Reflexes

Question 1

Functions of Reflexes:

Answer 1

• Automatically adjust posture
• Automatically adapt motor patterns to achieve a behavioral goal
• Adjust proper amount of force needed for task
• Provide fast-acting safety reactions to avoid hazardous situations

Question 2

General components of a reflex:
Need to sense status of muscle

Answer 2

• Muscle receptors: monitor length and tension
  
  • Muscle spindles, Golgi Tendon Organs
• Afferent neuron that innervates receptor: relay information about length and tension to spinal cord
  
  • Cell bodies are in the DRG, TG

Question 3

General components of a reflex:
Need to cause an effect on muscle

Answer 3

• Efferent neuron: Motor neurons in ventral horn of spinal cord

Question 4

General components of a reflex:
Modify information between sensing and causing effect

Answer 4

• Interneurons modify reflex locally in spinal cord
• Descending neurons from cortex, brainstem, send axons down through spinal tracts and modify spinal reflex
• Interneurons and descending neurons can be either excitatory or inhibitory

Question 5

What is the purpose of the muscle spindle & golgi tendon organ?

Answer 5

Give feedback to the CNS about status of muscle

Question 6

Muscle spindle:

Answer 6

• Parallel to extrafusal fibers (force-generating fibers)
• Sense muscle length
• Key for proprioception: Length of muscle is associated with angle of joints
  
  • Muscle spindles together with joint and cutaneous afferents help sense positions of limbs with respect to body
• Discharge best when muscle is stretched
  
  • Fall silent when muscle shortens
• Composed of intrafusal fibers which sense length of muscle

Question 7

Types of intrafusal fibers:

Answer 7

1. Dynamic nuclear bag fibers
   
   • Response to lengthening of muscle adapts over time
2. Static nuclear bag fibers & nuclear chain fibers
   
   • Response is steady over time

Question 8

Muscle spindle sensory afferents:
Spiral around the intrafusal fibers in the spindle

Answer 8

1. Ia afferents
2. II afferents

Question 9

Muscle spindle sensory afferents:

Ia afferents

Answer 9

innervate all 3 types of fibers

• Sense muscle length and rate of change of length
• Convey fast, phasic, dynamic responses of muscle fibers
• Velocity of stretch
  
  • Very sensitive to small changes

Question 10

Muscle spindle sensory afferents:

Il afferents

Answer 10

innervate static bag fibers and nuclear chain fibers

• Sense muscle length (not rate)
• Convey slow, tonic, static responses
  
  • Steady state, static length

Question 11

1. When intrafusal fibers are stretched:
2. When intrafusal fibers are unloaded:

Answer 11

1. sensory endings are stretched and increase firing rate
2. sensory endings stop firing

Question 12

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

Answer 12

• Dynamic fibers to sense when the muscle is changing
• Static fibers to sense when the muscle has stabilized at a new length
  
  • Both are very sensitive to small changes
  • provide info about unexpected changes in length
  • useful to generate quick corrective measures
• CNS uses muscle spindles to sense and correctly change the position of body segments

Question 13

Golgi tendon organ:

Answer 13

• Located at junction between muscle fibers and tendon
  
  • in series with extrafusal fibers
• Sense muscle tension
• Sensory afferent: single Ib afferent neuron
  
  • branches intertwine among collagen fasicles

Question 14

What happens when the Golgi tendon organ is stretched?

Answer 14

Stretching tendon organ:

1. straightens collagen fibers
2. compresses nerve endings
3. AP firing

• Discharge best when muscle connected to Golgi tendon organ contracts
• Very sensitive to muscle contraction
  
  • Precisely measures force in the contracting muscle

Question 15

Why do we need Golgi tendon organs?

