Exam #1: Spinal Reflexes

Question 1

In what region of the spinal cord are lower motor neurons found?

Answer 1

Ventral horn

Question 2

What are two different types of motor neurons in the ventral horn? What is the main difference between these two neurons?

Answer 2

1) Alpha
2) Gamma

The major difference between these 2x neurons is their size; alpha are large, gamma are smaller.

Question 3

Where do the axons of the two different types of lower motor neurons differ in their end points?

Answer 3

Alpha= extrafusal fibers i.e. normal fibers of skeletal muscle

Gamma= intrafusal fibers i.e. polar contractile fibers of the muscle spindle

Question 4

What are the two regions of an intrafusal fibers?

Answer 4

Contraticle & non-contractile regions

• Middle= non-contractile
• Polar ends= contractile

Question 5

What is a spinal reflex?

Answer 5

Stereotyped response to a specific sensory stimulus that is:

• Involuntary
• Instantaneous
• Nonconscious

*****Generally, reflexes serve a protective function

Question 6

What are the five parts of a spinal reflex?

Answer 6

1) Receptor
2) Afferent limb i.e. pseudounipolar sensory neuron that synapses on spinal cord gray matter
3) Interneuron (+/-)
4) Efferent limb i.e. motor neuron located in the ventral horn of the spinal cord that synapses at the skeletal muscle in the periphery
5) Effector muscle that performs the action

Question 7

What are the different classifications of reflexes dependent on levels involved?

Answer 7

1) Intrasegmental= involve only a single spinal segment
2) Intersegmental= involve multiple spinal segments
3) Suprasegmental= input from the brain that influences the reflex

Question 8

What spinal cord levels are tested with the biceps, triceps, quadriceps, and achilles tendon reflexes?

Answer 8

Biceps= C5,6 
Triceps= C6,7,8 
Quadriceps= L 2,3,4
Achilles= S1,2

Question 9

What is the muscle stretch reflex? Outline the muscle stretch reflex using the patellar tendon reflex as an example. What is the function of this reflex/ what is it compensating for?

Answer 9

Generally, these are the deep tendon reflexes e.g. the quadriceps or patellar tendon reflex

1) Tap patellar tendon
2) Stretch BOTH extrafusal & intrafusal fibers i.e. spindle
3) “Annulospiral” endings of Gamma Ia fibers stretched/ stimualted
4) Stretch caused sensory endings to fire, which synapse with alpha motor neurons in the spinal cord
5) Alpha motor neuron activation causes contraction of the quadriceps.

*Function is protective i.e. prevention of muscle fiber overstretching i.e. compensation for muscle stretch

Question 10

What is the difference between reciprocal & autogenic inhibition? Use the patellar tendon reflex as an example.

Answer 10

BOTH modulate the stretch reflex.

Reciprocal= simultaneous activation of one muscle & inactivation or its antagoinst

• Gamma Ia fiber bifurcates
• One has excitatory projections onto alpha motor neuron of the quadriceps
• One end has excitatory projection onto an INHIBITORY INTERNEURON of the hamstrings

Autogenic= Golgi Tendon Organ activation to cause relaxation of the contracting muscle

• Gamma Ib/ GTO activation has excitatory input on an INHIBITORY INTERNEURON of the quadriceps
• Function to limit muscle over-stretching & protects the tendon

Question 11

What is the flexor withdrawl reflex?

Answer 11

This is a nociceptive reflex; painful/harmful stimuli causes reflexive flexor muscle contraction & extensor muscle inhibition to withdraw the affected limb from the stimulus

Question 12

Outline the Flexor Reflex.

Answer 12

1) Painful stimuli triggers cutaneous or subcutaneous receptors
2) Lightly myelinated (A-delta) or Unmeylinated (C) fibers send afferent to Dorsolateal tract of Lissauer
3) Excitatory input on interneurons of the gray matter
- Excitatory interneurons on flexors
- Inhibitory interneurons on extensors
4) Efferents are alpha motor neurons of the flexors & extensors
- Flexors activated
- Extensors inactivated

*Note that this response is intersegmental i.e. it involves multiple spinal levels

Question 13

What is the crossed extension reflex?

Answer 13

This reflex occurs in conjunction with the flexor reflex; there is simultaneous contraction of the extensor muscles in the opposite limb to maintain balance

Question 14

Describe the mechanism of the crossed extension reflex.

Answer 14

1) Flexor Reflex (withdrawal) triggered
2) Commissural (crossing over) interneurons send excitatory & inhibitory projections to the opposite ventral horn
3) Excitatory interneuron stimulates the alpha motor neuron of the extensors & inhibitory interneuron stimulates the flexors

Question 15

How do skeletal muscles maintain muscle tone?

Answer 15

Muscle tone is generated by stimulation of gamma motor neurons & the stretch reflex, which is referred to as the “gamma loop”

Question 16

Outline the loop that maintains muscle tone.

Answer 16

1) Cerebral cortex & brainstem send descending motor tracts that synapse w/ gamma motor neurons in the ventral horn
2) Gamma motor neurons stimulate intrafusal fibers/ polar bodies in the muscle spindle
3) Non-contractile regions is stretched, stimulating annulospiral endings of Gamma Ia sensory fibers
4) Gamma Ia fibers synapse with alpha motor neurons in the ventral horn that innervate the skeletal muscle (specifically, the extrafusal fibers)

Question 17

What is the name of the loop that maintains muscle tone?

Answer 17

Gamma loop

Question 18

Do gamma motor neurons receive any input from annulospiral neurons?

Answer 18

NO–gamma motor neurons only receive input from the cerebral cortex/ brainstem

Question 19

What CNS structure coordinates muscle tone?

Answer 19

Cerebellum

No cerebellum= no muscle tone

Question 20

What does the Basal Ganglia do?

Answer 20

• Projections to thalamus
• Motor cortex

Basal ganglia inhibits excess muscle tone

Question 21

What does a lesion of the basal ganglia produce?

Answer 21

Excess muscle tone

–> Hypertonia

Question 22

What is alpha gamma coactivation?

Answer 22

This is the type of innervation seen with VOLUNTARY muscle contraction; descending motor tracts converge on alpha & gamma motor neurons and stimulate them simultaneously

• Alpha= extrafusal fiber contraction
• Gamma= intrafsual fiber contraction

Question 23

What is the function of alpha-gamma coactivation?

Answer 23

This is a mechanism by which the changes in gross muscle movement during voluntary contraction can be matched by changes in spindle senstivity, such that muscle spindles remain sensitive regardless of the length

Question 24

Without alpha-gamma coactivation, would muscle spindles shorten during muscle contraction?

Answer 24

No–they would remain in a relaxed state

*****Key point is that alpha-gamma coactivation pre-stretches the muscle spindle during muscle contraction

Question 25

What is the clinical presentation of a lower motor neuron lesion?

Answer 25

• Flaccid paralysis of denervated muscle
• Hypotonia
• Atrophy
• Diminished or absent tendon reflexes