Transcript Motor
Summarizing scheme of the interaction between different motor centers. The different major compartments of the motor system and their main pathways for interaction are indicated. The basic functions of the different compartments and descending tracks are summarized to the left and below the scheme. CS, corticospinal; RbS, rubrospinal; VS, vestibulospinal; RS, reticulospinal. MOTOR NEURON-MUSCLE RELATIONSHIPS Somatotopic organization of lower motor neurons in a cross section of the ventral horn at the cervical level of the spinal cord. Motor neurons innervating axial musculature are located medially, whereas those innervating the distal musculature are located more laterally. Two types of lower motor neuron are found in these neuronal pools: 1) α (alpha) motor neurons, which innervate extrafusal muscle fibers - the striated muscle fibers that generate the forces needed for movement. 2) small γ (gamma) motor neurons innervate specialized muscle fibers that are actually sensory receptors called muscle spindles. The muscle spindles are embedded within connective tissue capsules in the muscle, and are therefore referred to as intrafusal muscle fibers (fusal means capsular). The intrafusal muscle fibers are also innervated by sensory axons that send information to the brain and spinal cord about the length of the muscle. THE MOTOR UNIT REGULATION OF MUSCLE FORCE: TERMINOLOGY SUMMATION REGULATION OF MUSCLE FORCE THERE ARE 3 MAJOR TYPES OF MOTOR UNIT: 1) SLOW 2) FAST FATIGUE-RESISTANT 3) FAST FATIGABLE REGULATION OF MUSCLE FORCE THERE ARE 3 MAJOR TYPES OF MOTOR UNIT: 1) SLOW (“RED MEAT”; AEROBIC; MYOGLOBIN-RICH; MANY MITOCHONDRIA; DENSE CAPILLARIES; SMALL MNs) 2) FAST FATIGUE-RESISTANT (INTERMEDIATE CHARACTERISTICS; MEDIUM MNs) 3) FAST FATIGABLE (“WHITE MEAT”; ANAEROBIC; FEW MITOCHONDRIA; GLYCOGEN STORES CONVERT TO LACTIC ACID; LARGE MNs) MOTOR UNIT RECRUITMENT SIZE PRINCIPLE SLOW (S) FAST FATIGUE-RESISTANT (FR) FAST, FATIGABLE (FF) MUSCLE STRETCH RECEPTORS AND GOLGI TENDON ORGANS THE SPINAL CORD CIRCUITRY UNDERLYING MUSCLE STRETCH REFLEXES Stretching a muscle spindle leads to increased activity in Ia afferents and an increase in the activity of α motor neurons that innervate the same muscle. Ia afferents also excite the motor neurons that innervate synergistic muscles, and inhibit the motor neurons that innervate antagonists. ROLE OF GAMMA MOTOR NEURONS FLEXION WITHDRAWAL REFLEX/ CROSSED EXTENSOR REFLEX DESCENDING MOTOR PATHWAYS Corticospinal tract neurons (upper motor neurons) have multiple branches within the spinal cord i.e. not a point-to-point topography Divergence of M1 outputs to multiple muscles. (A) Tracing of a single corticospinal axon ramifying in the ventral horn of the spinal cord shows terminal fields in the motor neuron pools of four forearm muscles. From Shinoda et al. (1981). (B) Action potentials in a cortical neuron (top trace) are followed at a fixed latency by peaks of postspike facilitation in EMGs recorded from four of six recorded forearm muscles (lower traces), consistent with monosynaptic excitation of all four motor neuron pools by that cortical neuron. The EMGs are rectified and averaged responses to 7051 action potentials in the cortical neuron. From Fetz and Cheney (1980). (C) These anatomic and physiologic findings indicate that the output of single corticospinal neurons often diverges to influence multiple muscles. From Cheney et al. (1985). CORTICOSPINAL TRACT NEURONS CODE FOR FORCE CORTICOSPINAL TRACT NEURONS CODE FOR DIRECTION Activity of three neurons—one in M1, one in PM, and one in SMA—recorded as a monkey pressed three buttons in sequence. The sequence was first cued visually by lighting the buttons and was then cued internally. The M1 neuron showed similar activity whether the monkey performed from visual or internal cues. The PM neuron, however, was much more active in response to visual than internal cues, whereas the opposite was true for the SMA neuron. Modified from Mushiake et al. (1991).