Introduction+to+Motor+Systems

=**Introduction to Motor Systems**=


 * 1. Define concept of motor unit.**

Because the transmission of the neuromuscular junction is very reliable, an action potential in the axon of the motor neuron invariably causes a muscle action potential in each of the innervated fibers. Because of this cooperative and efficient action, the motor neuron and all its recipient muscle fibers are called a “motor unit.”


 * 2. Describe neural mechanisms regulating force production in muscles.**

The nervous system has several neural mechanisms that regulate which motor units become active, how many become active, and the level of activity within those units.

__Size Principle__ For low tension requirements, smaller motor neurons are first to activate; as the required tension increases, progressively larger motor neurons are recruited to those already active. That is, for a given amount of synaptic input, smaller motor neurons (which contact fewer muscle fibers) are more likely to reach action potential threshold than larger ones.

__Number Principle__ As muscle force requirements (and synaptic excitation) increase, the number of motor units becoming active also increases.

__Action Potential Frequency__ Low frequency firing produces distinct contracts at the same frequency; however, at higher frequencies, the slow decay of tension causes successive action potentials to produce a cumulative effect called “tetanus.” Partial tetanus refers to the summation of tension which still displays evidence of single contractions while fused tetanus refers to maximum tension production and no individual contracts are evident. During tetanic contractions, slight changes in action potential frequency can make a large difference in total tension produced. Tetanus results because Ca2+ release from action potentials is released at a faster rate than Ca2+ can be pumped back into sarcoplasm.

__Fiber Type__ Smaller motor units which are usually recruited first have muscle fibers that are relatively slow and non-fatiguing (oxidative metabolism). Fibers recruited later are fast twitch fibers which are powerful but fatigue quickly (anaerobic metabolism). Different muscles are composed of different proportions of fiber types but all fibers within each motor unit are of the same type.


 * 3. Recognize internal organization of motor pools (spinal cord & brainstem) and summarize somatotopic arrangement of motor neurons.**

Spinal motor neurons are found in the ventral horn of the spinal lamina IX. Motor neurons that innervate a single muscle are clustered together in motor neuron pools and have axons leaving the spinal cord through one ore more adjacent ventral roots. Motor neuron pools are columns of cells that extend 1 or more spinal segments. They are organized with a somatotopic arrangement in which more superior/proximal muscles are higher in spinal cord segment than inferior/distal muscles. Additionally, proximal and distal muscles are represented medially and laterally with extensors more ventral to flexor motor neuron pools. This is consistent with the locations of motor tracts in the spinal white matter.

Motor neurons involved in controlling somatic and visceral motor functions of cranial nerves are located in the brainstem and have nuclei arranged in functional columns like spinal cord motor neurons.

Surrounding motor neuron pools are interneurons in spinal laminal VII which typically have inhibitory inputs to motor neurons. Interneurons in lamina III-VI of the dorsal horne may be excitatory or inhibitory to motor neurons and may project to contralateral motor pools. Sensory inputs from dorsal root ganglia and descending pathways synapse to interneurons to provide polysnaptic reflex and voluntary control of motor neurons on both sides of the body.


 * 4. Describe classes of motor neurons.**

A-alpha motor neurons are larger and innervate skeletal muscles which move joints and are involved in motor units. Smaller gamma motor neurons innervate specialized intrafusal muscles located in muscle spindles that are sometimes called extrafusal muscles. Intermediate size A-beta motor neurons supply extrafusal and intrafusal fibers with collateral axons but are rare in humans.

A-alpha motor neurons receive numerous synaptic inputs that may be excitatory (from muscle spindle afferents) or inhibitory (layer VII interneurons). The size of an EPSP and its likelihood of producing an action potential depend on the size of the motor neuron, its passive electrical properties, distance of active synapse from initial segment of motor neuron axon, and presence/absence of concurrent IPSP from other synaptic sites.


 * 5. Learn locations of interneurons and how they are combined into functional units.**

Interneurons combine sensory afferent and descending control pathways form cerebral cortex or brainstem to provide a variety of motor functions. In addition to divergence and convergence, interneurons can also serve as gates to permit or block peripheral inputs. This gating function is an important feature of how pain information is processed in the nervous system. Similar gating function can be done from an intercalated interneuron circuit or by presynaptic inhibition (in which transmitter release is controlled by axo-axon synapses.

Reverberating circuits involve negative feedback loops in which activity from one neuron activates a circuit which eventually turns itself off, sequencing periods of activity and inactivity. This is a common feature in spinal cord and basal ganglia. Rhythmic alteration is a type of reverberating circuit in which groups of interneurons feed-forward inhibitory signals to sequence activity in other interneurons in a reciprocal interaction. Rhythmic alteration is important in locomotion and respiratory control.


 * 6. Gain a general overview of the contributions made to motor control by motor cortex, premotor cortex, cerebellum and basal ganglia.**

The thalamus receives information from cerebellum and basal ganglia and transfers it to the cerebral cortex. The cortex sends both sensory and motor information back to basal ganglia which is involved in the initiation and maintenance of motor activity (particularly repetitive, semi-automatic behaviors). The cortex also sends motor information to cerebellum by indirect pathways through the red nucleus, pontine and medullary nuclei. The cerebellum is involved in motor accuracy, learning, and highly skilled movement. Cerebellum can use sensory inputs to compare the intended motor pattern received from cerebral cortex with feedback from movements currently taking place.

Several motor centers in the brainstem send commands to spinal interneuronal networks including the red nucleus, two vestibular nuclei, and several regions in the pontine and medullary reticular formation. Motor cortex and cerebellum project to certain parts of these nuclei. Motor cortex also directly projects to spinal cord through corticospinal tracts.


