal cells and microglia. Astrocytes function to:
1) Regulate the extracellular environment.
2) Absorb excess neurotransmitter.
3) Form scar tissue in CNS when neural damage occurs.
4) Play a role in the blood brain barrier.
Macroglia are cells in the CNS including oligodendrocytes and astrocytes and are of ectodermal origin.
Microglia are small phagocytic cells that are of mesodermal origin. They are the only CNS cells not of ectodermal origin.
Ependymal cells are columnar cells that line the ventricles of the brain and spinal cord. The form the chroid plexus and produce cerebrospinal fluid (CSF).
In the PNS they are schwann cells and satellite cells.
The blood brain barrier has two layers that each forms a tight layer to prevent the passage of most materials between the CNS and the circulatory system.
1) The most important layer is a layer of endothelial cells with tight junctions in the capillaries that prevent the passage of material.
2) The second layer is made by the “end feet” of the astrocytes that is located deep to the endothelial layer.
All cells have a resting membrane potential (VM) and neurons are one of the few that make use of it in their function. In neurons it is about –70mV. In neurons when VM rises it causes them to fire the AP. The AP is important as it regenerates the signal with each step. That is essential because neurons are a very poor conductor.
The space between two neurons is the synapse and the signal can pass electrically with the flow or ions through gap junctions or chemically with the release of neurotransmitters from the presynaptic terminal (at the end of the axon) to the postsynaptic membrane (on either the dendrites or soma). Synapses are either called axosomatic or axodendritic based on where the postsynaptic membrane is located. There are few exceptions to this that will be discussed in neuroanatomy next semester.
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