Spinal Reflexes

Question 1

What are the types of fibers found in the skeletal muscles?

Answer 1

1) Extrafusal muscle fibers (Attached to the tendons and thus generate movement)

2) Intrafusal fibers (encircled by connective tissue capsule, they are not connected to tendons but rather act as proprioceptors “for the muscles, joints, ligaments)

Question 2

Which muscle fiber is attached to the tendon?

Answer 2

The extrafusal muscle fibers

Question 3

What is a muscle spindle?

Answer 3

A bunch of intrafusal muscle fibers

Question 4

What is the function of the extrafusal fibers?

Answer 4

They connect to tendons and thus they generate movement

Question 5

What is the function of the intrafusal fibers?

Answer 5

They are proprioceptive, carrying out information about the length and the velocity at which the muscle is stretching (degree and speed of muscle stretch)

Question 6

What are the types of intrafusal muscle fibers?

Answer 6

1) Nuclear bag fibers

2) Nuclear chain fibers

Question 7

What are the Nuclear bag fibers sensitive to?

Answer 7

1) Length of muscle fiber

2) Velocity of muscle fiber stretch

Question 8

What are the nuclear chain fibers sensitive to?

Answer 8

The length of muscle fibers

Question 9

What are the nerve fibers that pick up the degree of nuclear bag fiber stretch?

Answer 9

Type Ia fibers (sensory), AKA Annulospiral endings

Question 10

What are the nerve fibers that pick up the degree of nuclear chain fiber stretch?

Answer 10

1) Type Ia Fiber (Annulospinal endings)

2) Type II fibers (Flower spray endings) “mainly found in the nuclear chain fibers”

Question 11

What is the type of nerve fiber that stimulates the stretching of the intrafusal fibers?

Answer 11

Gamma motor neurons (send Ach, causing actin-Myosin contraction)

• Motor neurons of the intrafusal fibers

Question 12

What nerve supplies the motor innervation for the intrafusal muscle fibers?

Answer 12

Gamma motor neurons

Question 13

How do the Type 1a or Type 2 Nerve fibers get activated?

Answer 13

• Neuromuscular junction

1) Once the AP reaches the end of the gamma motor neurons, calcium will enter the nerve and release ACh to the muscles

2) ACh binds to the muscle, leading to the entry of sodium, which will cause an AP

3) The AP will travel down the sarcolemma and into the T-Tubules, releasing calcium into the muscle filaments = Contraction

4) The contraction of the muscle will lead to an AP that will travel down the sensory fibers into the brain/spinal cord

Question 14

How does the sensory signals enters the spinal cord?

Answer 14

Mostly through the sensory (posterior) roots

Question 15

What are the fates of the sensory roots after they enter the spinal cord?

Answer 15

1) One branch terminates immediately in the grey matter of the cord, and it elicits a local segmental cord

2) Another branch transmits signals to higher levels of the nervous system (higher levels in the cord itself, brain stem, or the cerebral cortex). “More interpretation depends on the degree of stimuli. A lot of pain means it should be interpreted by the brain. If it is something you are not sure about, it goes through the anonymous part of your brain; if it is something you experienced before, it goes through the memory center to validate that.”

Question 16

What is a dorsal root ganglion?

Answer 16

• It is a relay station that receives information, sorts it out, and then sends it

Question 17

What is a relay station?

Answer 17

They are neuronal cells that are involved in screening, filtering, and processing the neuronal signals

Question 18

Describe the structure of the spinal motor neurons

Answer 18

• The spinal motor neurons are located in each segment of the anterior (ventral) horn of the cord grey matter
• They give rise to the nerve fibers that leave the cord via the anterior roots and directly innervate the skeletal muscle fibers, these neurons are of two types (alpha motor neurons and gamma motor neurons

Question 19

What is a muscle sindle?

Answer 19

They are an encapsulated muscle fibers that are distributed throught the belly of the muscle

Question 20

What type of information does the muscle spindle sends?

