KIN 3515 Lecture : Unit 1 Notes
15 Mar 2019
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Unit 1 8/27/2013 4:08:00 PM
Each motor neuron synapses with numerous muscle cells (as little as 4 to
over 200).
• The motor neuron and all the muscle cells it services is called a
MOTOR UNIT.
o Size depends on size of muscle
o Larger the muscle, larger the unit
• The motor unit is the basic functional unit of the muscle.
• 1 muscle cell is about 2cm in length, very thin
o If you contract just one muscle cell, wouldn’t get much done
o Need multiple cells contracting at the same time
• 1 neuron will activate several muscle cells, as little as 4 and up to
200
o Called motor unit
• The motor neurons are connected to the rest of the nervous
system via a complex network of neurons.
Neurons and muscle cells are electrically charged
• Hence, neurons and muscle cells are “activated” by changing
their electrical charges.
• Resting membrane potential
o Cells are in a resting sense of negative
o Need to make positive to make them do anything
o Go from negative to positive
→
action potential
▪ Nerve and muscle are activated and do stuff
Electrical signals can not jump between cells
• Neurons communicate with other neurons and with muscle cells by
releasing neurotransmitters
• When you activate a nerve, it releases a neurotransmitter
• NEUROTRANSMITTERS = Chemicals released by a neuron which
cause either excitation, facilitation, or inhibition.
o NOTE: Drugs can also serve as neurotransmitters.
Neuron activations or excitations (aka nerve impulses) are really waves of
depolarization caused by Na+ influx.
• (NOTE: Muscle activation/excitation is also caused by Na+ influx)
• When a nerve goes from negative to positive
→
depolarization
• Caused by the fact when K on inside and Na on outside
o Na is positive and when it goes in, causes inside of cell to go
positive
▪ Purpose of activating a nerve, get Na to come in
o K on inside also positive
• No positive in cells, causes cramps
Sodium (Na+) Gates or Channels
1. Nerve and muscle cells have two types of Na+ gates/channels
which allow NA+ entry (influx) and depolarization (easier than
repolarization, Na wants to go in).
2. The first type are chemical gates, and are the least numerous
a. found mainly in the nerve cell body and dendrites
b. found in muscles at the nerve synapse
c. chemical gates are opened (unlocked) by the
neurotransmitters
d. need to make chemicals, can run out of them
3. The second type are electrical gates and are the most
numerous
a. found mainly in the neuronal axons
b. found distal to synapses in muscle cells
c. electrical gates are opened when the resting membrane
potential exceeds a specific threshold voltage (usually around
-60 mV) (when gates open, activates nerves)
4. It is the opening of the electrical gates that causes the necessary
level/wave of depolarization called the action potential.
Before another impulse can be sent down an axon, the nerve must
repolarize (return from positive to negative)
• Na+ (out) & K+ (in) are switched via the Na-K pump.
o Takes Na out of cell and brings in K into the cell
o This requires energy in the form of ATP (i.e. no ATP no
pumping) to turn them off
▪ No ATP, no repolarization
▪ Note: Repolarization also occurs in muscle.
• No energy to activate things, only requires energy to turn off
(repolarize)
o Aids with speed
o “Pays for it at the end of activity”
Facilitation
• Raising the resting membrane potential above the normal potential,
thereby increasing the likelihood that succeeding neurotransmitter
releases will cause the potential to exceed the threshold.
• This enhances the chances of the neuron firing and activating the
target.
• (i.e. greater chance of muscle excitation by facilitating the motor
neuron).
Inhibition
• Lowering the resting membrane potential below the normal
potential, thereby decreasing the likelihood of reaching the
threshold.
• This usual prevents the neuron from activating its target.
• (i.e. prevents motor neuron from activating the motor unit).
o Example: to shut down reflexes
Most Common Excitatory Neurotransmitters
• Norepinephrine & Acetylcholine (ACH) (nerve to muscle)
o When norepinephrine released, causes epinephrine to be
released to make sure things happen
Most Common Facilitory Neurotransmitters
• Norepinephrine & Acetylcholine (in lower amounts)
o NOTE: Norepinephrine packs a double wallop by activating
the adrenal medulla to release epinephrine (a sibling chemical
to norepinephrine) which can also cause facilitation and/or
excitation.
o NOTE: another name for epinephrine is adrenalin
o NOTE: In some parts of the body Norepinephrine &
Acetylcholine will act as inhibitors.
Most Common Inhibitory Neurotransmitter
• GABA (gamma-amniobutyric acid)
• Date rape drug, chemical form of that
• Take in large amounts, body becomes inhibited
• Cannot inhibit muscle cell, inhibit another neuron
DRUGS CAN ALSO HELP FACILITATION or INHIBITION
• Facilitation:
Document Summary
Each motor neuron synapses with numerous muscle cells (as little as 4 to over 200): the motor neuron and all the muscle cells it services is called a. 200: called motor unit, the motor neurons are connected to the rest of the nervous system via a complex network of neurons. 60 mv) (when gates open, activates nerves: it is the opening of the electrical gates that causes the necessary level/wave of depolarization called the action potential. Most common excitatory neurotransmitters: norepinephrine & acetylcholine (ach) (nerve to muscle, when norepinephrine released, causes epinephrine to be released to make sure things happen. Most common inhibitory neurotransmitter: gaba (gamma-amniobutyric acid, date rape drug, chemical form of that, take in large amounts, body becomes inhibited, cannot inhibit muscle cell, inhibit another neuron. Ach esterase, breaks down muscle activation: norepinephrine is sucked back into the synaptic bulb.
