lecture 16 motor- spinal cord mechanisms Flashcards
who found the motor system
charles sherrington, pioneering english neurophysiologist
discovered the basis of spinal reflexes like myotatic
coined the word synapse
who found the motor system
charles sherrington, pioneering english neurophysiologist
discovered the basis of spinal reflexes like myotatic
coined the word synapse
why is the motor system so complicated
behaviors require the coordinated actions of 700 muscles in a changing and sometimes unpredictable environment
why have insights into motor control of behavior lagged knowledge of sensory systems
easy to know muscle output but not origin of movement in the brain
why have insights into motor control of behavior lagged knowledge of sensory systems
easy to know muscle output but not origin of movement in the brain
skeletal muscles
striated (not cardiac or smooth muscles
mostly innervated by neurons in brain stem or spinal cord
skeletal muscles move bones around joints
flexion or extension or both (for rigidity)
three classes of skeletal muscle, each controlled differently
axial (trunk)- posture
proximal or girdle- carrying weight, not fine movements
distal and face- we can control finely
facial nerve
lots of muscles all innervated by the same nerve
muscles of face expression
each motor axon innervates…
multiple muscle fibers in one muscle
how to motor neurons generate muscle force
henneman’s size principle: motor unit recruitment order from small to large
lower motor neuron
final common pathway for all behavior
alpha motor neurons located in ventral horn of spinal cord
lower motor neuron
final common pathway for all behavior
alpha (pretty large) motor neurons located in ventral horn of spinal cord
lower motor neuron organized in three kinds of maps
1) segmental
2) mediolateral
3) dorsolateral
segmental
relates to rostrocaudal position of the muscle in the body (extra neurons in cervical and lumbar enlargements because of extra muscles in arms and legs)
mediolateral
relates to the proximal (near body axis) to distal (far from body axis): axial/proximal/distal muscles
dorsolateral
flexormuscles (dorsal) to extensor (ventral) muscles
who is activating alpha motor neurons
1) spinal interneurons (excitatory and inhibitory)
2) sensory input from muscle spindles (dorsal root ganglion cells)
3) input from upper motor neurons in the brain
myotatic stretch reflex
prevents falling (1a afferent neuron)
reverse myotatic
prevents tears in muscles by relaxing muscles (1b afferent neuron makes synapse on inhibitory interneuron)
reciprocal inhibition
allows joint movement by inhibiting antagonistic muscle
flexor reflex
avoid noxious stimuli before you have conscious awareness
crossed extensor reflex
stops us from falling over (one leg is stiffened to brace while other leg is pulled up if you like step on a thumbtack), or walking
spinal circuitry for locomotion
reflexive alternation of arms and leg swinging
horse walking, trotting, cantering, galloping
chicks hop but ducks waddle
rhythmic activity
central pattern generators
cats with high cervical spinal cord transection can still walk
cpg for locomotion is in the spinal cord
brain turns on behavior but doesnt dictate its every move
mike the headless chicken
head not attached but circuitry was still intact when head wasnt attached
how are cpgs activated
by tonic descending activity from brainstem
tonic (continuous) stimulation in brainstem or cortex gives rise to rhythmic activity of extensor and flexor motor neurons
Behavioral output
the motor system give rise to behavior. Movement is produced by the coordinated activation of motor neurons, which causes the coordinated contraction of skeletal
muscles
Skeletal muscle (Extrafusal muscle fibers)
type of muscle under the control of the motor
system and produces voluntary movement. Also known as striated muscle since it is made of
multiple long, thin, multi nucleated fibers that have a regular pattern of red and white lines.
Contractions in skeletal muscle fibers causes a shortening in the length of a fiber.
Tendon
fibrous collagen tissue that connects muscle to bone. Contains proprioceptive Golgi
tendon organs that sense changes in muscle tension. The golgi tendon system is a feed back
system that monitors and maintains muscle force
Flexion/Extension
Muscles of the body are often in opposing pairs. A flexor is a muscle
whose contractions bend a limb or other part of the body. An extensors is a muscle whose
contractions straighten a limb or other part of the body
(Lower) Motor neuron
neuron who’s cell body is in the gray matter of the spinal cord and
send its axon out of the spinal cord to innervate individual muscle fibers
Motor unit
The set of muscle fibers from a muscle that is innervated by a single lower motor
neuron. The size of a motor unit is determined by the number of fibers
Motor pool
a cluster of motor neurons that innervate the same muscle
Henneman’s Size Principle
motor neurons that have smaller motor unit sizes, which usually
have smaller axon diameters, fire before neurons with larger motor unit sizes. Enables the
incremental control of the magnitude of an individual
muscle’s contraction in response to varying inputs
received by its motor pool
Ventral horn of the spinal cord
location in the gray matter of the spinal cord that contains the cell bodies of the lower motor neurons. Motor neurons innervating the axial musculature are located medially, whereas those Innervating the distal musculature are located more laterally
Reverse myotatic reflex
revents tears. The lb
(“one B”) afferents from golgi tendon organs contact
inhibitory interneurons that decrease the activity of α
motor neurons innervating the same muscle.
Crossed extension reflex
provides postural support during withdrawal of the affected limb from the painful stimulus. Stimulation of
cutaneous receptors in the foot (by stepping on a tack, in this example) leads to activation of spinal cord local circuits that serve to withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory support
Central Pattern Generators (CPGs)
a circuit that is capable of producing rhythmic output for coordinated contraction of different muscles without sensory feedback
Primary Motor Cortex
provides commands to initiate voluntary movement and control complex movement. Primary motor cortex is topographically organized.
The motor cortex, in coordination with other areas of the brain integrates information from multiple sensory systems and sends depending axons to the motor control regions of the brainstem, to spinal cord interneurons, and to lower motor neurons themselves.
Neurons in motor cortex are tuned to different parameters of movement, including direction, velocity, joint angle etc. There is
some evidence that stimulation of specific sites in motor cortex can lead to coordinated movements across multiple joints
Premotor Cortex
The premotor cortex is a crucial part of the brain, which is believed to have direct control over the movements of voluntary muscles. The PMC is about six times larger than the primary motor cortex. The premotor cortex motor area lies anterior to the primary motor cortex.
The premotor cortex appears to be involved in the selection of appropriate motor plans for voluntary movements, whereas the primary motor cortex is involved in the execution of these voluntary movements. Premotor cortex neurons signal the preparation for movement.
Explain how all observable behavior is the result of the coordinated activity of muscles.
Apply the size principle to connect motor unit size to recruitment priority when activating a
muscle
Analyze reflex diagrams to predict behavioral output
Explain how a central pattern generator can control rhythmic behaviors like walking.
