13. Motor Reflexes (Karius)

Question 1

Cortical reflexes include

Answer 1

placing reaction

hopping reaction

Question 2

Spinal reflexes include

Answer 2

stretch

golgi tendon reflex

crossed extensor

Question 3

Brainstem/Midbrain reflexes

Answer 3

vestibular

righting reflex

sucj=kle

yawn

eye/head movements

Question 4

What are the purposes of a reflex?

What are the characteristics of reflexes?

Answer 4

protective

no voluntary motor control needed

FAST

specific and fast

Question 5

What must reflexes have in order to be effective that also makes them look like volitional movement?

how would you tell the difference between volitional movement and reflexes?

Answer 5

PRECISION!

Reflexes are faster

Question 6

How do you tell the difference between reflex and volitional motion besides speed?

Answer 6

Reflexes:

happen at any CNS level, doesn’t need cortical involvement

direct responses to stimuli, protective

fixed circuitry

high specificity

Volitional:

cortical and subcortical involvement

response to need/desire

variable circuitry

high specificty

Question 7

Describe the myotactic reflex:

appearance:

purpose:

characteristics:

Answer 7

contraction of muscle

protect from tearing due to stretch

initiated by muscle spindle, causes monosynaptic, segmental refex

Question 8

Describe the muscle spindle

Answer 8

found with skeletal muscle, embedded in fusiform capsule

parallel to muscle fibers

both afferent and efferent

Question 9

Describe Intrafusal fibers

Answer 9

capsule is fusiform, so fibers are called intrafusal

muscles around it are called the extrafusal fibers

contain both motor and sensory components

Question 10

What are the characteristics for the sensory part of the muscle spindle

Answer 10

not contractile

sensitive to length

contains nuclear bag and nuclear chain fibers

Question 11

Describe the Ia fibers (primary afferent)

Answer 11

innervates nuclear bag and chain

myelinated, large

sensitive to length of muscle and rate of change

Question 12

Describe the characteristics of the secondary afferent

Answer 12

smaller, mylinated

Group II fiber

innervates only nuclear chain

sensitive only to length of muscle

Question 13

Describe the motor portion of the intrafusal fibers

Answer 13

histologically same as skeletal muscle

innervated by gamma motor neuron

controls length of sensory portion (intrafusal fibers)

causes sensory protion to become more sensitive to stretch

Question 14

Controlling the sensitivity of the myotactic reflexes

Answer 14

even though the overall length of the msucle spindle remains the same, contraction of the intrafusal contractile fibers stretches the sensory portion and increases the sensitivity of the Ia and II fibers to stretch

Question 15

What are the main features of alpha motor neurons?

Answer 15

large, very myelinated

innervates skeletal muscles (extrafusal)

activate muscle

lead to motion

Question 16

What are the main features of gamma motor neurons?

Answer 16

smaller, slower, still fast tho

innnervates the intrafusal muscle spindle

causes contraction

controls sensitivty of muscle spindle

does not DIRECTLY lead to motion

Question 17

Describe the pathway of the reflex

Answer 17

• within the spinal cord, the Ia afferent from the muscle spindle synapses onto the alpha motor neuron innervating the stretched muscle
• the motor neuron is excited by the activation and contraction relieves the stretch, returning the Ia discharge rate back to normal
• simultaneously, the antagonist gets stretched
• another Ia afferent sunapses onto the muscle and activates another a-motor neuron innervating the stretched muscle but EAA causes a release of GABA onto that a-motor neuron causing fewer action potentials (relaxation) of the antagonist
• thus there is a decrease in the activity of the a motor nueron of the antagonist allowing it to relax and lengthen

Question 18

Describe the golgi tendon reflex

appearance

purpose

characteristics

Answer 18

sudden relaxation of a contracted muscle

protect due to excessive force

initiated by golgi tendon organ

polysynaptic, segmental reflex

Question 19

What does the golgi tendon organ innervate?

what kind fiber?

Answer 19

the tendon via bare nerve endings with lots of branches

AP increases with tension

Ib fiber to spinal cord

Question 20

path of golgi tendon reflex

Answer 20

Ib afferent from golgi tendon to alpha motor neuron of contracting muscle

fewer AP, less tension in muscle

Question 21

The golgi tendon reflex inhibits the motor neuron by activating the spinal interneuron thus causing

Answer 21

abrupt relacation of the muscle, returning the golgi tendon organ discharge rate back to normal

Question 22

Normal Reflex Arc

Answer 22

muscle stretches

AP increases on the 1a fiber

EAA released and synapse occirs on the a-motor neuron with more EAA release

AMN synapses on NMJ

releases Ach

shortens muscle, does reflex

Question 23

How does a myotactic hyperreflexia arc happen?

Answer 23

cortex can modulate reflex activity

if the cortex is damaged: there is decreased inibition and unoppsed excitation in the brain stem so hyperreflexia and clonus can occur

this input goes to yMN

the antagonist stretches and the ymn are effected and trigger the reflex there, so you have opposing reflexes causing that alternating occilation seen in clonus

Question 24

What is the mechanism by which rigidity occurs (like in decerebrate posturing)?

Answer 24

results from maintained muscle contraction due to continual activation of a-MN

brainstem is activaing a-MN but not clear on why

Occurs in the extensor muscles

Question 25

What is the mechanism by which spatisicty occurs (like in decorticate posturing)?

Answer 25

myotactic reflex is hyperactive due to continual activation of g-MN contracting intrafusal muscles leads to lengthening of nuclear bag/chain fibers by signals from brainstem

Question 26

How does the cortex modulate reflexes?

Answer 26

there is the brainstem faciliatory region that makes teh muscle spindles more sensitive and it becomes spontaneously active (does not rely on input from other parts of the brain) the brainstem inhibitory region inhibits g-MN, making the muscle spindle less sensitive this region requires activation from cortical regions damage to cortex inhibits the inhibitory action thus causing spacitiy from uncontrolled brainstem facillitatory regions

Question 27

identify occurrence and presentation of spasicity

Answer 27

pt resists passive stretch of muscles contraction doesn't start until the stretch occurs hyperactive myotactic reflex due to increased gamma motor neuron firing cause: damage to cortex that abolishes activation of the brainstem inhibitory region (brain arousal systems)

Question 28

identify the pt presentation and occurence of rigidity

Answer 28

contraction of muscle in absence of other stimuli a-motor neurons continually active cause: loss of cortical influence that inhibits a medullary input to the alpha motor neurons

Question 29

Describe decorticate posturing

Answer 29

flexion of upper limbs due to disinhibition of red nucleus and its control of UE flexors extensino of lower limbs due to disinhibition of reticulospinal and vestibulospinal pathways Dependent on head position (releases postural reflexes) likely from stroke in near internal capsule, common site of stroke bilateral or unilateral loss of cortical inputs

Question 30

Describe decerebrate posture

Answer 30

contarction of all antigravity muscles (arms in extension) loss of input from all structures rostral to the pons (caudal to red nucleus) usually indicative of severe brain injury

Question 31

Describe spinal shock

Answer 31

caused by transection of spinal cord all reflexes gon, even if circuit is intact caused by hyperpolarization of spinal neurions due to loss of excitatory input from cortex

Question 32

Remember that Ia fbers are faster than Ib fibers because

Answer 32

1a fibers are larger and more myelinated