lecture 16 motor- spinal cord mechanisms

Question 1

who found the motor system

Answer 1

charles sherrington, pioneering english neurophysiologist

discovered the basis of spinal reflexes like myotatic

coined the word synapse

Question 1

who found the motor system

Answer 1

charles sherrington, pioneering english neurophysiologist

discovered the basis of spinal reflexes like myotatic

coined the word synapse

Question 2

why is the motor system so complicated

Answer 2

behaviors require the coordinated actions of 700 muscles in a changing and sometimes unpredictable environment

Question 3

why have insights into motor control of behavior lagged knowledge of sensory systems

Answer 3

easy to know muscle output but not origin of movement in the brain

Question 3

why have insights into motor control of behavior lagged knowledge of sensory systems

Answer 3

easy to know muscle output but not origin of movement in the brain

Question 4

skeletal muscles

Answer 4

striated (not cardiac or smooth muscles

mostly innervated by neurons in brain stem or spinal cord

Question 5

skeletal muscles move bones around joints

Answer 5

flexion or extension or both (for rigidity)

Question 6

three classes of skeletal muscle, each controlled differently

Answer 6

axial (trunk)- posture
proximal or girdle- carrying weight, not fine movements
distal and face- we can control finely

Question 7

facial nerve

Answer 7

lots of muscles all innervated by the same nerve

muscles of face expression

Question 8

each motor axon innervates…

Answer 8

multiple muscle fibers in one muscle

Question 9

how to motor neurons generate muscle force

Answer 9

henneman’s size principle: motor unit recruitment order from small to large

Question 10

lower motor neuron

Answer 10

final common pathway for all behavior

alpha motor neurons located in ventral horn of spinal cord

Question 11

lower motor neuron

Answer 11

final common pathway for all behavior

alpha (pretty large) motor neurons located in ventral horn of spinal cord

Question 12

lower motor neuron organized in three kinds of maps

Answer 12

1) segmental

2) mediolateral

3) dorsolateral

Question 13

segmental

Answer 13

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)

Question 14

mediolateral

Answer 14

relates to the proximal (near body axis) to distal (far from body axis): axial/proximal/distal muscles

Question 15

dorsolateral

Answer 15

flexormuscles (dorsal) to extensor (ventral) muscles

Question 16

who is activating alpha motor neurons

Answer 16

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

Question 17

myotatic stretch reflex

Answer 17

prevents falling (1a afferent neuron)

Question 18

reverse myotatic

Answer 18

prevents tears in muscles by relaxing muscles (1b afferent neuron makes synapse on inhibitory interneuron)

Question 19

reciprocal inhibition

Answer 19

allows joint movement by inhibiting antagonistic muscle

Question 20

flexor reflex

Answer 20

avoid noxious stimuli before you have conscious awareness

Question 21

crossed extensor reflex

Answer 21

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

Question 22

spinal circuitry for locomotion

Answer 22

reflexive alternation of arms and leg swinging

horse walking, trotting, cantering, galloping

chicks hop but ducks waddle

rhythmic activity

Question 23

central pattern generators

Answer 23

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

Question 24

mike the headless chicken

Answer 24

head not attached but circuitry was still intact when head wasnt attached

Question 25

how are cpgs activated

Answer 25

by tonic descending activity from brainstem

tonic (continuous) stimulation in brainstem or cortex gives rise to rhythmic activity of extensor and flexor motor neurons

Question 26

Behavioral output

Answer 26

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

Question 27

Skeletal muscle (Extrafusal muscle fibers)

Answer 27

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.

Question 28

Tendon

Answer 28

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

Question 29

Flexion/Extension

Answer 29

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

Question 30

(Lower) Motor neuron

Answer 30

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

Question 31

Motor unit

Answer 31

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

Question 32

Motor pool

Answer 32

a cluster of motor neurons that innervate the same muscle

Question 33

Henneman’s Size Principle

Answer 33

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

Question 34

Ventral horn of the spinal cord

Answer 34

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

Question 35

Reverse myotatic reflex

Answer 35

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.

Question 36

Crossed extension reflex

Answer 36

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

Question 37

Central Pattern Generators (CPGs)

Answer 37

a circuit that is capable of producing rhythmic output for coordinated contraction of different muscles without sensory feedback

Question 38

Primary Motor Cortex

Answer 38

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

Question 38

Premotor Cortex

Answer 38

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.

Question 39

Explain how all observable behavior is the result of the coordinated activity of muscles.

Question 40

Apply the size principle to connect motor unit size to recruitment priority when activating a
muscle

Question 41

Analyze reflex diagrams to predict behavioral output

Question 42

Explain how a central pattern generator can control rhythmic behaviors like walking.