Spinal Control of Movement

Midterm 2 Results

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Types of Muscles Smooth – digestive tract, arteries Striated:

Cardiac – accelerates or slows heart rate Skeletal – moves bones around joints, moves

eyes, facial expression, respiration, speech Skeletal muscles are the somatic motor

system and are under voluntary control.

Types of Movement Extension – takes limb away from body (opens

penknife) Flexion – brings limb toward body (closes penknife) Muscles cannot push so any movement requires

coordination Synergists – muscles that work together Antagonists – muscles that pull in opposite directions

Muscles are also named by location: Axial (trunk), proximal (shoulder, elbow, knee), distal (toes)

Motor Units Each muscle fiber is innervated by an alpha motor

neuron. Bundles of fibers form large and small motor units.

Small motor units act first and control fine motor movement.

Finer control is possible under lighter loads. Fast contracting, fast fatiguing white fibers form

“fast” motor units (slow ones are red). Alpha neuron firing rate makes a fiber/motor unit fast or

slow.

Input to Alpha Motor Neurons Dorsal root ganglion input to muscle spindle.

Provides feedback about muscle length. Upper motor neurons in the motor cortex and

brain stem. Voluntary control of muscles.

Interneurons

Muscle Fiber Structure Muscle fibers are enclosed by an excitable

cell membrane called sarcolemma. Within the muscle fiber are cylindrical

structures called myofibrils which contract in response to the sarcolemma’s action potential.

The action potential releases Ca++ from the sarcoplasmic reticulum which starts the chemical reaction resulting in contraction.

How Muscles Contract Myofibrils are divided into segments by disks

called z lines. The myofibril in between two disks is called a sarcomere.

Each sarcomere includes both thick and thin filaments.

During contraction, thin filaments slide along the thick filaments bringing the z lines closer together. This is reversed during relaxation.

Chemical Reaction Calcium binds with troponin (in the thin

filament). This exposes myosin binding sites on the thin filament’s actin.

Myosin heads in the thick filament bind with actin in the thin filament, causing the myosin heads to rotate. This results in the sliding. This continues as long as calcium is present.

An ATP-driven pump returns calcium to the sarcoplasmic reticulum and muscles relax.

Reflexes Reciprocal inhibition – cannot flex and extend

the same muscle Myotatic (knee-jerk)

Opposes gravity Uses spindle sensory feedback

Reverse myotatic (knife-clasp) Relaxes overloaded muscle Responds to golgi tendon organ feedback

More Reflexes Flexor reflex – response to pain Crossed-extensor reflex – compensates for

flexor reflex One side extends as the other flexes The circuit for coordinated control of walking

resides in the spinal cord. Circuits called “central pattern generators” give

rise to rhythmic motor activity.