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).