Spinal Reflexes

Question 1

Myotatic reflex

Answer 1

stretching any striated muscle will evoke a contraction of that muscle

Question 2

longitudinal structure embedded within muscle, oriented parallel with extrafusal muscle fibers

Answer 2

muscle spindle

Question 3

What do the ends of the muscle spindles attach to?

Answer 3

extrafusal fibers (outside the spindle)

Question 4

The number of spindles is directly related to what?

Answer 4

The amount of control required of the muscle (finger muscle will have more spindles than back muscle per unit)

Question 5

What are the two types of receptors within a muscle spindle–how many of each?

Answer 5

1) nuclear bag, 2

2) nuclear chain fibers, 5

Question 6

What are nuclear bag and nuclear chain fibers collectively called?

Answer 6

Intrafusal fibers (within the spindle)

Question 7

Two important qualities of nuclear bag and nuclear chain fibers

Answer 7

1. both have contractile elements at their distal ends

2. both are oriented along the long axis of the spindle

Question 8

The intrafusal fibers are afferently innervated by these nerve fibers

Answer 8

Ia and II (one each per spindle)

Question 9

These nerve fibers have annulospiral endings which coil around the central region of all nuclear bag and all nuclear chain fibers in a spindle

Answer 9

Ia fibers

Question 10

There is no synapse between Ia fibers and nuclear bag and nuclear chain fibers. How does it work?

Answer 10

Mechanical perturbation caused by stretch opens Na channels in the annulospiral endings (mechanoreceptors)

Question 11

These nerve fibers have flower spray endings that innervate the pericentral regions of only one type of nerve intrafusal. Which one?

Answer 11

Type II fibers, only innervate nuclear chain fibers

Question 12

Why are Ia deemed primary over II?

Answer 12

Greater innervation and conduction velocity

Question 13

The strong, short lasting, phasic response to a brief stretch (tapping patellar ligament), is mediated by which fiber type?

Answer 13

Nuclear bag fibers

Question 14

The weaker, longer lasting tonic response is mediated by what type of fiber?

Answer 14

Nuclear chain fibers

Question 15

This type of fiber is important in correcting abrupt changes in muscle length

Answer 15

Nuclear bag fibers

Question 16

This fiber type is important for maintaining muscle length, such as posture

Answer 16

Nuclear chains

Question 17

How can the stretch reflex be evoked at different muscle lengths?

Answer 17

Primary afferents synapse on alpha motor neurons and on smaller gamma neurons (via interneurons)

Question 18

How do the axons of the gamma motor neurons work?

Answer 18

They innervate the contractile elements on the distal poles of the intrafusal fibers-has the effect of stretching the central region of intrafusal fibers, thus activating Ia and II afferents.

Question 19

Alpha motor neurons innervate what type of muscle

Answer 19

Extrafusal muscle fibers

Question 20

Spindle receptors transmit information about”

Answer 20

The length of the spindle relative to muscle length

Question 21

Knee-jerk reflex demonstrate a myotatic reflex, but what is an example where the reflex is always occurring? Explain.

Answer 21

Standing; gravity stretches quadriceps, activates Ia afferents, activates quads via monosynaptic connection to alpha motor neurons, keeps repeating, providing continuous tone to quads

Question 22

The myotatic reflex is a mechanism for ________, a necessary condition for standing

Answer 22

muscle tone

Question 23

Gamma motor neurons innervate what type of fibers?

Answer 23

Intrafusal

Question 24

Consequence of spinal cord injuries that results in inactivation of gamma motor neurons

Answer 24

intrafusal fibers can no longer contract, resulting in hypotonia
-changes in muscle length can no longer adequately stretch flaccid muscle spindle to activate primary and secondary endings

Question 25

Why might hypertonia occur in spinal cord injuries?

Answer 25

removal of inhibitory influences of gamma motor neurons, muscle is continuously activated. Muscle spindle is no very sensitive to stretch, taping patellar ligament results in exaggerated reflex

Question 26

Clonus

Answer 26

repetitive triggering of myotatic reflex in hypertonic muscles (inertia on way down of leg swinging causes muscle to stretch again, resulting in another myotatic reflex)

Question 27

These fibers carry information about dynamic (phasic) and static (tonic) changes in muscle length

Answer 27

Ia afferent fibers

Question 28

Function of efferents to intrafusal fibers

Answer 28

To adjust the length of the muscle spindles relative to the length of the muscle

Question 29

The number of golgi tendon organs is somewhat (lower/higher) than muscle spindles in a given muscle

Answer 29

lower

Question 30

While muscle spindle detects muscle length, the golgi tendon organ detects

Answer 30

Muscle tension/force

Question 31

The GTO (other major muscle receptor), is embedded in

Answer 31

the musculotendinous sheath

Question 32

During passive stretch, the muscles fibers are (more/less) elastic than the tendinous sheath

Answer 32

More. Thus, muscle spindles are stretched, but there is miminal stretch applied to the GTO

Question 33

When the muscle contracts, it becomes ____ and stretches the _____ thus activing ____

Answer 33

inelastic, tendon, GTO

Question 34

Each golgi tendon organ is innervated by a single __ fiber

Answer 34

Ib

Question 35

Ib have their cell bodies located in _________ and ______

Answer 35

Dorsal root ganglion and in the cord synapse via an interneuron (disynaptic) onto alpha motor neurons

Question 36

Activation of a signle Ib results in inhibition of ______ and excitation of __________

Answer 36

homonymous motor neurons; motor neurons of antagonistic muscles

Question 37

Golgi tendon organ is exquisitely sensitive (low threshold) to what? It has a high threshold for what?

Answer 37

Sensitive to muscle contraction; high threshold to passive stretch

Question 38

True or false: More than one single motor unit is required to activate the golgi tendon organ receptor.

Answer 38

FALSE. Activity of a SINGLE MOTOR UNIT is sufficient.

Question 39

The function of muscle spindle and GTO afferents is not solely for mediating spinal cord reflexes. For example, Ia and Ib afferents participating in reflexes also synapse in _____

Answer 39

Clarke’s column (then form the dorsal spinocerebellar tract)

Question 40

Ia also travel in the dorsal column and synapse in the lower medulla in nuclei related to _____ and _____

Answer 40

gracilis and cuneatus (then they travel with the medial lemniscus and synapse in thalamus–>cortex)