NSCI 175 Lesson 12

Flexion

Bending movement around a joint that lessens the angle between bones of the arm or leg at that joint.

Extension

Bending movement around a joint that increases the angle between bones of the arm or leg at that joint.

Axial Muscles

Responsible for trunk movement

Proximal Muscles

Controls shoulder, elbow, pelvis, and knee movement

Distal Muscles

Control hands, feet, and digit movment

Motor unit

A lower motor neuron and all the muscle fibers it innervates

Motor neuron pool

All alpha motor neurons that innervate a single muscle

Alpha motor neurons

Directly trigger muscle contraction through the release of achetylcholine

Alpha motor neurons receive input from...

1. Upper motor neurons: important for the initiation and control of voluntary movement
2. Muscle spindles: these receive input from dorsal root ganglion cells and provide feedback about motor length
3. Interneurons in the spinal cord

Slow motor units

Slowly fatiguing red muscle fibers

Fatigue-resistant motor units

Moderately strong and fast contractions. Relatively resistent to fatigue. Small-diameter, slowly conducting axons.

Fast Fatigable Motor Units

Strongest, fastest contractions, but are quickly fatigued. Largest diameter.

Process of muscle contraction

Initiated by the release of acetylcholine from the axon terminals of alpha motor neurons. This produces an EPSP, evoking an action potential in the muscle fiber. This action potential triggers the release of calcium from internal organelles which leads to fiber contraction.

How does muscle contract work

An action potential occurs in the an alpha motor neuron. Achetylcholine is released at the neuromuscular junction. Nicotinic receptor channels in the sarcolemma open, and the postsynaptic sarcolemma depolarizes. Voltage-gated sodium channels in the sarcolemma open and an action potential is generated in the muscle fiber. This sweeps down the sarcolemma and into the T tubules. Depolarization of the T tubules causes calcium release from the sarcoplasmic reticulum.

The steps of contraction are...

Calcium binds to troponin. Tropomyosin shifts position and myosin binding sites on actin are exposed. Myosin heads bind actin and then pivot. ATP binds to each myosin head, causing it to detach from actin. This cycle continues as long as there is calcium and ATP present.

Relaxation

Occurs when EPSPs end and sarcolemma and T tubules return to resting potentials. Calcium is moved into the sarcoplasmic reticulum. Myosin binding sites on actin are covered by tropomyosin.