3 types of muscles:

1) Smooth muscle - controlled by the autonomic nervous system; may either be generally inactive and then respond to neural stimulation or hormones or may be rhythmic

2) Cardiac muscle - found in the heart, acts like rhythmic smooth muscle, modulated by neural activity and hormones

3) Skeletal muscle - move us around and responsible for most of our behavior; most attached to bones at each end via tendons

movement:

• flexion - contraction of flexor muscles, drawing in of a limb
• extension - opposite of flexion, produced by contraction of extensor muscles (antigravity)

• extrafusal muscle fibers - served by axons of the alpha motor neurons (serve multiple muscle fibers); contraction of these muscles provides movement - extrafusal muscle fibers and associated alpha motor neurons are called a motor unit
• intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles
• gamma motor neuron - efferent axon causes the intrafusal muscle fiber to contract, but contributes little force; serves to modify the sensitivity of the fiber’s afferent axon to force
• myofibrils contain actin & myosin - proteins that provide the physical basis for muscular contraction myosin attaches to actin, lets go, then reattaches lower on the strand, etc. - rowing motion produces muscular contraction

• neuromuscular junction - synapse between efferent terminal button and the membrane of a muscle fiber
• motor endplates (postsynaptic membrane)
• acetylcholine - released by efferent axon’s terminal buttons, result in depolarization at endplate; endplate potential all or none (no threshold), produces contraction or "twitch" of the muscle fiber (calcium channels open - trigger rowing action)
• single impulse produces single twitch, need series of action potentials to produce a sustained contraction of the muscle fiber

• intrafusal muscle fibers - stretch when the muscle lengthens and relaxed when it shortens - detect muscle length
• Golgi tendon organ - stretch receptors located within the tendons, detecting the amount of stretch exerted by the muscles on the bones to which they are attached; encode degree of stretch by the rate of firing; don’t respond to length, but to how hard it is pulling
• passive movement - someone lowering your relaxed arm while he holds it - muscles lengthen passively - effect on Golgi tendon organ?
• arm dropped quickly - effect on Golgi tendon organ?
• weight dropped into hand held parallel to the floor - effect on Golgi tendon organ?

• stimulating patellar tendon causes knee to kick - occurs in 50 milliseconds, too fast for brain to be involved
• if asked to move leg when touched on knee, would be slower

1) afferent impulses from the muscle spindle are conducted to the terminal buttons in the gray matter of the spinal cord

2) terminal buttons synapse onto an alpha motor neuron

3) alpha motor neuron synapses on motor endplate on the extrafusal muscle fibers of the same muscle

4) if arm starts to drop, then muscle spindle afferent neurons start to fire as they detect muscle lengthening, they then synapse on alpha motor neurons and rate of firing increases, and then muscle contraction increases

example: posture - if pushed forward, muscles in back of calves stretch, causing contractions in the toes

Gamma motor system:

• allows for adjustment of sensitivity of muscle spindles to muscle length
• when muscle spindles are relaxed, they are relatively insensitive to stretch; but when already taught, they feel stretch quicker
• gamma motor neurons contract muscle spindles, making them more sensitive

1. alpha motor neuron and gamma motor neurons activated
2. alpha motor neurons start the muscle contracting
3. if no resistance, both extrafusal and intrafusal muscle fibers contract at the same rate, sending little info from muscle spindles
4. if resistance, then extrafusal muscle fibers are halted, but intrafusal continue to contract, as told to by the gamma motor neuron
5. then, sensory info from the intrafusal fibers goes to the spinal cord, where there is a synapse onto the alpha motor neuron, which then increase muscular contraction

Golgi tendon organs have 2 kinds of receptors:

• more sensitive - tell how hard the muscle is pulling

• less sensitive - their terminal buttons synapse onto an interneuron in the spinal cord gray matter which then synapse onto the relevant alpha motor neuron, producing inhibitory (glycine) potentials

• decreases muscular contraction, prevents injury

Muscle spindles send terminal buttons to:

1. alpha motor neurons
2. the brain
3. inhibitory interneurons

• homunculus - disproportionate amount of motor cortex devoted to fingers and speech muscles
• primary motor cortex - stimulation produces movement; connected to primary sensory cortex
• monkeys respond faster when trained to pull a lever following a stimulus to the hand, rather than sight or sound
• frontal association cortex - stimulates primary motor cortex; receives input from association areas of the occipital (visual), parietal (spatial), and temporal (auditory) lobes

Pathways that originate in the cortex:

1) corticospinal tract - axons terminate in gray matter of spinal cord, mostly originating in primary motor cortex, through pyramidal tracts, then at the end of the medulla they cross and descend through the contralateral spinal cord, forming the lateral corticospinal tract (control distal part of limbs); the remaining fibers stay on the same side and form the ventral corticospinal tract (control upper legs and trunk)

2) corticobulbar tract - projects to the medulla, ending at cranial nerves which control movements of the face and tongue

Pathways that originate in the brainstem:

1) rubrospinal tract - originates in the red nucleus, which receives info from motor cortex and cerebellum; axons terminate on motor neurons in the spinal cord (control arms and legs, but not fingers)

2) ventromedial pathways - terminate in gray matter of spinal cord; include vestibulospinal, tectospinal, and reticulospinal tracts (control movement of the truck and proximal limb muscles, such as walking, head turning, autonomic functions)

apraxia - inability to properly execute a learned skilled movement

1. limb apraxia - moving wrong part of limb, moving correct part in the wrong way, or correct movements in the wrong sequence (assessed by pantomiming)

• callosal apraxia - apraxia of the left limb caused by damage to the anterior corpus callosum (think about pathway from hearing speech to following command)
• sympathetic apraxia - apraxia of left hand due to damage to the anterior left hemisphere (can’t communicate to right if can’t process info from verbal channels); why was it called sympathetic?
• left parietal apraxia - apraxia of both limbs due to lesions of the posterior left parietal lobe (left verbal area sends info to left parietal, which gets info re environment from right parietal, and then calculates movement; also, acalculia)

Basal ganglia - know Parkinson’s & Huntington’s

Cerebellum - know definitions of terms in bold and section about lesions of the lateral zone

Reticular formation - controls activity of the gamma motor system - regulates muscle tone (remember association with arousal and autonomic functioning)