Answer 20

They sense specialized information about the length and rate of change of the muscle

• Rate/length = Muscle Spindle

Question 21

What is a Golgi tendon organ?

Answer 21

They are found in the muscle tendon, and they transmit information about the tension and force (rate of change) of the tension/force

Tension/Force = Golgi tendon

Question 22

What forms a muscle spindle?

Answer 22

• It is composed of 3-12 tiny intrafusal fibers (Gamma) that are surrounded by large extrafusal fibers (Alpha)

Question 23

What are the types of intrafusal fibers?

Answer 23

1) Nuclear bag fibers (1-3 in each spindle), nuclei are in the middle

2) Nuclear chain fibers (3-9 in each spindle), nuclei are in a row/line “Majority”

• They are differentiated by the distribution of the nuclei

Question 24

What are the types of sensory endings of the muscle spindle?

Answer 24

1) Primary endings (annulo-spiral ending)

2) Secondary endings (flower spray endings)

Question 25

Which intrafusal fiber is innervated by the primary ending, and what is its function?

Answer 25

It is innervated by Ia nerve fibers, where it wraps the middle of the nuclear bag and the chain nerve fibers (mainly bag), and they react very quickly, sending a message to the brain once the muscle starts stretching - Primary endings = quick dynamic actions "beginning of the stretch"

Question 26

Which intrafusal fiber is innervated by the secondary ending, and what is its function?

Answer 26

- They are mainly found at the end of the nuclear chain fiber, mainly type 2 nerve, telling the brain how long the muscle is stretched. They send a message about how long the muscle is stretched, but not as fast as the primary - Secondary endings = Steady, static position "sustained stretch"

Question 27

What is the general classification of the nerves?

Answer 27

1) Group A: - Alpha (Fastest) - Beta - Gamma - Delta (Slowest) 2) Group C

Question 28

What are the types of motor neurons that we have?

Answer 28

1) Alpha (Extrafusal) 2) Gamma (Intrafusal) - It is very defined, so the system knows exactly what to do

Question 29

Which intrafusal fibers are supplied by the primary endings?

Answer 29

1) Nuclear bag 2) Nuclear chain fibers

Question 30

Describe the primary ending nerve fiber

Answer 30

They are: 1) Thick 2) Myelinated 3) Type Ia (A alpha Type I, 17 micrometer in diameter) 4) They are rapidly conducting fibers (120m/sec)

Question 31

Type Ia are very sensitive to what?

Answer 31

- The velocity of change in the length of the muscle during a stretch (This is a dynamic response) - They provide information about the speed of movements and allow for a quick corrective movement

Question 32

Summary of intrafusal nerve fibers

Answer 32

- Motor output (Gamma efferent) - Sensory input (1a for both bag and chain, and II for chain mainly)

Question 33

Which afferent nerve fiber is responsible for sensing the dynamic changes?

Answer 33

Ia

Question 34

Which nerve fiber is responsible for sensing the static changes?

Answer 34

Type II - Static means something that is happening but not very exciting

Question 35

What does the secondary fibers supply?

Answer 35

They supply only/mainly the nuclear chain fiber, encircling the sides of the nuclear chain - These fibers are smaller in diameter, and they mainly sense static changes in the muscle length

Question 36

What is the function and innervation of the type Ia fiber?

Answer 36

1) Muscle spindle 2) Primary annulospinal ending - SENSE DYNAMIC CHANGES IN THE LENGTH (mainly nucleus bag)

Question 37

What is the function and innervation of a type Ib nerve fiber?

Answer 37

- Found in the golgi tendon - Senses the overstretching of the tendon

Question 38

What is the innervation and function of the type-II nerve fiber?

Answer 38

- Innervates the muscle spindle - Flower spray secondary ending (mainly the nuclear chain) - Senses the static changes

Question 39

What is the function of type III nerve fiber?

