Unit 3 - Lectures 20-28

Planning and strategic phase

Thinking about what actions you need to do to reach a certain goal.

Execution phase

The brain sending commands to the muscles to _______ the plan.

Flexors

Muscles that decrease the angle of a particular limb.

Flexion

The movement to close.

Extensors

Opposes the action of the flexor by increasing the angle of a limb.

Extension

The movement to open.

Antagonists

A pair of muscles that oppose each other (flexors and extensors).

Alpha motor neurons

Directly trigger the generation of force by the muscles by contracting them.

Motor unit

An AMN and all the muscle fibers that it innervates.

Motor neuron pool

All of the AMNs that innervate a muscle.

Acetylcholine

A neuromodulator that has dual affects on both ionic and metabotropic receptors.

• Released by LMNs at the neuromuscular junction and bind to receptors that are directly linked to ion channels. This causes depolarization (Na+ rushes into the muscle) and a contraction/twitch to occur.

Larger in some areas

The size of the ventral horn is ______ __ ____ _____ because the distribution of lower motor neurons is not even. Some levels of the body need more control, and so the ventral horns contain more neurons.

Summation

When multiple action potentials fire in the alpha motor neuron and stimulate muscle fibers, the contraction is sustained over a period of time because the relaxation window is not being allowed due to the temporal input.

Tetanus

When the sustained contraction reaches a plateau.

Sources of input for muscle contraction

• firing rate of muscle contraction

• size of the motor unit

• type of muscle fibers

Motor unit control of muscle contraction

The size of the motor unit recruited determines the force applied to the muscle. Smaller motor units = smaller forces (used for precise movement) while larger motor units = a large force being applied to the muscle.

Fast twitch fibers

These are white muscles and use anaerobic respiration. They generate larger amounts of force, but get exhausted quickly.

Slow twitch fibers

These are red fibers and use aerobic respiration. They also contain a lot of mitochondria. They generate less force, but they fatigue more slowly.

Fatigue-resistant fibers

Generate moderately strong and fast contractions.

Fast fatigable fibers

Generate the strongest, fastest contractions, but get exhausted quickly.

Muscle spindles

specialized organs that are located within muscle fibers.

1A fibers

Sensory axons that wrap around muscle spindles, which allow changes in them to be sensed and transmitted to the spinal cord. They are highly myelinated.

Gamma lower motor neurons

Innervate muscle spindles, and they adjust their length so that they can stretch more or less depending on the position of the muscle.

Reflex

Automatic, involuntary responses to stimuli are nearly instantaneous. Important for maintaining body stability and reacting to harmful stimuli. Many can occur through neural circuits in the spinal cord and can bypass the brain.

Step 1 of the Knee jerk reflex arc

Banging on the knee stretches the quadricep muscles and muscle spindles.

Step 2 of the Knee jerk reflex arc

Muscle spindle causes increased firing in the 1A sensory axons, which project thorugh the dorsal root ganglion into the spinal core.

Step 3 of the Knee jerk reflex arc

1A axons branch and synapse on the alpha motor nerons and interneurons in the ventral horn.

Step 4 of the Knee jerk reflex arc

Firing of the 1A axon causes excitation of the motor neurons projecting to the quads (flexor; causes them to contract) and inhibition (through interneurons) of the hamstring muscle (extensor; stretching), which causes the knee to fling out.

Circuits for locomotion

_______ in the spinal cord generate ______ (flexion and relaxation of muscles). This is hardwired within specialized _____ within the spinal cord and happens with constant input from the brain. Rhythmic commands from the motor cortex are not needed.