2.3

Question 1

Spinal reflexes

Answer 1

Involuntary, instantaneous, and usually protective

Question 2

Steps for spinal reflexes

Answer 2

1. Stimulus activates peripheral sensory receptor associated with pseudounipolar neuron.
2. Sensory neuron projects to spinal cord where it synapses with interneuron or motor neuron.
3. Motor neuron joins peripheral nerve that carries it to skeletal muscle
4. The motoneuron stimulates the muscle and causes it to contract.

Question 3

Categories of spinal reflexes

Answer 3

1. Intrasegmental - same level
2. Intersegmental - 2 of more levels
3. Suprasegmental - to brainstem

Question 4

Intrasegmental reflex

Answer 4

Sensory branches allow motor response from same level

Question 5

Suprasegmental reflex

Answer 5

Influenced by neurons of the cerebral hemispheres or brainstem (thalamus or sensory cortex)
Example: postural reflexes

Question 6

Components of spinal reflexes

Answer 6

Sensory receptors
Sensory nerves
Motor nerves
Skeletal muscle

Question 7

Extrafusal muscle fibers are innervated by

Answer 7

alpha motor neurons

Question 8

Intrafusal muscle fibers are innervated by

Answer 8

gamma motor neurons

Question 9

Alpha motor neurons

Answer 9

The largest neurons innervate typical skeletal muscle fibers, referred to as extrafusal fibers
When stimulated, cause contraction of the gross muscle.

Question 10

Gamma motor neurons

Answer 10

Gamma motoneurons are smaller neurons and innervate the contractile portions of the intrafusal fibers of muscle spindles.
The main function of the gamma motoneurons is to increase the sensitivity of the intrafusal fibers to changes in muscle length and indirectly cause the reflex stimulation of the alpha motoneurons that stimulate contraction of the gross muscle

Question 11

Muscle spindle function

Answer 11

Change in tension
Gamma motor neurons provide proprioceptive feedback
- length (degree of stretch)
- velocity (speed of stretch)
Muscle spindles are sensors of deep tendon reflexes.
Unique to skeletal muscle that continually monitor changes in muscle length.
They have a key function during movement and in the maintenance of muscle tone
Muscle spindles are oriented parallel to the longitudinal axes of the extrafusal muscle fibers

Question 12

Skeletal muscle receptors in muscle spindle

Answer 12

within the muscle

Question 13

Skeletal muscle receptors in the golgi tendon organ

Answer 13

within muscle tendon to detect tension

Question 14

Golgi tendon organ function

Answer 14

Detect tension
Muscle shortens –> compresses GTO receptors –> signal travels to CNS
When the muscle contracts, it produces movement by puling a bone. The pull of the contracting muscle stretches the tendon, triggering GTO response.
It has no contractile element of motor neuron.
They receive sensory innervation.
Arranged parallel to the skeletal muscle fibers and contain intrafusal fibers

Question 15

Two types of intrafusal fibers

Answer 15

There are two morphologically and functionally distinct types of intrafusal fibers: Nuclear bag fibers and nuclear chain fibers.
- Skeletal muscle fibers (extrafusal fibers) contain multiple nuclei. Nuclear bag fibers and nuclear chain fibers are also multinucleated cells, and the organization of their nuclei is their most distinctive characteristic.

Question 16

Specific spinal reflexes

Answer 16

Muscle stretch reflex (simplest)
Reciprocal inhibition
Autogenic inhibition
Flexor reflex
Crossed extension reflex

Question 17

Muscle stretch reflex = myotatic

Answer 17

Deep tendon reflex
Monosynaptic
Enhanced by reciprocal inhibition

Question 18

Reciprocal inhibition

Answer 18

Primary mover contracts, antagonist muscle relaxes
Involved inhibitory interneuron
One muscle (e.g., the extensor) is stimulated to undergo contraction, while another muscle (e.g., the flexor, it’s antagonist) is simultaneously inhibited from undergoing contraction and, therefore, relaxes.

Question 19

Autogenic inhibition

Answer 19

GTO fine tunes movement and inhibits further contraction
Sends inhibitory signal when too much tension is being placed on one muscle.
GTOs are specialized receptors that monitor the tension applied to tendons generated mainly by muscle contraction.
Muscle contraction causes the sensory endings terminating on tendons (the GTOs) to discharge.

Question 20

Flexor reflex

Answer 20

Withdrawal reflex
Response of an entire limb to noxious stimulus resulting in flexion/ withdrawal
The flexor reflex is elicited by stimulation of cutaneous receptors and includes a reflex response across several joints involving an entire limb.

Question 21

Crossed extension reflex

Answer 21

Maintains balance/posture
The crossed extension reflex occurs to maintain balance and upright posture following contact with a noxious stimulus.
The “crossed extension reflex” occurs in the contralateral limb that does not encounter the noxious stimulus.

Question 22

Radiculopathy

Answer 22

compression of the nerve at the level of the nerve root

Question 23

A motor unit that innervates only three muscle fibers is likely to innervate muscle in which of the following?

Answer 23

Thumb
Motor units are smallest in the parts of the body that have the most precise motor control, such as thumb, fingers, and tongue.

Question 24

Which of the following effects is caused by activation of gamma motor neurons during active contraction of extrafusal muscle fibers?

Answer 24

Maintained sensitivity of the Ia afferents during unexpected stretch
Coactivation of gamma motor neurons with alpha motor neurons shortens the intrafusal muscle fibers during contraction of extrafusal fibers so that sensitivity of the Ia stretch receptors in the intrafusal fibers is maintained when unexpected stretch occurs during the extrafusal muscle contraction.

Question 25

Which of the following observations would suggest that an upper motor neuron lesion rather than a lower motor neuron lesion is present?

Answer 25

Spasticity
Upper motor neuron lesions produce spasticity, involving hyperactive stretch reflexes. Lower motor neurons don’t.