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Nervous System
Part 3:
Neurons & Nerve Impulses
Neuron Structure
• A neuron is a nerve cell
• The nucleus of a neuron and most of its
organelles are located in the cell body
• Dendrites are membrane-covered extensions
that extend from the cell body in different
directions
– They receive information form other neurons or other
cells and carry the info toward the cell body
• An axon is a long, membrane-bound projection
– It transmits info away from the cell body via action
potentials
Neuron Structure Continued
• Neurons may have a single axon or branching
axons
• The end of an axon is called the axon terminal,
which may contact and communicate with a
muscle cell, a gland cell or another neuron
• Most axons are covered with a lipid layer called
the myelin sheath
• The myelin sheath speeds up transmission of
action potentials
Neuron Structure Continued
• Schwann cells, which are found in neurons not
of the brain or spinal cord, surround the axon
and produce myelin
• In the CNS, myelin is produced by a type of
neuroglia called an oligodendrocyte
• Gaps in the myelin sheath along the length of
the axon are called nodes of Ranvier
• On top of the myelin sheath is the neurilemma
(neurilemmal sheath), but it is not present in the
brain or spinal cord.
Neuron Classification
• Neurons are classified in two ways: structural
differences and functional differences
• There are 3 structural classifications:
multipolar, bipolar and unipolar
• There are also 3 functional classifications:
sensory, interneuron and motor
• How they are connected is found on p.368 in
table 10.1 of your book
Neuron Classification
Neuron Structural Classification
1. Multipolar neurons:
– have many processes arising from their
cell bodies
– Only one process is an axon & the rest
are dendrites
– Found mostly in the brain and spinal
cord
– Picture on p.367
Neuron Structural Classification
2. Bipolar neurons:
– The cell bodies have only two
processes, one on each end
– One process is an axon and the other
is a dendrite
– They are found in specialized parts of
the eyes, nose and ears
Neuron Structural Classification
3. Unipolar neurons:
–
–
–
–
–
Have a single process extending from their cell bodies
A short distance from the cell body, this process divides
into two branches, which function as a single axon
One branch (peripheral process) is associated with the
dendrites near a peripheral body part
The other branch (central process) enter the brain or
spinal cord
The cell bodies of some unipolar neurons aggregate in
specialized masses of nerve tissue called ganglia,
which are located outside of the CNS
Neuron Functional Classification
1. Sensory Neurons:
– Also known as afferent neurons
– Conduct impulses from peripheral body
parts to the brain or spinal cord
– At their distal ends, the dendrites or
specialized structures act as sensory
receptors
– Most are unipolar, but some are bipolar
Neuron Functional Classification
2. Interneurons:
– Also known as association or internuncial
neurons
– They lie within the brain or spinal cord
– They are multipolar and form links with other
neurons
– They relay information from one part of the
CNS to another part
– They direct incoming sensory information to
appropriate regions for processing
Neuron Functional Classification
3. Motor Neurons:
– Also known as efferent neurons
– Are multipolar and conduct impulses
out of the brain or spinal cord to
effectors
– Motor neurons of skeletal muscles are
under voluntary control
– Motor neurons of cardiac and smooth
muscles are involuntarily controlled.
Neuron Communication
• Neurons communicate with other neurons and
other cells at special junctions called synapses
• Neurons usually do not touch each other or
other cells
• A small gap, called a synaptic cleft, is present
between the axon terminal and the receiving cell
• Electrical activity in the neuron usually causes
the release of chemicals called
neurotransmitters into the synaptic cleft
Synaptic Terminology
• At a synapse, the transmitting neuron is
called a presynaptic neuron
• The receiving cell is called a postsynaptic
cell
Nerve Impulses
• All cells, including neurons, have an electrical
charge inside the cell that is different from the
electrical charge outside the cell
• This difference in electrical charge across a
membrane is called a membrane potential
• Membrane potentials are produced by the
movement of ions across a cellular membrane
Resting Potential
• A neuron is at rest when it is not receiving
or sending a signal
• In most neurons, the resting potential is
-70 millivolts
Action Potential
• When a dendrite or cell body is stimulated,
the permeability of the neuron’s
membrane changes suddenly
• This action begins an action potential
At Membrane Level
• At the resting potential, sodium channels
are closed and some potassium channels
are open
• During an action potential, sodium
channels open, allowing sodium ions to
move into the axon
Nerve Impulse
• Animation!!!!
• The essential steps are outlined in the
animation