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Neural Activation of Skeletal Muscle. Review Neuron Anatomy Resting Membrane Potential Action Potential Synapse: Facilitation and Inhibition Neuromuscular Junction Control of muscle fiber properties by the α -motoneuron a. Action potential pattern and quantity

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Neural activation of skeletal muscle
Neural Activation of Skeletal Muscle

  • Review

  • Neuron Anatomy

  • Resting Membrane Potential

  • Action Potential

  • Synapse: Facilitation and Inhibition

  • Neuromuscular Junction

  • Control of muscle fiber properties by the α-motoneuron

    • a. Action potential pattern and quantity

  • b. Neurotrophic influences


Muscle mechanics isometric contractions
Muscle mechanicsIsometric contractions

  • Length-tension relationship (Po, Lo)

  • Lo – optimal length

    • Sarcomere length that provides for optimal overlap of the thick and thin filaments

    • Length < Lo – maximal force is production impaired

    • Length > Lo – tension does not drop appreciably until the length is extended by 10-15%

  • Po – maximal isometric force

Brooks et al.


Muscle mechanics isometric contractions1
Muscle mechanicsIsometric contractions

Fast Twitch

Slow Twitch

Force

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

Time


Dynamic contractions
Dynamic contractions

Eccentric

  • Have to Compromise

  • Max force = loss in velocity

  • Max velocity = loss in force

Isometric

  • Po – max isometric tetanic tension. Occurs when force curve crosses the y-axis and velocity becomes zero

  • ↓force → ↑Velocity

  • Eccentric force is 50-100% due to more force needed to detach crossbridges

    • Also causes muscle damage

  • Power = Load x Velocity

    • “0” power when there is no load or when load is too heavy to be moved

Concentric

McMahon, Muscles, Reflexes, and Locomotion, Princeton, 1984


The larger the load the less shortening
The larger the load, the less shortening

S

H

O

R

T

E

N

I

N

G

V max

No load

Small load

Medium load

Large Load

Very Large Load (no velocity)

Time (from onset of stimulation)


Muscle mechanics
Muscle mechanics

To review, determinants of force/power production by a muscle

1. # of motor units recruited (i.e., the cross-sectional area of the active muscle)

- recruitment rule – smaller units first followed by larger

2. frequency of stimulation (i.e., rate coding)

3. length of the fibers relative to Lo

4. velocity (shortening and lengthening)

a. myosin ATPase activity

b. SR concentration

5. muscle architecture (consider pennation)

a. orientation of fibers to the long axis

b. the # of sarcomeres in series


Neuron anatomy axon structure
Neuron anatomy:axon structure

  • Cells body

    • Makes neurotransmitters

  • Axon - distribution

    • Myelin Sheath

    • Nodes of Ranvier

  • Dendrite

McComas, Skeletal Muscle, Human Kinetics, 1996


Neuron anatomy classification
Neuron anatomy: classification

Advantage?

Advantage?

Vick, Contemporary Medical Physiology, Addison-Wesley, 1984


Generating resting membrane potential
Generating resting membrane potential

(150,000 Na+)

Resting ~ -70mv

Electrochemical Gradient High  Low

Three major factors contribute to the resting membrane potential:

1. Na2+-K+ pump

2. Differential permeability of ions

3. Non-permeable ions (e.g., proteins)

Brooks et al. – Fig. 18-2


Resting membrane potential

Na+/K+ Pump

Resting Membrane Potential

  • Maintained primarily by active transport

  • Pump opens to the cytoplasm where it binds Na+, hydrolyses ATP

  • Decreased affinity for Na+ binding

    • uncovered binding sites for K+

    • pump cavity closes to cytoplasm, opens to external solution

  • When K+ ion bind, phosphate dissociates and things return to normal

  • Each cycle pumps out 3 Na, 2 K in and uses 1 ATP


Neuron anatomy ion channels general model
Neuron anatomy: ion channels – general model

