Chapter 1

Question 1

Muscle Heterogeneity

Answer 1

The relation between muscle fiber type composition and contractile performance during stimulation. This is in composition of dissimilar parts that muscles contract.

Muscles are not isometric, fibers do not respond simultaneously.

Question 2

Orderly Motor unit recruitment

Answer 2

“Hennemans Size Principle”
When a muscle is stimulated the motor units recruited are according to size. Smallest to largest, and grow in action potential. Greater the stretch greater the number of units recruited.
Larger motoneurons posses larger axons with larger spike amplitudes and faster conduction velocities.

Question 3

Motoneuron size &

How is it determined?

Answer 3

Can be estimated by:
Hennemans size can be estimated different ways taken at the axon level. Increasing amplitude of the axonal spike.
Estimate size by measuring total surface membrane area histologically. Estimate combined volumes of the cell body & trunk dendrites near the soma then using these measurements to make assumptions about the length of dendrites.
Electrophysiologically measuring cell capacities. Measure the time constant of the voltage change (assuming cylinder)can determine cell capacitance.

Question 4

Membrane resistivity and motoneuron size

Answer 4

Rin=the input resistance-> measure the resistance to a current injected into a cell
Cell surface area is the prime determinant.
Smaller cells have high resistance per unit of membrane area.
Larger cells have lower resistances-more current can flow out across the conductance channels.
Variation of resitivity is based on the difference in the range of motoneuron size and range of cell Rin.

Question 5

Factors determining Action Potential generation

Answer 5

-Amount of current that is coming into the cell
-Resting membrane potential (RMP)
-The depolarization amplitude required to reach the threshold for spike generation-threshold depolarization (Vth).
The amount of current that produces and action potential= rheobase current (Irh)
-Irh=Vth/Rin
Low rheobase cells require less depolarization to generate an action potential.
-Nonlinerarity in membrane response between RMP and voltage at which the action potential is generated.This can be caused by persistent inward current (PIC)- calcium mediated. This is more significant in smaller cells which is an advantage of larger cells with regard to their susceptibility to be recruited.

Question 6

Afterhyperpolarization and impact on firing rates

Answer 6

AHP is the prolonged hyper polarization that follows the action potential. Duration varies among the motoneurons of different types. AHP is attributed to the activation of calcium activated potassium conductance that gradually declines after the spike. AHP is important because it has an effect on the motoneuron firing patterns in response to excitation. AHP has a stronger correlation to minimal firing rates than max. The min firing rates show sustained excitation.

Question 7

Effect of stimulation of late adaptation

Answer 7

Takes ten seconds to minutes to occur, not rapid.
Maintained contraction must gradually increase excitation to the motor pool to compensate for late adaptations to increase the recruitment of motor units to maintain the force.
Late adaptation varies among the different motor unit types; more developed in motoneurons that are less excitable and that innervate fast muscle units.

Question 8

Motoneuron PICs and impact on excitability

Answer 8

PIC=persistent inward currents
These currents last a long time and are generated by direct intracellular/synaptic excitation. Last several seconds after excitation has ended (due to slow activation of channels). If a cell is already firing, increase in firing rate that can out last original stimulation.
Considered a type of warmup method.