Answer 39

They sense: 1) pain and cold 2) Touch receptor

Question 40

What is the function of type IV nerve fiber?

Answer 40

It senses: 1) Pain 2) Temperature 3) ETC

Question 41

What type of nerve fibers innervate the large extrafusal skeletal muscle fibers?

Answer 41

Type A-alpha motor nerve fibers - Stimulating one A-alpha nerve fiber could excite 3-100+ skeletal muscle fibers (AKA motor unit)

Question 42

What does the A-Gamma nerve fibers innervate?

Answer 42

The intrafusal fibers, and they have two types: 1) A-Gamma dynamic: Mainly the nuclear bag intrafusal fibers 2) A-Gamma static: Mainly the nuclear chain intrafusal fibers

Question 43

How does the muscle spindle (AKA Group of intrafusal fibers) operates?

Answer 43

1) Stretching the muscle activates the muscle spindle = increased rate of AP travelling through Type Ia fibers 2) Contracting the muscle reduces the tension on the muscle spindle = decreased rate of AP traveling through the Type Ia fiber

Question 44

What is a reflex?

Answer 44

Instantaneous and involuntary movement in response to a stimulus

Question 45

What are the components of a reflex arc?

Answer 45

(SRACER) 1) Stimulus 2) Receptor 3) Afferent neurons 4) CNS integration 5) Efferent neurons 6) Response

Question 46

What are the different classifications of reflexes?

Answer 46

1) By development: - Innate reflexes (genetic/developmentally acquired) - Acquired (learned) 2) By response: - Somatic (controls the skeletal muscle) - Autonomic (controls smooth and cardiac muscle and the glands) 3) By complexity of the circuit: - Monosynaptic: One synapse - Polysynaptic: Multiple synapses 4) Processing site: - Spinal reflexes: Processed in the spinal cord - Cranial reflexes: Processed in the brain

Question 47

What is a monosynaptic reflex?

Answer 47

It is a simple reflex where the sensory neurons (afferents) directly synapse into the motor neurons (efferent) - The outcome of this reflex depends on the temporal summation of the impulses at the cell body of the afferent neurons

Question 48

What is a polysynaptic reflex?

Answer 48

- It is when there is one or more interneurons involved in the reflex

Question 49

Is the stretch reflex a mono or a polysynaptic reflex?

Answer 49

Monosynaptic reflex (like tapping the patellar tendon)

Question 50

Describe the stretch reflex

Answer 50

When a skeletal muscle with an intact nerve supply gets stretched, the muscle spindle will sense this stretch and cause the muscle to contract via the motor nerve

Question 51

What is the neurotransmitter at the central synapse?

Answer 51

Glutamate

Question 52

What is meant by a static stretch reflex?

Answer 52

- When a stretch is fixed or maintained, it stimulates the primary and secondary endings that supply the nuclear chain fibers - The secondary nerve fibers will slowly and continuously discharge signals as long as the stretch is maintained - An example of a static stretch is the muscle tone

Question 53

What are some examples of a polysynaptic reflex?

Answer 53

1) Withdrawal/flexor reflex - For example, when you withdraw your leg from a painful stimulus, the pain stimulus and postural adjustment are involved in actually withdrawing your foot 2) Crossed extensor reflex - In here, the same thing as before, but it also involves the body shifting the weight away from the painful stimulus so that we can balance

Question 54

What is the inverse stretch reflex (AKA Golgi tendon reflex)?

Answer 54

- It is a protective reflex that prevents the muscle from generating too much tension - Stimulus: When the muscle tension becomes too high - Receptor: Golgi tendon organ located in the tendons (near the muscle-tendon junction) - Afferent: The afferent fiber of the Golgi tendon (Type Ib) sends signals to the spinal cord - Integration: Type Ib fibers inhibit the a-motor neurons = muscle relaxation - The antagonist muscle will get activated