Motor Control: Reflexes

Question 1

What are brainstem/midbrain reflexes?

Answer 1

Vestibular
Righting reflex
Suckle
Yawn
Eye/head movements

Question 2

What are spinal reflexes?

Answer 2

Stretch (myotactic)
Golgi tendon reflex
Crossed extensor

Question 3

What is an importance difference between a reflex movement and voluntary movement?

Answer 3

Reflex movement occurs faster than the fastest voluntary motion

Question 4

Distinguish between reflex activity and volitional movement

Answer 4

Reflex activity:
Precise motions in response to afferent stimuli
Mediated at all levels of CNS
Rapid initiation
Many elicited even during unconsciousness

Volitional activity
Originates in cortical areas associated with judgement, initiative, and motor control
Longer onset latency due to processing
Require conscious awareness

Question 5

Describe myotatic reflex. Appearnce, purpose, and characteristics

Answer 5

Appearance: contraction (shortening) of a stretched muscle
Purpose: protect muscle from tearing due to stretch
Characteristics: initiated by muscle spindle. Monosynaptic (afferent fibers synapse directly onto efferent), segmental reflex

Question 6

Describe muscle spindle

Answer 6

Parallel to extrafusal fibers
Several different types of fibers within muscle spindle
Afferent (sensory) and efferent (muscle
Intrafusal fibers of the muscle spindle have both motor and sensory components

Question 7

Describe the sensory portion of a muscle spindle

Answer 7

Is not contractile
Is the portion sensitive to length
Is actually two sensors with different afferents: nuclear bag fiber and nuclear chain fiber

Question 8

Describe the primary afferent of a muscle spindle

Answer 8

Innervates both nuclear bag and nuclear chain
Large, myelinated Ia fiber
Sensitive to both:
-length of muscle (increases in length cause increases in firing rate of neurons)
-how fast length is changing (faster the change, the greater the firing rate)
At rest, there is some discharging with the neurons

Question 9

How are action potentials generated for primary afferents in muscle spindle?

Answer 9

They are generated when deformation (stretching) opens a member of the epithelial sodium channel (ENaC/amiloride sensitive degenerin family). Colocalization of this channel with a neuronal form of the acid-sensitive sodium channel (ASIC2) has also been found.

Question 10

Describe the secondary afferent for a muscle spindle

Answer 10

Smaller, myelinated group II fiber
Innervates only nuclear chain fiber
Sensitive only to length of muscle

Question 11

Compare and contrast the functions of the primary and secondary afferents associated with the muscle spindle

Answer 11

Primary:
Large Ia fiber (heavily myelinated, fast velocity, low threshold)
Innervate both nuclear bag and nuclear chain fiber
Detects length and change in length

Secondary
Group II fiber (smaller diameter, less myelin, still pretty fast)
Innervates only nuclear chain fiber
Detects only length

Question 12

Describe the motor portion of the muscle spindle

Answer 12

Intrafusal contractile elements
Same as skeletal muscle
Innervated by a gamma motorneuron
Control the length of the sensory portion
By contracting the intrafusal muscles, the sensory portion stretches. This renders the sensory portion more sensitive to a superimposed stretch

Question 13

Compare and contrast the functions of the alpha and gamma motorneurons

Answer 13

Alpha:
Large, heavily myelinated fiber
Innervates (via NMJ) skeletal muscle
Responsible for activating muscle
Activity directly leads to motion

Gamma:
Slightly smaller, slower than alpha. Still fast overall
Innervates contractile component of the muscle spindle via NMJ
Activity causes contraction
Controls sensitivity of muscle spindle
Activity does NOT directly lead to motion

Question 14

What channel/receptor is found on the motor end plate?

Answer 14

Nicotinic cholinergic receptors

Question 15

What ion directly produces Ach release from alpha-motoneurons?

Answer 15

Calcium

Question 16

An axon enters spinal cord via dorsal root. Its cell body is located in dorsal root ganglion. What is its most likely function?

Answer 16

Afferent neuron relaying information about muscle length to brain

Question 17

A patient accidentally ingests a substance that enzymatically cleaves VAMP, a protein that is required for vesicular fusion in alpha motoneurons. What is the effect of this cleavage?

Answer 17

Decreased Ach release from terminal

Question 18

You accidentally touch a hot iron with your right hand. The withdrawal reflex is activated. When do you feel pain: before or after withdrawing hand? What would be the consequence if you had to think about the act of withdrawing your hand?

Answer 18

Most people feel pain concurrently with withdrawing the hand or slightly after the hand is withdrawn.
Fibers conveying the thermal/pain information reaches the spinal cord, while afferent information continues up to thalamus and cortex in order to be processed into sensation of pain.
Since motor output causing the withdrawal is conveyed using large neurons, it beats signal conveying sensation to brain

If you had to think about withdrawing your hand, there would be more damage to the hand

Question 19

What is the difference between alpha and gamma motoneurons? Group Ia and Ib or Group II?

Answer 19

alpha motoneuron is bigger (and has more myelin) than gamma.
Group I>II>III>IV
Ia>Ib
IV not myelinated. III only sparsely

Question 20

Touch left thumb to left shoulder. Agonist muscle is biceps. As muscle is contracting, what happens to length of muscle spindle? What is the reflex effect? What effect will this have on ability to touch shoulder?

Answer 20

As muscle length shortens, muscle spindle also shortens.
By shortening muscle spindle, action potential number traveling up Ia afferent decreases. This will lead to less activation of alpha motoneuron.
With less activation of the alpha motoneuron, biceps are less contracted, slowing or preventing me from touching my shoulder.

