3. Spinal Cord: Motor Unit And Reflexes 2

Front 1

What is a Reflex?

Back 1

An involuntary stereotyped motor response to a sensory input.

Front 2

2 Functions of Reflexes:

Back 2

1. Fast-acting safety reactions to avoid hazardous situations
2. Automatically adapt motor patterns to achieve a behavioral goal

Front 3

Are spinal reflexes entirely limited to only involve the spinal cord and peripheral nerves?

Back 3

No; descending pathways from the brainstem and motor cortex modulate spinal reflexes.

Front 4

Areflexia

Back 4

Complete loss of a reflex

Front 5

Hyporeflexia

Back 5

Reduced strength of a reflex

Front 6

Hyperreflexia

Back 6

Overactive reflex

Front 7

What are the 2 general components of information in a reflex?

Back 7

1. Sensation of muscle status
2. Effect caused on muscle

Front 8

What 2 muscle receptor types sense muscle status?

Back 8

-Muscle spindles
-Golgi tendon organs

Front 9

What do the muscle receptors monitor ABOUT the muscle?

Back 9

-Length
-Tension

Front 10

Where are the cell bodies of the muscle sensory neurons located?

Back 10

In the DRG

Front 11

What are the efferent neurons for spinal reflexes?

Back 11

Lower motor neurons in the spinal cord ventral horn.

Front 12

What MODIFIES information between the sensing and effect?

Back 12

-Interneurons locally in SC
-Descending neurons from cortex and brainstem.

Front 13

What can interneurons and descending neurons be?

Back 13

Either EXCITATORY or INHIBITORY

Front 14

2 Types of muscle efferents:

Back 14

1. Alpha motor neurons
2. Gamma motor neurons

Front 15

What do alpha motor neurons innervate?

Back 15

Extrafusal - work muscle fibers

Front 16

What do gamma motor neurons regulate??

Back 16

Sensitivity of muscle spindle intrafusal fibers

Front 17

What about the muscle do Muscle Spindle receptors sense?

Back 17

Length

Front 18

What about the muscle do Golgi tendon organ receptors sense?

Back 18

Tension

Front 19

What does length of muscle tell?

Back 19

The angle of a joint

Front 20

What 2 structures do the muscle Spindle receptors work with to sense position of limbs wrt body?

Back 20

-Joint afferents
-Cutaneous afferents

Front 21

When do Spindle receptors discharge best?

Back 21

When muscle is stretched.

Front 22

What happens to Spindle receptors when the muscle shortens?

Back 22

They fall silent.

Front 23

What are muscle spindle receptors composed of?

Back 23

Intrafusal fibers

Front 24

Where are intrafusal fibers located within muscle?

Back 24

Parallel to extrafusal fibers.

Front 25

2 General types of intrafusal fibers:

Back 25

1. Dynamic nuclear bag fibers
2. Static nuclear bag fibers

Front 26

What is the response of Dynamic nuclear bag fibers to lengthening muscle?

Back 26

It adapts over time

Front 27

What is the response of static nuclear bag fibers to lengthening muscle?

Back 27

Steady over time - so it senses at all muscle lengths.

Front 28

How are muscle spindle afferents associated with the intrafusal fibers?

Back 28

They wrap and spiral around them.

Front 29

What type of afferents innervate the dynamic nuclear and static nuclear muscle spindles?

Back 29

Ia afferents - innervate both dynamic and static nuclear bag fibers.

Front 30

What do Ia afferents sense?

Back 30

Info about:
-Muscle length
-Rate of change of length

Front 31

What does each afferent convey?
-Ia
-II

Back 31

-Ia conveys fast phasic dynamic responses of muscle fibers
-II conveys slow tonic static responses

Front 32

So when are Type Ia afferents responsive?

Back 32

During small changes of muscle

Front 33

When are Type II afferents responsive?

Back 33

When the muscle length is static

Front 34

What happens to the spindle afferents when intrafusal fibers are stretched?

Back 34

Sensory endings are stretched and increase their firing rate.

Front 35

What happens to spindle afferents when intrafusal fibers are unloaded?

Back 35

Sensory endings stop firing.

Front 36

What are the Muscle Spindle Efferents and what do they innervate?

Back 36

Gamma motor neurons that innervate both static and dynamic nuclear bag fibers.

Front 37

2 things that happen when gamma motor neurons fire:

Back 37

-Intrafusal fibers shorten
-Central region of intrafusal fibers stretch

Front 38

What is achieved by gamma efferents firing?

Back 38

It increases the firing of afferent fibers from spindles.

Front 39

What is the role of gamma motor efferents for spinal reflexes?

Back 39

To adjust the dynamic and static sensitivity of the muscle spindle and their afferents.

Front 40

How is it that by shortening the ends of intrafusal fibers it causes their central regions to stretch?

Back 40

Their central regions are not contractile.

Front 41

Why do we need dynamic and static intrafusal fibers in muscle spindles?

Back 41

To sense two things:
1. When muscle length changes
2. What the muscle is doing when not changing - new stable length

Front 42

What process occurs in muscle efferents during voluntary movements?

Back 42

Alpha-gamma co-activation

Front 43

Why are both alpha and gamma efferents coactivated?

Back 43

To keep the spindles sensitive as the contracting muscle shortens, preventing them from falling silent.

Front 44

Response of a Ia afferent fiber to sustained tension:

Back 44

Steady firing

Front 45

Response of a Ia afferent fiber to stimulation of only an alpha motor neuron:

Back 45

Ongoing discharge pauses because muscle shortens as it contracts.

Front 46

Response of a Ia afferent fiber to stimulation of an alpha AND a gamma motor neuron:

Back 46

Spindle won't be unloaded by contraction so it keeps on firing.

Front 47

What do Golgi tendon organ muscle receptors detect?

Back 47

Tension of the muscle

Front 48

Where are Golgi tendon organ receptors located?

Back 48

At junction between muscle fibers and tendon.

Front 49

Are Golgi tendon organ receptors free?

Back 49

No, they're encapsulated.

Front 50

What innervates a Golgi tendon organ receptor?

Back 50

A single Ib afferent

Front 51

What stimulates the Golgi tendon organ receptor?

Back 51

Stretch

Front 52

How does stretch stimulate the Golgi tendon organ receptor

Back 52

By straightening the collagen fibers and compressing the nerve fibers underlying them.

Front 53

How are Golgi tendon organ receptors associated with the rest of muscle?

Back 53

In series with 15-20 extrafusal fibers.

Front 54

When do Golgi tendon organ receptors discharge BEST?

Back 54

When muscle contracts

Front 55

What do Golgi tendon organ receptors measure?

Back 55

Force in contracting muscle

Front 56

Why do we need Golgi tendon organ receptors?

Back 56

To protect against too much muscle tension and damage.

Front 57

Other than protecting against muscle damage what do Golgi tendon organ receptors do?

Back 57

Signal very minute changes in muscle tension to precisely inform CNS about the state of a muscle contraction.