 * 7. Recognize the various types of sensory receptors involved in spinal reflexes.**

__Muscle Spindles__ Muscle spindles are groups of specialized (intrafusal) muscle fibers with primary and secondary sensory endings that detect velocity of stretching and absolute length of muscle, respectively. Muscle spindles are arranged in parallel with extrafusal muscle fibers and surrounded by connective tissue that connects the spindle to the tendon insertion of the muscle. Spindle length is a surrogate index of skeletal muscle length and is coded in action potential frequency of primary afferent fibers contacting the spindle.

Nuclear bag fibers are larger fibers characterized by many cell nuclei congregated in an expanded bag in the central portion of the receptor area. Nuclear chain fibers are thin and have nuclei aligned in series along the fiber’s length.

Primary sensory ending of annulospiral ending forms a spiral around the nuclear bag and nuclear chain fibers. This ending gives off to a fast afferent fiber ending monosynaptically on motor neurons of the muscle of origin and synergistic muscles. Collateral branches form the primary afferent fiber pass up the posterior column and connect by interneuron to motor neurons of antagonistic muscles. Secondary sensory ending or flower spray ending are generally on nuclear chain fibers and give off slower afferent axons which connect with motor neurons only through interneurons.

Both primary and secondary sensory endings are sensitive to stretch of intrafusal fibers. Both primary and secondary endings are responsive to the slow static phase information when stretch is maintained. However, only primary endings are responsive to fast dynamic phase information when the muscle elongates.


 * 8. Describe the anatomic and functional pathways involved in the stretch reflex, withdrawal reflex, and the golgi tendon (or lengthening) reflex.**

__Stretch Reflex__ In phasic stretch (deep tendon) reflexes, the action potential from type A-alpha muscle spindle afferents are excitatory to motor neurons of the homonymous muscle (monosynaptic), to synergistic muscle motor neurons (polysynaptic), and to interneurons that lead to inhibition of antagonistic muscles. This reflex type involves reciprocal inhibition.

Tonic stretch reflexes are initiated by group II afferents in response to any maintained change in length of muscle. This type of reflex is polysynaptic and provides continuous background of excitatory inputs to alpha motor neurons. This activity is responsible for muscle tone (slight tension that can be felt in a relaxed muscle).

Gamma motor neurons modulate the sensitivity of intrafusal muscle fibers by producing tension changes in these fibers. During or just prior to controlled movement, activation of gamma motor neurons by higher brain areas can set the sensitivity of the muscle spindles to a level appropriate to the amount of contraction that will be needed. There are both dynamic and static gamma motor neurons that control spindle sensitivity by type of contraction and contraction speed. Gamma motor neuron control during movement can continually adjust spindle sensitivity to assure the spindle provides a steady flow of signals to the nervous system while the extrafusal muscle length changes until the movement is completed. //Gamma motor neurons have no connections with extrafusal muscle fibers and can affect reflexes only through the sensory portion of the process.//

__Withdrawal Reflex__ The flexion or withdrawal reflex is a polysynaptic reflex associated with various types of noxious stimuli. Receptors involved may be nociceptors, touch, or pressure receptors which conduct their impulses through A-delta and C fibers. The reflex withdrawal is the withdrawal of a portion of the body from a noxious stimulus. This reflex is distributed to motor pools in multiple segments by collateral axons of dorsal root ganglia. It can be elicited even when the spinal cord is transected above the reflex center but disappears in deep unconsciousness such as during anesthesia.

__Golgi Tendon (Lengthening) Reflex__ Golgi tendon organs (GTO) are encapsulated sensory receptors with A-beta axons that are sensitive to tension and located in a tendon. Muscle spindles detect muscle length changes while GTO detects muscle tension. Because GTO is in series with extrafusal fibers, GTO increases its discharge rate in response to increase tension.

When GTO of a muscle are stimulated by increased muscle tension, signals are transmitted by A-beta axons to the spinal cord to cause reflex effects. This reflex is entirely inhibitory (opposite to muscle spindle reflex). Central process of the A-beta fiber terminates on interneurons that inhibit motor neurons of the muscle origin (homonymous and synergist muscles) and facilitates the antagonist muscles. This provides negative feedback that prevents development of too much tension on a muscle. When tension in a muscle and its tendon becomes extreme, inhibitory effect form GTO can signal instantaneous relaxation of the entire muscle in the lengthening reflex. This prevents the tearing of muscle or avulsion of the tendon from its attachments. At lower levels of tension, GTO provides proprioception input to the cerebral cortex and cerebellum by brainstem relay centers which allow monitoring of muscle tension in relation to planned and ongoing movements.


 * 9. Recognize the importance of supraspinal influences and Renshaw cell circuits which regulate spinal reflexes.**

__Superspinal Influences__ Spinal neuron activity is strongly influenced by supraspinal neurons, both excitatory and inhibitory, that converge on motor neurons. e.g. descending control produce a movement directly or indirectly by acting on gamma motor neurons. Contraction of somatic muscles indirectly by activation of gamma motor neurons is called the gamma loop. Activation of gamma motor neurons would contract the intrafusal muscles, causing increased A-alpha fibers firing from the muscle spindle. This activates the homonymous and synergistic motor neurons which produce a contraction of the extrafusal fibers of the muscle.

__Renshaw Cells Circuits__ A-alpha motor neurons send collateral axons to synapse with inhibitory (glycine-containing) interneurons called Renshaw cells. These cells inhibit the homonymous and/or synergist motor neurons. Activity in such a loop temporarily reduces the motor neuron’s activity immediately after firing in a negative feedback loop and is called “recurrent inhibition.” The function of the Renshaw loop is to stabilize the discharge frequency of motor neuron pools, regularize timing of action potentials in efferent nerves, and generally prevent neurons from discharging at excessive rates beyond useful ranges for muscle force regulation.