Question 21

Describe reciprocal inhibition

Answer 21

Full reflex contains a second efferent limb.
In order for muscle length of stretched muscle to change, the antagonist muscle must also chnage.
Ia afferent also activates as interneuron in spinal cord that inhibits alpha motoneuron innervating antagonist muscle.

Question 22

Describe control of muscle spindle sensitivity.

Answer 22

One function of gamma motoneurons (they also innervate intrafusal contractile elements)
When intrafusal fibers contract, they stretch muscle spindle.
By stretching, nuclear bag and nuclear chain are sensitized to additional stretching.

Question 23

Describe golgi tendon reflex. Appearnce, purpose, characteristics

Answer 23

Appearance: sudden (abrupt) relaxation of a contracted muscle
Purpose: protect muscle from damage due to excessive force
Characteristics: initiated by golgi tendon organ. Polysynaptic, segmental reflex

Question 24

Describe golgi tendon organs

Answer 24

Innervate tendon
Bare nerve ending with lots of branches
Action potentials increase with tension (mech to open sodium channels not known)
Ib fiber to spinal cord

Question 25

Compare and contrast functions of the muscle spindle and golgi tendon reflexes

Answer 25

Myotactic reflex
Initiated by muscle spindle 
Passive stretch of muscle 
Monosynaptic reflex
Contraction of stretched muscle back to normal length

Golgi tendon (reverse myotatic)
Initiated by golgi tendon
Active contraction of muscle 
Polysynaptic reflex
Abrupt relaxation of contracted muscle to prevent damage

Question 26

Describe spinal shock and recovery from spinal shock

Answer 26

Even though neurons producing reflex are below level of transection (and are intact), the reflex fails to occur

Recovery can result from:
Axonal sprouting below level of transection
Expression of receptor phenotypes that are self-activating (5HTC receptor). This causes persistent influx of calcium into motoneuron. If input from the brain is reestablished, this aids in motor recovery. If input cannot be reestablished, muscle spasms result.

Question 27

What causes decerebrate posturing?

Answer 27

Results from loss of all structures rostral to the pons

Question 28

Describe rigidity

Answer 28

Resists motion in all directions
Results from maintained muscle contraction
Continual activation of alpha-motoneurons
Not due to reflexes
Brainstem is activating alpha-motoneurons, but not clear why
Cutting afferents does not affect rigidity

Question 29

Describe spasticity

Answer 29

Resists motion in a given passive direction
Myotatic reflex hyperactive
Continual activation of gamma-motoneurons
Contracts intrafusal muscle
Lengthens nuclear bag/chain fibers
Brainstem

This leads to increased sensitivity to stretch reflex

Question 30

Describe brainstem facilitatory region, brainstem inhibitory region, and spasticity.

Answer 30

Facilitatory region: by activating gamma-motoneurons, makes muscle spindle more sensitive. This region is spontaneously active.

Inhibitory region: inhibits gamma motoneurons, making muscle spindles less sensitive. This region requires activation from cortical regions.

With the loss of the cortex, the brainstem (-) region is not activated, leaving the (+) region to dominate
The end result of the uncontrolled brainstem facilitatory region is stretch reflexes that fight any passive motion -> spasticity

Question 31

Identify occurrence and cause of spasticity or rigidity

Answer 31

Spasticity
Patient resists a passive stretch of their muscles
Contraction doesn’t start until stretch occurs
Hyperactive myotatic reflex due to increased gamma motoneuron firing
Cause: damage to cortex that abolishes activation of brainstem inhibitory region (brain arousal systems)

Rigidity
Contraction of muscles in absence of other stimuli
Alpha-motoneurons continually active
Cause: loss of cortical influence that inhibits a medullary input to alpha motoneurons

Question 32

Describe decorticate posturing

Answer 32

Flexion of upper limb joints
Extension of lower limbs
Dependent on head position

Unlike decerebrate posturing, decorticate posturing is dependent on head position and will change as the head is moved passively

Question 33

Describe unilateral decorticate posturing

Answer 33

Results from strokes in vicinity of internal capsule

Relatively common site for a stroke

Question 34

Identify spinal shock, decorticate, and decerebrate posturing and their lesions

Answer 34

Spinal shock
Transection of spinal cord
All reflexes abolished, even if circuit is intact
Cause: hyperpolarization of spinal neurons due to loss of excitatory input from cortex

Decorticate posture
Internal rotation of legs in extended position, flexion of arms, dependent on head position
May be bilateral or unilateral
Loss of cortical inputs -> most commonly caused by lesion of internal capsule in humans

Decerebrate posture
Contraction of all anti-gravity muscles (arms in extension)
Loss of input from all structures rostral to pons
Usually bilateral and indicative of severe brain injury

Question 35

Which of the following reflexes requires cortical input?
A. Withdrawing hand from hot stove
B. Turning head towards a sound
C. Hopping reaction
D. Turning eyes towards a motion
E. The righting response

Answer 35

C. Hopping reaction

Question 36

What innervates intrafusal muscle fibers (contractile elements?)

Answer 36

Gamma-motoneuron

Question 37

Dr. Karius tries to lift 47 cases of diet Pepsi into her car in one lift. A reflex is elicited, resulting in complete relaxation of her muscles. Activation of what fiber type produced this response? How many synapses in the spinal cord are required for this reflex? What receptor is most likely found on the alpha-motoneuron as part of this reflex?

Answer 37

Ib fiber

2

Glycine

Question 38

What are cortical reflexes?

Answer 38

Placing reaction

Hopping reaction