D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MOTOR SYSTEMS, SPINAL CORD
Lecture 17: Spinal Reflexes
Motor systems of spinal cord – cord reflexes: (chapter 54)
• organized into a hierarchy (ppt. 1): many muscular movements are
controlled by circuitry of the spinal cord (fig. 54 – 1 & ppt. 2), which varies from
simple (monosynaptic) pathways to more complex (polysynaptic) pathways that
control groups of muscles and may be called 'central pattern generators' or CPGs
- higher levels, brainstem & cerebral cortex, represent upper motor
neurons of direct system (corticospinal or pyramidal), from the motor cortex and
motor neurons of the indirect system (rubrospinal, etc.), from brainstem nuclei; send
‘commands’ to activate spinal cord circuits
- ultimate regulatory control of motor behavior resides with cerebellum &
basal ganglia, which receive input from direct & indirect systems (regarding
intended motor behavior) as well as input from motor activity; these issue corrective
adjustments to higher motor neurons that send altered commands to spinal cord
1.
Spinal Cord Organization (ppt. 3):
• gray matter
- dorsal horns (posterior) receive sensory input and input from higher brain
levels, (e.g. corticospinal tract); input synapses excite local segmental circuitry as well
as neurons communicating with other regions of the nervous system (other spinal
levels, brain levels)
- lateral horns contain the motor neurons of autonomic nervous system
- ventral horns (anterior) contain the motor neurons that generate efferent
signals to muscles; large motor neurons include A (larger, faster – alpha motor
neurons) and A fibers (gamma motor neurons) (fig. 54 - 2 & ppt. 4)
- alpha motor neurons innervate motor units of the muscles
- gamma motor neurons innervate muscle spindle organs
Bio 659, lec 17
- p. 2 -
- interneurons (most abundant neurons in the gray matter) constitute the
neuron pools that govern the patterns of spinal cord circuits (CPGs), e.g., divergence
(propriospinal fibers that communicate with other cord segments & Renshaw cells
that produce lateral inhibition), convergence (afferents as well as various CNS levels
converge on alpha motor neurons), repetitive discharge (reverberating circuits
extend contractile effort after expiration of sensory input)
- sensory input includes somatosensory and proprioceptive information
regarding state of muscle (provided by muscle spindles & Golgi tendon organs that signal
spinal cord, cerebellum & cerebral cortex) (ppts. 5 & 6)
2.
Spindle Organs: (figs. 54 – 2, 54 – 3 & ppts. 4 & 7) muscles contain two
types of muscle fibers – extrafusal (which deliver the contraction) & intrafusal (which
are the muscle’s sensory organ)
• intrafusal fibers: clustered together in spindle organs; lack contractile
myofibrils in the center = sensory area; only the ends contract
• sensory fibers – nerve fibers of two types (Ia & II) monitor static stretch and
dynamic change of length (rate of length change), respectively, of the muscle
• character of signal: both sensory fibers signal slow stretch with signal that
increases in proportion to stretch as long as spindle stays stretched (static response)
- primary sensory fibers (Ia) signal a rapid change in length with very
strong signal that disappears the instant the change stops (dynamic response);
superimposed upon static response signal
- unstimulated spindles generate a continuous rate of discharge, so that
stimuli can produce positive (increased rate of discharge) or negative (decreased rate of
discharge) signals corresponding with direction of length change in a muscle (ppt. 8)
• gamma motor neurons: contractions of the ends of the intrafusal fibers are
stimulated by gamma motor neurons; cause stretch of sensory region – tune
sensitivity of spindle organ (ppt. 9)
Bio 659, lec 17
3.
- p. 3 -
Golgi Tendon Organs (fig. 54 - 8 & ppt. 10):
• muscle tension: Ib afferent fibers in tendon produce dynamic and static signals
(like spindle organs), but respond to changing tension, not length
- character of signal is proportional to change or to rate of change of tension;
activates reflex that inhibits muscle contraction (protection against excessive tension
& distribution of load to more motor units) (ppt. 11)
4.
Reflexes:
• monosynaptic: represents one branch of afferent fiber synapsing directly with
alpha motor neuron; other branches distribute input to other destinations
- simple stretch reflex: Ia afferent synapses with alpha motor neuron to
same muscle (monosynaptic pathway) (fig. 54 - 5 & ppt. 12); instant resistance to
changing length ‘smoothes out’ jerkiness that may result with oscillating nerve
signals to muscle (fig. 54 - 6)
• circuits: interneurons participate in most reflexes producing different
patterns of circuitry:
- divergent: distribution of signals to contralateral muscle groups; lateral
inhibition (sharpens focus of input signal); distribution of inputs to various brain
regions; excitation of gamma fiber concurrently with alpha fiber (maintains spindle
sensitivity during contraction)
- convergent: several separate inputs delivered to single motor neuron or
group of motor neurons (e.g. signals from other regions of CNS modify primary
input to alpha motor neurons)
- repetitive discharge: output continues to cause muscle contraction after
stimulus has ceased; due to reverberating circuit pattern
• other reflexes exemplify some of these patterns (fig. 54 – 9 & ppts. 13 to 16)
- reciprocal innervation: divergent circuit excites agonist muscle and
inhibits antagonist; divergent circuit excites two muscle groups to produce
coordinated actions
Bio 659, lec 17
- p. 4 -
- flexor reflex (withdrawal reflex): withdrawal from painful stimulus
- crossed extensor reflex: opposite limb pushes away from withdrawal
reflex stimulus
- walking reflexes: forward movements of one limb coordinated with
backward movements of opposite limb; backward movements & reciprocal forward
movements follow after delay
- postural reflexes: pressure on foot stimulates muscle contractions to
support body; reflexes preserve body orientation to gravity
- Babinski's sign: abnormal response to testing of plantar reflex; toes should
normally curve under (flexion) but if corticospinal tract is injured, great toe extends and
other toes abduct (Babinski's sign)