Muscle Innervation And Spinal Reflexes

Question 1

What is the physiological roles of the muscle spindle?

Answer 1

• Participates in stretch (myotatic) reflexes
• Provides proprioceptive information to the CNS
• Regulates muscle contraction under the influence of descending motor pathways and afferent inputs

Question 2

What are the physiological roles of the myotactiv reflexes?

Answer 2

A tap of the patellar tendon stretches quadriceps muscle briefly

The intrafusal fibers in the muscle spindle are deformed briefly, and initiate action potentials

The Ia afferent fibers conducts this excitatory impulse to the alpha motor neurons of the quadriceps causing contraction

Reciprocal inhibition via an interneuron promotes relaxation of the antagonist muscle

Question 3

How do muscle spindles affect proprioception?

Answer 3

Muscle spindles and Golgi tendon organs communicate with spinal dorsal horn and cerebellum

Question 4

What is the golgi tendon organ?

Answer 4

Located between tendon and muscle fibers (in series with muscle fibers)

• High-threshold receptor that detects contraction-dependent muscular tension

Question 5

How does the golgi tendon function?

Answer 5

Ib afferents from the organ react to tension in a muscle (related to contraction of extrafusal fibers)

1. High tension in the muscle vigorously activates Ib afferents
2. Ib afferents activate interneurons that inhibit alpha motor neurons communicating with overactive extrafusal fibers
3. Reduced activity in alpha motor neurons relaxes over active extrafusal fibers, thus reducing chances of injury and/or redistributing load to less strained extrafusal fibers
4. At lower muscular tensions, the apparatus exerts excitatory influences that aid in rhythmic muscular contractio

Question 6

What is the inverse myotactic reflex?

Answer 6

• The Ib fiber makes an excitatory synapse with an interneuron that inhibits the alpha motor neuron
• The net effect is that the period of contraction of the muscle in response to a stretch is reduced

Question 6

What is the inverse myotactic reflex?

Answer 6

• The Ib fiber makes an excitatory synapse with an interneuron that inhibits the alpha motor neuron
• The net effect is that the period of contraction of the muscle in response to a stretch is reduced

Question 7

How does the golgi tendon organ limit stretch reflex?

Answer 7

• When the muscle tension gets high, the Golgi tendon organ reflex limits the contraction of the muscle
• Synonym: Inverse myotatic reflex

• Afferent limb: Golgi tendon organ and Ib
afferents

• CNS unit: Inhibitory interneuron

• Efferent limb: Alpha motor neuron and
homonymous muscle

Question 8

What is the flexion reflex?

Answer 8

• Noxious stimuli trigger ipsilateral flexor (withdrawal) responses
• Signals diverge within the cord to recruit muscles into response
• Ipsilateral extensor responses are suppressed

Question 9

What is the flexion crossed extension reflex?

Answer 9

• Afferent signals of the flexion reflex cross the cord to promote contralateral extensor responses while suppressing contralateral flexion

Question 10

What is the motor control system?

Answer 10

Coordination between contraction and relaxation of muscle groups is controlled via:

1. Higher-order executive systems
2. Reflex mechanisms

Question 11

What are the reasons for hyporeflexia and areflexia?

Answer 11

Hyporeflexia and areflexia (diminished or absent
reflexes) can have several causes:

• Lesion of the afferent limb (sensory loss)
• Peripheral nerve lesion
• Lesion of a spinal cord segment
• Lesion of the efferent limb (lower motor neuron lesion)
• Diseases of the NMJ
• Lesion of the muscle

Question 12

What are the features of lower motor neurkn syndrome?

Answer 12

Lower Motor Neuron Syndrome:
– Arises with damage to cells derived from the basal plate that communicate with somatic muscle cells

• Features:
– Paresis/paralysis
– Hyporeflexia
– Hypotonia
– Fasciculations
– Fibrillations
– Muscular wasting/atrophy

Question 13

What is hypotonia?

Answer 13

Hypotonia
• Pathologically reduced level of sustained partial contraction of extrafusal fibers
• Assessed through palpation
• Absence of resistance to passive movement
• May reflect damage to alpha motor neurons or Ia afferents
– Mild hypotonia can arise from other lesions (e.g., cerebellar), but other accompanying signs would
permit differential diagnostics

Question 14

What is spinal shock?

Answer 14

• Acute transection of the spinal cord leads to a period of spinal shock
• During this period, which starts immediately after the sectioning, spinal (including autonomic) reflexes mediated caudal to the lesion are suppressed
• It can take several weeks until spinal reflexes return via a poorly understood process

Question 15

What is a motor unit?

Answer 15

Up to 100 motor neurons per muscle

• Each muscle fiber receives only the ending of one branch of a motor axon
– Motor unit = Alpha motor neuron and innervated extrafusal fibers

• Fine movements (e.g., extraocular muscles) require small motor units (e.g., about 10 extrafusal fibers)

• Coarsely controlled muscles utilize large motor units (e.g., about 1000 extrafusal muscle fibers per alpha
motor neuron

Question 15

What is a motor unit?

Answer 15

Up to 100 motor neurons per muscle

• Each muscle fiber receives only the ending of one branch of a motor axon
– Motor unit = Alpha motor neuron and innervated extrafusal fibers

• Fine movements (e.g., extraocular muscles) require small motor units (e.g., about 10 extrafusal fibers)

• Coarsely controlled muscles utilize large motor units (e.g., about 1000 extrafusal muscle fibers per alpha
motor neuron

Question 16

What are the types of motor units?

Answer 16

Type I: Slow-twitch fibers; low tension; fatigue-resistant; aerobic; innervated by relatively small motor neuron and axon

• Type IIA: Fast; relatively fatigue-resistant; large tension; some aerobic capacity; innervated by relatively large motor neuron and axon
• Type IIB: Fast; fatigue-susceptible; large tension; anaerobic; innervated by relatively large motor neuron and axon

Question 17

What are the factors affecting contraction?

Answer 17

The are two crucial factors:
1. The firing rates of the motor neurons

2. The number of motor units recruited

Question 18

How to examine neurological examination for LMN or UMN lesion?

Answer 18

– Knee jerk and ankle jerk bilateral very brisk

– Bilateral extensor plantar response

– Muscle tone in legs bilaterally elevate

Question 19

Describe HTLV1 Associated Myelopathy

Answer 19

Tropical Spastic Paraparesis (TSP)

• A chronic degenerative neurological disease that causes the demyelination of the spinal cord.
– The disease involves hyperreflexia, spasticity and weakness of the lower limbs, and gait abnormality

Question 20

What is the etiology of HTLV1?

Answer 20

• Also known as HTLV-1–associated myelopathy (HAM). The virus can be spread through the placenta, blood transfusions, breastfeeding, contaminated needles, and sexual contact

Question 21

What is the epidemiology of HTLV1?

Answer 21

HAM/TSP usually occurs in tropical locales. It is most frequently found in Caribbean, Japan, the Seychelles Islands, regions of South America,
western Africa.

Question 22

What is the treatment of HAM/TSP?

Answer 22

Treatment

• Incurable; significant improvement has been seen with corticosteroids.

Differential Diagnoses:
• HIV- and MS-related myelopathy. Clinicians should be sensitive to coinfection with HIV.