Ch 55_Motor Functions of the Spinal Cord; the Cord Reflexes

Flashcards about motor functions of the spinal cord and cord reflexes.

Breakdown the importance of each in regards to the nervous system

1. Spinal Cord

2. Brain stem

3. Cerebrum

1. Relatively simple reflexes.

2. more complicated reflexes

3. most complicated reflexes

What is the Gray Matter?

The area where cord reflexes are integrated.

• sensory signals enter the cord through the posterior root (afferent) and will terminate in the gray matter cord OR be transmitted to higher levels of nervous system

Which two neurons are found within the grey matter?

anterior motor neurons and interneurons

Describe Anterior Motor Neurons

• Located within the anterior (ventral) horns of the cord gray matter & innervate skeletal muscles.

• consist of alpha motor neurons (large type) and gamma motor neurons (innervate the intrafusal fibers (muscle spindle and fibers)

α-motor neurons

Large type Aα (motor units).

γ- motor neurons

½ as many as α-motor neurons; Smaller type γ fibers innervate the intrafusal fibers.

Describe interneurons

• located in the dorsal and ventral horns of the gray matter

• small and highly exciteable

• consist of renshaw cells

Which neuron of the grey matter is responsible for most of the integrative functions of the spinal cord?

Interneurons

What are Renshaw cells

Inhibitory neurons located in the anterior horns of the spinal cord close to motor neurons

inhibits adjacent motor neurons (lateral inhibition - for fine precision and controlled movements) to focus or sharpen its signal.

1. What are Sensory Receptors?

2. What are the purpose of them?

3. Name the two types

1. Provide continuous feedback of sensory information indicating functional status of each muscle.

2. intrinsic muscle control

3. Muscle spindles and Golgi Tendon Organs (GTO)

How is information sent to the brain from the spinal cord using sensory receptors?

Spinal cord, cerebellum, and cerebral cortex

What are Muscle Spindles?

Sensory receptor providing information regarding muscle length or rate of change of length.

Which sensory receptor has no actin and myosin but the ends are activated by gamma motor nerves called gamma efferents

muscle spindles

• receptor is stimulated by stretch

Muscle spindle (sensory receptor) have sensory fibers that encircle the midportion of the receptor. What are the two types?

BOTH ARE SENSORY

• Primary ending - large type Ia fiber (fast)

• Secondary ending - smaller type II innervates on the side of the primary ending

Within the muscle fiber are two types of intrafusal muscle fibers. Define both

1. nuclear bag fibers - innervated by primary ending

2. Nuclear chain fibers - innervated by primary ending and secondary ending

What responses are intrafusal fibers responsible for?

Response is based on the type of stretch

• static response - when spindle is stretched slowly (both primary and secondary endings respond)

  • we are always at static response because info is always being relayed and incoming because of muscle tone

• dynamic response - when length increases suddenly (primary ending only responds)

  • once stretch is over, static response will occur

Muscle Stretch reflex

Stretch of the muscle causes excitation of the spindle causing reflex contraction of skeletal muscle. What two pathways can the stretch reflex occur in?

What are the two reflex components

1. Monosynaptic and Polysynaptic (more synapse = relay takes longer)

2. The reflex has two separate components

   1. Dynamic stretch reflex and static stretch reflex

Which stretch reflex is continued for prolonged periods of length change and responsible for maintaining contraction and tone?

Static stretch reflex

What are Golgi Tendon Organs (GTO)?

Sensory receptors that detect changes in muscle tension located at the end of muscles

• use dynamic and static response

Once change in muscle tension is detected, impulses will be sent to the dorsal horn of the spinal cord AND cerebellum a cerebral cortex with large type Ib fibers (afferent)

GTOs are inhibitory - when activated they inhibit the anterior motor neurons from firing

What does it mean that GTO are inhibitory?

1. when activated they inhibit the anterior motor neurons from firing

2. They participate in a negative feedback loop

   1. in an event of extreme tension is placed on a muscle, the lengthening reaction (instantaneous relaxation of muscle) may occur as a protective mechanism (when you left heavy and can no longer hold the weight, signal is sent to the brain to let it go”

What are the primary and secondary functions of GTO

1. Negative feedback loop for lengthening reaction

2. Equalize contractile forces of separate muscle fibers = fibers with excessive tension are relaxed and fibers with too little tension become more excited

Nuclear bag fibers

Nuclei are congregated; contains primary ending.

Nuclear chain fibers

Nuclei arranged in chain fashion; contains primary and secondary endings.

Static response

When spindle is stretched slowly, both endings respond.

Dynamic response

When length increases suddenly, primary ending only responds.

Muscle Stretch Reflex

Simplest manifestation of muscle spindle function.

What is Damping?

Important function of stretch reflex - Smoothing of jerky body movements.

• muscle spindles act to dampen the signals for smooth contraction

1. What is Coactivation?

2. What is the purpose?

1. Allows for contraction of intrafusal (alpha motor neurons) and extrafusal fibers (gamma motor neurons) of the muscle spindle simultaneously.

2. Keeps length of receptor portion from changing during contraction AND maintains proper dampening function

Gamma motor system

1. Where are impulses from?

2. Why are they necessary?

1. bulboreticular facility of brain stem

2. Necessary for stabilization of body position during motor actions (gross and intricate) where muslces on both sides of the joint are activated

what is the Patellar tendon reflex

Stretches quad muscle and excites a dynamic stretch reflex; used to asses the degree of facilitation of spinal cord centers.

What is Clonus?

Oscillation of a muscle.

• toe raises are used to assess the presence of upper motor neuron lesions, where the muscle involuntarily contracts and relaxes repeatedly.

What is the Flexor reflex?

Cutaneous sensory stimulus from a limb causes (flexor) muscle contraction, withdrawing the limb from the object.

aka nociceptive reflex or pain reflex

What are the 3 types of circuits in neuronal mechanism of flexor reflex?

Diverging-Spread reflex to necessary muscles for withdrawal.

Reciprocal inhibition - Inhibit antagonists.

Afterdischarge - After stimulus is over; repetitive firing of excited interneurons (immediate); recurrent pathways that initiate oscillation in reverberating interneurons (prolonged).

T/F: both flexor reflex and crossed extensor reflex use polysynaptic pathways

true

Crossed Extensor Reflex

Extension occurs in the limb opposite of the flexor reflex very quickly after flexor reflex.

• has a longer afterdischarge than flexor reflex

What is reciprocal inhibition?

when a stretch reflex excites one muscle or group of muscles while simultaneously inhibiting the antagonist muscle or group