Motor Function of the Nervous System

Muscle tissue contracts in response to appropriate nervous impulses.
- impulse travels along a motor neuron to the nerve-muscle synapse (neuromuscular junction)
- at the synapse, neurotransmitters (acetylcholine) are released into the synaptic cleft
- neurotransmitters bind to receptors on the muscles, initiating a depolarization process in the muscle, causing actin filaments to pull in between the myosin filaments, resulting in contraction of the muscle
- once the action potential has passed, the muscle will relax

http://www.hoops.co.il/?p=14856

http://www.zoology.ubc.ca/~gardner/chemical_synapses%20-%20postsynaptic.htm

In skeletal muscle:
- one impulse results in one contraction - this is called muscle twitch (izomrángás)

http://howmed.net/physiology/skeletal-muscle/

- since the action potential has passed by the time the muscle contracts, a new action potential can innervate the muscle before the period of relaxation begins.

In the figure below, in (a) we can see a series of twitches, where the impulses are spaced out in time

Staircase effect
- if impulses arrive one after the other in more rapid succession, we can observe what is referred to as the staircase effect, where each twitch is a bit stronger than the one before.  See (b).  Basically, the second twitch came so quickly that the first one never fully relaxed, so there is leftover calcium in the sarcoplasm.  Since there is some force leftover from the first twitch, they build on each other.  This is called summation of force

Tetanus
- In (c), the twitches are created by such a rapid succession of impulses (at least toward the end), that the individual twitches summate and blend into a smooth sustained contraction, called tetanus.  This is also called frequency summation

http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/unit9/summatio.htm

- sustained contractions are normal in living organisms, individual twitches are rare.

Muscle tone/Tonus
- skeletal muscles are constantly in a state of passive, partial contraction, which is called muscle tone
- it helps maintain posture and it decreases during REM sleep
- muscles are always ready to respond to sudden changes (eg. in balancing)
- muscle tone is primarily controlled at the level of spinal reflexes, but some aspects also have higher control.
eg. knee-jerk reflex

http://kids.britannica.com/comptons/art-156424/Knee-jerk-reflex-reaction-and-motor-neuron-connection-to-spinal

- hitting the hammer to the knee causes a slight stretch in the quadriceps (thigh muscle on front of leg)
- the stretch receptor in the quadriceps sends an impulse along the sensory neuron to the spinal cord
- the axon branches to two synapses: one to an inhibiting interneuron which synapses with a motor neuron that leads to the biceps muscle and prevents it from contracting; the other branch of the axon synapses directly with a motor neuron which sends and impulse to the quadriceps causing it to contract.
- lower leg kicks forward

- reflexes play a role in more complex motions, but control over most motion occurs at higher levels of the central nervous system
- the organization and coordination of our motions occurs in the cerebrum, in the posterior region of the frontal lobe
-motor tracts descend through one of 2 systems:  pyrmidal and extrapyramidal motor systems

Pyramidal motor system:
- the pyramidal tracts descend from the cerebrum through the brainstem to the grey matter of the spinal cord
- in the brainstem some fibres connect to motor neurons of brain nerves, but the majority cross-over in the pyramids of the medulla oblongata and continue into the spinal cord where they synapse with motor neurons
- a few nerves don't cross-over in the medulla oblongata, but continue straight down into the spinal cord , where they cross-over right before synapsing with motor neurons
- in the end, the motor innervation of the right side of the body is controlled by the left side of the brain and vice versa.
- the pyramidal motor system controls all intentional movement and learned fine motor coordination (like writing)

http://teachmeanatomy.info/neuro/pathways/descending-tracts-motor/

Extrapyramidal motor system:
- includes parts of the cerebrum, the mesencephalon and the cerebellum, as well as the brainstem
- this system controls automatic learned movements, large motor movements and motions that reflect emotions, as well as taking part in maintaining muscle tone
- the extrapyramidal system is very ancient and 3 of the 4 tracts found in humans are shared by salamanders

-well-controlled movement requires the coordinated function of both the pyramidal and the extrapyramidal systems

- the contraction of smooth muscle is also controlled by stimulation from vegetative motor neurons.
- contraction of the heart muscle does NOT require external stimulation.  The heart contains small muscle cells that are capable of spontaneous stimulation.  These cells are more permeable to sodium than the others and are found grouped at the sino-atrial node and at the atrio-ventricular node.