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Neurons
Neurons- also called ‘nerve cells”
- the structural unit of the nervous system
- conduct messages in the form of impulses from one part of the body to another
Special Characteristics:
1. Extreme longevity - given adequate nutrition, can function for a lifetime
2. Amitotic - do not undergo mitosis, can’t be replaced if they die
3. High metabolic rate - require continuous supply of glucose and O2, cannot
survive more than a few minutes without O2
Functional Components:
1. Receptive region
2. Conduction component -generates and transmits an action potential
3. Secretory region - releases neurotransmitters
Neuron anatomy:
Cell body - Oval shape, contains a large spherical nucleus and a nucleolus, and
granular cytoplasm
- contains typical organelles with the exception of centrioles
- the major biosynthetic center of the neuron
- Protein- and Membrane-making system is the best and most active of any cell
* free ribosomes and rough endoplasmic reticulum (RER) make up the above. The
RER is also referred to as Nissl bodies
- the Golgi Apparatus (packaging center) is also expanded
- many mitochondria are present - make ATP, the cell’s usable energy
- Microtubules and neurofilaments - give the cell shape and function in transport
within the cell
- may or may not contain pigments: melanin, lipofuscin, and iron-containing
pigments
- plasma membrane is also a receptive surface
Where can these highly specialized cell bodies be found?
In the CNS, clusters of cell bodies are called Nuclei.
- Nuclei are abundant and are protected by the skull and spinal column.
In the PNS, clusters of cell bodies are called Ganglia.
- Ganglia are not very common in higher animals.
Neuron Processes
Processes = cytoplasmic extensions
* The CNS contains neuron cell bodies and processes; the PNS contains mainly
neuron processes.
Tracts - bundles of nerve processes in the CNS
Nerves - bundles of nerve processes in the PNS
* Please memorize Figure 11.4 - Structure of a motor neuron
Dendrites - short, tapering, diffusely branching extensions. Most motor neurons
have hundreds attached to the cell body
- all organelles in cell body are also found in dendrites
- correspond to the ‘input’ region of the neuron
- convey ‘graded potentials’ toward the cell body
- have a high surface area, are spiny
Axon - each neuron has only one
- forms at the axon hillock and is uniform for the rest of its length
*Any long axon is called a nerve fiber
- substances may move in either direction along the axon
- usually branches at its end, forming telodendria, also called terminal branches
(10,000 or more per neuron is not unusual!)
- telodendria end in synaptic knobs
* Generate nerve impulses and transmit them away from the cell body
*When the impulse reaches the axon terminal, it causes chemicals stored in
vesicles to be released into the extracellular space
Neurotransmitters - excite or inhibit neurons with which they have contact
Myelin Sheath Myelin - fatty, segmented sheath
- formed by Schwann cells in the PNS
- formed by Oligodendrocytes in the CNS
- protects and electrically insulates neurons from one another
- increases the speed of transmission of nerve impulses 150X or more
- always on axons, never on dendrites
Nodes of Ranvier - spaces between the myelin, to which electrical signals ‘jump’
*Multiple Sclerosis is one disease that causes ‘demyelinating’
* Regions of the brain and spinal cord containing dense collections of myelinated
fibers are referred to as white matter and are primarily fiber tracts. Gray matter
contains mostly nerve cell bodies and unmyelinated fibers.