MOMMidtermReview2 - Motor systems have been proven to...

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Motor systems have been proven to reside both in the brain and in the spinal cord, though both are required in order to respond critically-- in accordance with environmental parameters-- to a stimulus. In the brain, the cerebral cortex, basal ganglia, cerebellum, and brainstem have been linked to movement. Types of movement involve voluntary, reflexive, and locomotive (repetitive rhythmical movements such as locomotion or mastication). The brain contains a number of feedback loops that occur throughout the brain that are responsible for initiating motor movement, but the final coded message for movement-- including the parameters that govern the movement’s generation-- is relayed to the motor cortex, where it is transmitted to the spinal cord. In the spinal cord, the movement is processed both at the interneuronal level and at the motoneuron level. Once the interneurons process the information, they transmit the integrated action potential to motoneurons. The spinal nerves contains both sensory and motor axons (mixed nerve), though before they enter the spinal cord they diverge, with the sensory axons entering through the dorsal root via the dorsal root ganglion, and the motor neurons exiting via the ventral root. After the afferent sensory nerves enter the dorsal root, they pass through white matter until they reach lamina 9, a section of gray matter located in the anterior grey area (or ventral horn). There, they synapse either with an inhibitory interneuron, which hyperpolarizes the exiting motoneuron, thereby preventing it from activating the antagonist muscle (leading to reciprocal inhibition, where the antagonist muscles are passively stretched during agonist contraction), or with the alpha motoneuron that is responsible for activating muscle fibers in the agonist. This is evidence that the stretch reflex is disynaptic to some degree; however, the stretch reflex is typically considered a monosynaptic reflex because it can function correctly with only the synaptic connection occurring between the 1a afferent and the alpha motoneuron. A motor unit describes a single motoneuron and the collection of muscle fibers that it innervates. A motor pool describes the collection of motoneurons that innervate the muscle fibers in a single muscle. Only one motoneuron innervates a single muscle fiber (at birth we have multi- innervation of a single muscle fiber, but the extra connections are pruned away). Muscles are connected via tendon to bone; the point of attachment is called the origin, and the opposite end where it binds to another tendon or bone is called the point of insertion. When both the agonist and the antagonist contract, it’s called an isometric contraction. Typically when an agonist muscle contracts, the antagonist passively lengthens (non-active, “nonspontaneous” lengthening) to accommodate the contraction of the agonist.
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