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1) Which factors determine the velocity of an action potential propagation?
Action potentials travel fastest in axons that are large diameter and myelinated .
What determines the direction that action potential propagates to?
The refractory period of an action potential causes that region of membrane be temporarily unresponsive to another stimulus. This ensures that action potentials migrate in one direction, namely, away from the soma .
2) What is the main differences between an action potential and a graded potential?
- Action potential is an " all-or-none " response vs. graded potential which is of variable intensity. For any given neuron, all
- action potentials are of the same intensity. This is referred to as the " all-or-none " principle. In order to alter the intensity of a given neuronal stimulus, the rate of firing of action potentials is increased.
3) Describe the how the repolarization stage of an action potential is generated and terminated.
- After the first phase of the action potential, the Na+ channels becomes inactivated while the potassium (K+) channels begin
- to open.
This occurs when the membrane potential reaches approximately +30mV . The opening of these channels results in the outward movement of K+ . This second phase of the action potential is the repolarization phase.
The second phase of the action potential ends when the membrane potential reaches about -50mV which triggers the inactivation of the K+ channels.
4) Describe a synaptic transmission event.
- At the axon terminals, voltage-gated calcium (Ca+) channels open in response to the arriving action potential. This triggers synaptic vesicles
- to fuse to the pre-sunaptic plasma membrane, and release neurotransmitters into the synaptic cleft . The neurotransmitter then binds to post synaptic receptors eliciting a response in the post synaptic cell.
5) What allows for the generation of a resting membrane potential?
The ion permeability of the membrane.
What allows the maintenance of the resting membrane potential even in the presence of multiple action potentials?
The Na/K ATPase pump restores the ion composition, to allow the maintanace of the resting potential.
1. The spinal cord is at the bottom of the motor control hierarchy of the CNS, and pre-frontal cortex is at the top.
a. Why does the spinal cord specialize in feedback control?
- The spinal cord does not have appropriate information to predict or anticipate disturbances. Its sensory inputs are from the surface of the body, muscles and joints (proprioception rather than exteroception). Although it does have the wiring to do sophisticated control (central
- pattern generators for locomotion, for example), it doesn’t have the inputs to allow feedforward control.
b. Why does the pre-frontal cortex specialize in feedforward control?
- The prefrontal cortex is the site of executive control. For its executive functions, it needs to predict the future and anticipate disturbances. Also, feedback control requires up-to-date feedback, and the prefrontal cortex delays due to complex processing would make the feedback control too slow. Prefrontal cortex is like a command center for controlling action at a great distance (e.g. for driving the Mars rover from earth) – the delays are so long that feedback control won’t work, so NASA sends commands such as maps of the Martian surface to local (feedback)
- controllers on the rover, which take care of the moment-to-moment steering.
2. The motor servo is a spinal negative feedback control system that uses force and muscle stretch sensors to regulate stretch, force
and other variables that can be derived from these sensations.
a. How might stretch sensors be used to control joint angle?
If the joint has multiple muscles (and they typically do), then stretch signals from all the muscles can be combined to yield a joint angle measure.
b. One commonly regulated variable is joint stiffness. How can it be estimated from the available sensations?
Stiffness is the ratio of force change (from GTOs) to position change (see part a).
c. Why is it useful to have such low-level feedback control?
Delays are shorter than they would be if the controller were in the brain, and local control can be modulated from more predictive high-level areas (like the Mars rover).
3. State the Size Principle.
As an animal increases force from a muscle, recruitment is in the order of smaller motor neurons before larger ones.
4. a. What is recruitment?
Motor unit recruitment is a means of increasing force from a muscle by bringing in more and more motor units as the force requirement increases
b. What is rate coding?
Rate coding is a means of increasing force from a given motor unit by increasing the frequency of action potentials from the motor neuron.
c. Given that rate coding of human motor units in everyday activities almost never increases to tetanus, explain how we are able to
produce small smooth forces.
Small motor units are recruited asynchronously, so their forces, although bumpy, are not synchronized with each other. The sum of small bumpy forces that are asynchronous can be smooth if there are enough of them (enough motor units are recruited).
5. Give examples of structural features that specialize skeletal muscle for force production.
- 1.most common proteins are the motor protein myosin and the track it runs on, actin.
- 2.Myosin and actin filaments run parallel to each other for the length of a myofibril, so that force production is in one direction, and many small changes in length can sum to a large change in length (and high velocity of shortening).
- 3.To produce more force, myofibrils are arranged in parallel with each other.
6. Kinesin and other cargo-carrying motor proteins are processive, but skeletral muscle myosin is not.
a. What is the advantage of being processive?
Processive motor proteins can stay on track.
b. Why would processiveness be a disadvantage for skeletal muscle myosin?
Conversion of ATP to work is slow, so processive motors, which stay attached for tenths of seconds, are slow-moving compared to myosin.
c. How does skeletal muscle maintain force, given that an individual cross-bridge only remains attached for milliseconds?
There are lots of professors on the scooter, and they act independently!
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