Lecture 12: Neural Control and Reflexes (Hayward)

Question 1

where are motor neurons located?*

Answer 1

ventral horn of spinal cord

Question 2

2 types of motor neurons that have control over muscle system*

Answer 2

1) alpha motor neurons

2) gramma motor neurons

Question 3

contrast alpha motor neurons with gamma motor neurons*

Answer 3

alpha motor neurons are larger and send axons down to muscle fibers and activate muscles in general. Gamma motor neurons are smaller and innervate muscle fibers assoc. with intricate sensory endings in the muscle spindles to regulate the sensitivity of the muscle spindle

Question 4

extrafusal vs. intrafusal muscle fibers*

Answer 4

extrafusal muscle fibers are fast/slow twitch muscle fibers and innervated by alpha motor neurons. Intrafusal muscle fibers are muscle fibers assoc. with the muscle spindles and innervated by gamma motor neurons

Question 5

kinds of orientations/locations of receptors in a muscle*

Answer 5

1) in parallel with muscle (muscle spindles; provide info on muscle stretch)
2) in series with muscle (sensitive to force production)
3) proprioceptors (provide info on where limb is in space)
4) receptors in the skin

Question 6

what are considered upper motor neurons?*

Answer 6

cortex, subcortical nuclei, and cerebellum

Question 7

what are considered lower motor neurons?*

Answer 7

spinal cord (alpha and gamma motor neurons)

Question 8

what could be the cause of lower motor neuron disease?*

Answer 8

dysfunction in alpha/gamma motor neurons, how the motor neurons attach to a muscle, or the muscle fibers themselves

Question 9

Describe a typical reflex mechanism*

Answer 9

1) input from muscle enters spinal cord via afferent nerve
2) synapses on alpha motor neuron either monosynapticaly or polysynaptically (if polysynaptic, must synapse through interneurons before reaching motor neurons, and signal can be stopped. Monosynaptic synapses directly on alpha MN and cannot be stopped)
3) alpha MN sends output to muscle fiber via efferent nerve to cause effect

Question 10

general characteristics of reflex arcs*

Answer 10

• genetically programmed and fully developed at birth
• may be segmental or intersegmental (involves one or many segments of spinal cord)
• monosynaptic or polysynaptic

Question 11

which is more common in the nervous system: monosynaptic or polysynaptic reflexes? Give an example of system that uses monosynaptic reflex*

Answer 11

polysynaptic. Motor system

Question 12

what happens to central region of muscle spindle when outside region contracts? *

Answer 12

stretches

Question 13

What kind of neural control is outer region of muscle spindle under?*

Answer 13

gamma motor neuron system

Question 14

In which layer of muscle do muscle spindles lie?*

Answer 14

perimysium. They are functionally connected to the tendons by the connective tissue of the muscle

Question 15

Describe monosynaptic stretch reflex*

Answer 15

muscle stretched, which causes muscle spindle to send signal to spinal cord and cause excitation of the motor neurons from where spindle was stretched to contract the muscle. Contraction of opposing muscle is inhibited to allow for muscle contraction to be unopposed

Question 16

are intrafusal fibers part of the stretch reflex?*

Answer 16

NO, because the gamma MN system has to be activated during descending command, not via the stretch reflex

Question 17

what does lack of stretch reflex indicate?*

Answer 17

problem in lower motor neuron

Question 18

what does exaggerated stretch reflex indicate?*

Answer 18

problem in descending command (upper motor neuron)

Question 19

what does alpha-gamma MN co-activation allow for?*

Answer 19

muscle length detection during shortening contractions

Question 20

golgi tendon organ*

Answer 20

has free nerve endings intertwined in tendons that sense force production in muscle. During high force production, inhibits alpha MN from whence input came. Is fast conducting and lies in series with the muscle fibers. Discharge increases with increasing force production

Question 21

which is faster conducting: type 1 or 2 afferents?*

Answer 21

type 1

Question 22

golgi tendon organ reflex*

Answer 22

a protective mechanism that inhibits capacity to produce so much muscle force that the bone is damaged. An polysynaptic inhibitory reflex.

Question 23

when do muscle spindles “unload”?*

Answer 23

when they are contracted. Discharge when stretched.

Question 24

which types of muscle receptors modulate length and force production?*

Answer 24

type Ia, II. They are large diameter, myelinated and fast-conducting

Question 25

which types of muscle receptors sense muscle fatigue?*

Answer 25

type III, IV. They are small diameter, unmyelinated and slow-conducting.

Question 26

flexion withdrawal reflex*

Answer 26

A polysynaptic body-crossed extension reflex. Extensor muscle inhibition and flexor muscle excitation.

Question 27

where is problem if one limb is pinched and the opposite limb extends?*

Answer 27

in descending control pathway (upper MN)

Question 28

where is problem if no flexion withdrawal reflex occurs?*

Answer 28

between pre and postsynaptic site (mostl likely in MN)

Question 29

4 conditions leading to muscle contraction*

Answer 29

1) agonist stretch
2) muscle spindle activation
3) antagonist contraction
4) gamma MN system stimulation

Question 30

how does agonist stretch cause muscle contraction?*

Answer 30

reflex response is to cause excitation of surrounding agonists, and inhibition of surrounding antagonists.

Question 31

how does muscle spindle activation cause muscle contraction?*

Answer 31

causes AMN activation, which sends signal to contract muscle

Question 32

how does gamma MN system stimulation cause muscle contraction?*

Answer 32

activates end region of the muscle spindle and stretches the central sensory region

Question 33

pyramidal system*

Answer 33

component of descending control responsible for voluntary actions

Question 34

extra-pyramidal system*

Answer 34

component of descending control responsible for postural system

Question 35

symptoms of upper MN lesions*

Answer 35

inappropriate movements, inability to know limb position, abnormal segmental reflexes

Question 36

symptoms of lower MN lesions*

Answer 36

paralysis, atrophy, loss of segmental reflexes

Question 37

doe extrafusal fibers contract spontaneously?**

Answer 37

NO! They only contract when signaled to contract

Question 38

fx of sensory/afferent n.

Answer 38

carries sensory signals to the CNS, entering via the dorsal roots of the spinal cord

Question 39

fx of a receptor

Answer 39

converts environmental energy into action potential

Question 40

fx of motor/efferent n.

Answer 40

transports the action potentials from the CNS to the target organ. Leaves via the ventral roots of the spinal cord.

Question 41

effector organ

Answer 41

organ which causes the reflex response

Question 42

is the flexion withdrawal reflex intersegmental or segmental?

Answer 42

intersegmental

Question 43

what do type III, IV muscle fibers respond to?

Answer 43

mechanical forces (compression and muscle contraction) and chemical changes within the muscle that are assoc. with prolonged muscle activation (fatigue)

Question 44

what reflex do type III, IV muscle fibers activate?

Answer 44

exercise pressor reflex (a cardiovascular response to exercise). Has an inhibitory effect on alpha MN

Question 45

How do flexion withdrawal and crossed extension reflexes work together?

Answer 45

their combined action provides a quick shift of body support away from a painful stimulus. Both reflexes are POLYSYNAPTIC.

Question 46

what is the “final common pathway” through which the CNS regulates muscle?

Answer 46

the alpha motoneuron. Therefore, clinical signs of lower MN disease can be explained by the loss of the alpha MN or associated motor nerve.