Download Readings to Accompany “Nerves” Worksheet (adapted from France

Readings to Accompany “Nerves” Worksheet (adapted from France pp 324-328)
Types of Nervous Tissue
Nervous tissue is composed of two main cell types: neurons and neuroglial cells.
Neurons transmit nerve messages. Neuroglial cells are in direct contact with neurons
and often surround them. They serve to insulate, support and protect the neurons.
The Neuron
The neuron is the functional unit of the nervous system. Neurons possess the
characteristics of being able to react when stimulated (irritability) and of being able to
pass an impulse on to other neurons (conductivity). While variable in size and shape,
most neurons have three parts. Dendrites receive information (from the environment or
another cell) and transmit the message to the neuron cell body (AKA soma). The cell
body contains the nucleus, mitochondria and other organelles. After reaching the cell
body, the message is transmitted down the axon and can then be transmitted to
another neuron or to a muscle or gland. Axons can be covered with a substance called
“myelin” which greatly increases the transmission speed of impulses. An axon can
branch at its end and can thus contact many other cells. The terminal end of an axon is
called the terminal bouton (or terminal knob). Most terminal boutons contain
neurotransmitters (chemicals involved in the transmission of nerve impulses).
Types of Neurons
Functionally, there are three types of neurons. Sensory neurons (also called afferent
neurons) carry messages from sensory receptors in the periphery (such as those in the
skin) to the central nervous system (spinal cord and brain). Motor neurons (efferent
neurons) transmit messages from the central nervous system to the muscles or glands. .
Interneurons are found in the central nervous system where they connect neurons. The
vast majority of neurons in the human body are interneurons (approximately 99%).
Nerves
A nerve is a group of neuron fibers (axons and dendrites) which is bundled together. A
nerve can be strictly a motor nerve (a bundle of motor neurons which stimulates only
muscles or glands), a sensory nerve (a bundle of sensory neurons which is associated
with only sensory organs) or a mixed nerve (a bundle of both motor and sensory
neurons).
Structure of a Nerve
Two Neurons & Synapse between them
Illustration of Neuromuscular Junction
(AKA Synapse btn neuron & muscle fiber)
Nerve Impulse Transmission & Muscle Fiber Contraction
To understand how impulses are carried along nerves or throughout a muscle to cause
contraction, we need to learn a little more about membrane excitability. Nerves
transmit impulses by movement of electrically charged particles. Neurons have a
membrane that separates the cytoplasm inside from the extracellular fluids outside the
nerve cell, thereby creating two chemically different areas. Each area has differing
amounts of potassium and sodium ions and other charged substances, with the inside of
the cell being more negatively charged than the outside when the neuron is in a resting
state.
When dendrites of a neuron receive sufficient stimulation, the axon hillock of the
neuron will transmit that impulse toward the axon. This is the first step in transmitting a
stimulus called the action potential.
Sodium (Na+) ions will rush into the axon through Na+ channels resulting in a change in
the neuron’s charge from negative to positive with respect to the outside of the neuron.
This change in polarity is called depolarization. This depolarization will occur
sequentially at adjacent regions of the axon (or at nodes of Ranvier in myelinated
axons). The depolarization is quickly followed by diffusion of K+ ions out of the neuron
to “repolarize” the axon (re-establish a negative charge inside the axon as compared
with the outside). The term “diffusion” means the movement of ions from an area of
greater concentration to an area of lesser concentration. Following repolarization of the
neuron by diffusion of K+, a re-establishment of ion concentrations to that of the resting
state of the neuron is achieved by an active transport mechanism called the Na+ / K+
pump. The sodium/potassium pump actively pumps 3 Na+ ions out of the neuron for
every 2 K+ ions it pumps back into the neuron. The sodium potassium pump helps
return the ion concentrations inside and outside of the neuron back to their original
resting states.
When the depolarization reaches the terminal bouton of a neuron, Calcium (Ca ++) ions
rush into the terminal bouton and cause the release of neurotransmitter into the
synaptic cleft. Neurotransmitter will cross the synaptic cleft and bind to receptors on
the post-synaptic membrane (which can be a muscle fiber, gland, or another neuron).
The neurotransmitter (depending on its type) can either have an inhibitory or excitatory
effect on the post-synaptic membrane. If the post-synaptic membrane is a muscle fiber
and the neurotransmitter has an excitatory effect on it, Na+ ions will rush into it and
down tubes which traverse the muscle fiber called t-tubules. This causes the release of
Ca++ ions from another organelle in the muscle fiber called the sarcoplasmic reticulum
which is located adjacent to the t-tubules. These Ca++ ions bind to troponin molecules
found on thin myofilaments within the muscle fiber. This causes the protein
tropomyosin to move off of binding sites on the actin. Myosin heads attach to the
binding sites on actin and use energy from ADP + P to “ratchet”, sliding the thin filament
and resulting in muscle fiber contraction. This is called a “power stroke”. If ATP is
available, it will bind with the myosin head and cause it to detach from the actin
molecule. Energy from ATP will be used to “re-cock” the myosin head in preparation for
another muscle fiber contraction.
Nerve Damage
Nerves are fragile and can be damaged by compression, tension, or cutting. Injury to a
nerve can stop signals to and from the central nervous system, causing impaired muscle
function and loss of (or abnormal) sensation in the injured area. When a nerve is cut,
both the nerve and its insulating myelin sheath are disrupted. Compression or tensile
injuries can cause nerve fibers to break without damaging the insulatory sheath
surrounding the fibers.
When nerve fibers are damaged, the end of the fiber distal to the site of injury dies,
although the insulation (myelin sheath) stays healthy. The end of the nerve fibers
proximal to the injury (and thus closer to the brain and spinal cord) does not die. After
some time damaged nerve fibers will begin to heal and may grow down their empty
myelin sheath until they reach a muscle, gland or sensory receptor. If both the neurons
and the myelin sheath have been disrupted and the nerve sheath is not repaired
surgically, the new nerve fibers may grow into a ball at the location of injury, forming a
scar or neuroma. A neuroma is a ball-like growth of nerve fibers that can be painful and
cause an “electrical” sensation when touched.
The treatment for a cut nerve is to sew together the myelin sheath that is around both
ends of the nerve. The goal in fixing the nerve is to save the sheath (cover) so that new
fibers may heal and function may be regained. Once the myelin sheath is fixed, the
nerve generally begins to heal within it. Nerves usually grow one inch every month,
depending on the patient’s age and other factors. This means that with an injury to a
nerve in the arm above the fingertips, it may take up to a year before feeling returns to
the fingertips.