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Lecture 15
W3005/4005y
Darcy B. Kelley
Review of muscles and of central pattern generators
Review: The basic building block of movement is the motor unit, the motor
neuron and all the muscle fibers that it innervates. Study of certain elementary
reflexes (the stretch reflex, the crossed flexor/extensor reflex) reveals an
intrinsic set of connections between interneurons and motor neurons in the spinal
cord. Different kinds of sensory input (stretch via Ia fibers and the spindle
system (muscle length), muscle tension via Ib fibers and the Golgi tendon
organ) can activate different reflex pathways by accessing specific interneurons
with characteristic connectivity patterns (Ia and Ib interneurons). Some
inhibitory interneurons are activated by axon collaterals of motor
neurons(Renshaw cells).
Introduction to pattern generation: The interneurons and motor neurons of the
spinal cord generate rhythmic activity that is independent of sensory input; they
serve as central pattern generators. In locusts, Wilson observed rhythmic
output from the spinal cord to wing muscles in the absence of any sensory input.
Sherrington’s study of the scratch reflex demonstrated that stimulus intensity did
not affect rhythmic activity of muscle groups. The rhythmic activity generated by
CPGs is responsible for rhythmic motor patterns such as walking, flying and even
singing and is seen in ventral roots disconnected from the muscles themselves
(fictive walking etc). Even though sensory input does not create rhythmic
patterns it can modify them.
This rhythmic activity is due to cyclical changes in the firing patterns of spinal
cord neurons; each limb or segment has a pair of independent pattern generator
whose activity is co-ordinated with those of other limbs to produce locomotion.
Rhythmic activity can also be set off by application of L-Dopa or by stimulation of
supraspinal centers such as the mesencephalic locomotor region
How do these circuits generate rhythmic patterns?
• Models
• Lamprey and tadpole swimming, case studies
Supraspinal motor systems
The execution of voluntary movement requires the co-ordinated activity of three
supraspinal systems: the cerebellum, the basal ganglia and the motor cortices;
the first of which is the subject of today's lecture. Roughly speaking, the motor
cortices are involved in the planning of movement, the generation of motor
programs that result in the execution of complex movements involving multiple
muscle groups and multiple joints. The basal ganglia are required to maintain the
flow of movement, to supress unwanted movements, to insure the rapid initiation
of wanted movements and to maintain them during execution.