Nicholas Price; Week 1 MED1022; Physiology
Movement requires planning, initiation, coordination, refinement and optimisation. Lower motor neurons (alpha neurons) are in the spinal column, proximal limb muscle neurons are located medially and distal laterally. Flexors are typically ventral to extensors. Motor unit is composed of one motor neuron and the muscle fibres that it innervates, a pool of motor units innervates a single muscle. Inputs to LMN are input from interneurons, sensory input from muscle spindles, input from UMN. Motor recruitment is use of many motor units and dependent on size of motor units, which creates graded muscle contractions.
Group Ia sensory neurons synapse on alpha motor neurons and interneurons to form a monosynaptic myostatic reflex arc. Ia neurons target extrafusal fibres shortening bulk of muscle, gamma neurons target intrafusal fibres, shortening the muscle spindle. Alpha activation decreases Ia activity, gamma increases Ia activity.
Golgi tendon organs are innervated by Ib sensory axons, which synapse on interneurons in the ventral horn. Some interneurons target alpha motor neurons of the same muscle. It acts to maintain tension within a certain range by the reverse myotatic reflex- increased muscle tension inhibitors alpha motor neuron (through Ib axons) causing a reduced rate of muscle contraction.
Reciprocal inhibition is achieved by sensory input from Ia axons used to inhbit contraction of agonist (contraction of one muscle relaxes the second). This prevents resistance against intentional movement.
Flexor and crossed extensor reflexes allow flexion of ipsilateral limb to be compensated with extension of the contralateral limb.
Electromyograms can be used to observe muscular activity; can assess nerve conduction velocity associated with myelination, temperature, axon diameter and resistance as well as compression. It can diagnose motor nerve disease, NMJ problems and muscle fibres. Denervation leads to fibrillations (ACh is sensitised, spontaneous APs can spread across fibre, twitches); and degeneration leads to fasciculations (twitch contractions from spontaneous motor neuron discharges). Fasciculations can be seen and felt with the naked eye. LMN lesions lead to atrophy, flaccid paralysis (no voluntary movement), areflexia.
Command signals are generated in UMN, Betz cells are primary output neurons. They are pyramidal, project directly from anterior horn cells in the spinal cord. In the brain, the premotor cortex is in front of the primary motor cortex. Primary somatosensory cortex is behind this.
Sensory neurons have an enlarged layer 4 (inputs), motor has enlarged layer 5 (outputs) in the brain. Mirror neurons respond when an action is performed or observed. Basal ganglia is involved in movement selection and initiation, cerebellum coordinates timing of sequences of muscle contractions, integrates sensory feedback and is involved in motor learning.