McComas, Skeletal Muscle, Human Kinetics, 1996


Neuron anatomy na channel
Neuron anatomy: Na+ channel

At least 4 subunits for any Na+ channel to make a passage pore

McComas, Skeletal Muscle, Human Kinetics, 1996


Generating the resting potential
Generating the Resting Potential

  • Gated channels closed,

    non-gated channels open allowing for diffusion

  • Diffusion Gradients

    • K+ normally diffuses out of cell

    • Na+ normally diffuses into the cell

  • Na+ K+ pump

    • to maintain a negative resting membrane potential

    • pumps 3 Na+ out, 2 K+ in

Equilibrium potentials (Veq): Nernst equation

For ion X: VeqX = 61.5 log10 [X]o/[X]i

For K+: VeqK = 61.5 log10 [K]o/[K]I

VeqK = 61.5 log10 [4]o/[135]I = 93.5


Action potential
Action potential

First step is that Na+ channels open allowing Na+ into the cell. The membrane potential to become positive.

A positive membrane potential opens voltage gated K+ channels allowing the K+ to flow out of the cell.

Next the Na+ channels close, K+ channels are still open it allows the outflow of positive charge.

When the membrane potential begins reaching its resting state the K+ channels close.

The Na+ /K+ pump return cell to Resting Membrane Potential

3

4

Hyperpolarization

Why under resting?

Lost chemogradient

Have electrogradient

5


Action potential1
Action Potential

  • Resting membrane potential - permeability for K+ is 25x> Na+ so membrane potential is near equilibrium potential of K+ (-92mV)

  • Action potential - 20x more Na+ channels open (inward) than K+ channels (outward) so the membrane potential changes in a positive direction



Rate coding
Rate Coding

  • Frequency Coding of Stimulus Intensity

    • The greater the generator potential, the greater the number of action potentials produced per unit time (i.e., the higher the frequency)


Refractory periods
Refractory Periods

  • Absolute Refractory Periods

    • time at which a 2nd depolarization cannot occur regardless of the stimulus strength

      • Na+ channels inactive and cannot respond

      • Membrane is not repolarized

  • Relative Refractory Periods

    • a greater than normal stimulus can create an action potential

    • Na+ inactivation gates must have returned to normal (open position) and K+ channels (non-gated & gated)


The role of refractory periods
The Role of Refractory Periods

  • Prevent ‘backfiring”

    • wave of depolarization spreads in all directions but cannot depolarize an area from which the stimulus is coming - otherwise there would be a continuous cycle of depolarization

  • Sensory Coding

    • frequency of action potentials from the sensory neuron to the CNS

      • the greater the stimulus, the earlier in the relative refractory period will be the next action potential


Synapses facilitation and inhibition
Synapses: facilitation and inhibition

Transmission at synapses involves release of a chemical neurotransmitter from the pre-synaptic terminal and binding to receptors on the post-synaptic neuron

Brooks et al.


Neuromuscular junction
Neuromuscular Junction

Brooks et al.



Control of muscle fiber properties by the α-motoneurona. Action potential pattern and/or quantity b. Neurotrophic influences (substances released from the α-motoneuron that influence gene expression in the muscle fiber)


Control of muscle fiber properties by the motoneuron evidence for the control
Control of muscle fiber properties by the α-motoneuron: evidence for the control

Cross-innervation

Has the potential for complete myosin ATPase remodeling

Adapt to match motor unit

Fast Slow

McComas, Skeletal Muscle, Human Kinetics, 1996


Control of muscle fiber properties by the motoneuron evidence for the control1
Control of muscle fiber properties by the α-motoneuron: evidence for the control

Chronic low electrical stimulation

Created slow fiber type

McComas, Skeletal Muscle, Human Kinetics, 1996


Control of muscle fiber properties by the motoneuron evidence for the control2
Control of muscle fiber properties by the α-motoneuron: evidence for the control

Chronic electrical stimulation - soleus

Many pulses

Low Hz

few pulses

High Hz

Many pulses

High Hz

McComas, Skeletal Muscle, Human Kinetics, 1996


Control of muscle fiber properties by the motoneuron integrated model
Control of muscle fiber properties by the α-motoneuron: integrated model

McComas, Skeletal Muscle, Human Kinetics, 1996


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