SPINAL REFLEXES LECTURE

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.

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.

MOTOR NEURON-MUSCLE RELATIONSHIPS

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) SLOW2) FAST FATIGUE-RESISTANT 3) FAST FATIGABLE

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)

REGULATION OF MUSCLE FORCE

MOTOR UNIT RECRUITMENTSIZE PRINCIPLE

SLOW (S) FAST FATIGUE-RESISTANT (FR) FAST, FATIGABLE (FF)

MUSCLE STRETCH RECEPTORS AND GOLGI TENDON ORGANS

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.

THE SPINAL CORD CIRCUITRY UNDERLYING MUSCLE STRETCH REFLEXES

ROLE OF GAMMA MOTOR NEURONS

FLEXION WITHDRAWAL REFLEX/ CROSSED EXTENSOR REFLEX