Answer 15

• Continuously measure tension from force in a contracting muscle
• Provide nervous system with precise information about state of contraction of the muscle.
• Protective against too much muscle tension
  
  • protects against muscle damage

Question 16

Muscle Efferents: Lower motor neurons that control muscle

Answer 16

1. Alpha motor neurons:
   
   • Neural control of “workhorse” force-generating extrafusal muscle fibers
2. Gamma motor neurons:
   
   • Neural control that regulates sensitivity of muscle spindle intrafusal fibers

Question 17

Gamma motor neurons

Answer 17

1. Dynamic: innervate dynamic nuclear bag fibers
2. Static: innervate static nuclear bag fiber and nuclear chain fibers

• Activation of gamma motor neurons causes:
  
  1. shortening of intrafusal fiber
  2. stretches central region of intrafusal fiber
  3. increases firing of afferent fibers

Question 18

What is the role of the gamma motor neurons?

Answer 18

Role:

• Adjust dynamic and static sensitivity of the muscle spindle and their afferents
• Response of muscle spindles can be tuned:
  
  • gain can be turned up for either dynamic or static information

Question 19

Describe alpha-gamma co-activation:

Answer 19

During voluntary movements:

• Gamma motor neurons and alpha motor neurons are “co-activated”:
  
  • both extrafusal and intrafusal fibers contract
• Maintains spindle sensitivity as the muscle shortens
• Prevents spindle sensory fiber from falling silent when muscle shortens from active contraction
  
  • Automatically maintains muscle spindle sensitivity over all muscle lengths

Question 20

**Stretch Reflex: Monosynaptic excitatory reflex **

“deep tendon reflex”

• **Speed: **
• **Stimulus: **
• **Afferent: **
• **Efferent: **

Answer 20

• Speed: very fast < 20 ms latency
• Stimulus: Stretch of muscle increases its length
  
  • activation of muscle spindles
• Afferent: Ia afferent from spindle is activated
• Efferent: α-motor neurons fire and cause contraction of homonymous and synergist muscles

Question 21

What happens in the spinal cord during a monosynaptic excitatory reflex?

Answer 21

• Spinal cord:
  1. Ia afferent projects to and excites the α-motor neurons that control the same muscle
  2. Also project to and excite synergist muscles
  3. Ia afferent also projects to inhibitory interneurons that inhibit motor neurons that project to antagonistic muscles (reciprocal inhibition)

Question 22

What are monosynaptic excitatory reflexes used for clinically?

Answer 22

Use this to detect hyperactive or hypoactive reflexes

• Decreased reflex ⇒ indicates disease of muscle, nerve or spinal cord
• Increased reflex ⇒ indicates loss of descending inhibitory input to lower motor neurons

Question 23

Why do we need the monosynaptic reflex?

Answer 23

Roles:

1. Maintain muscle tone for posture
2. Allows us to hold still
3. Smooth out movements
4. Increase efficiency for locomotion

Question 24

Flexion and crossed-extension reflex: Flexor Reflex

• Role:
• Stimulus:

Answer 24

• Role: withdrawal from painful stimulus
• Stimulus: activation of nociceptors (myelinated Aδ)

Question 25

What happens in the spinal cord during a flexor reflex?

Answer 25

1. Activation of interneurons that excite ipsilateral flexor muscle and inhibit extensor muscle ⇒ withdraw leg from nail (reciprocal inhibition)
2. Activation of interneurons that cross spinal cord and excite contralateral extensor muscle and inhibit contralateral flexor muscle ⇒ gives body support, keep standing

Question 26

Golgi Tendon Reflex:

• Stimulus:
• Roles:

Answer 26

**Golgi tendon organ inhibits α-motor neuron via an inhibitory interneuron **

• Stimulus: tension on Golgi tendon organ activates Ib inhibitory interneurons
  
  • Ib inhibitory interneuron also receives convergent input from: muscle spindle afferents, low-threshold cutaneous afferents, joint afferents and excitatory and inhibitory input from descending pathways
• Roles: Signal minute changes in muscle tension to precisely control fine adjustments in muscle contraction in ordinary activites

Question 27

Give an example of when the Golgi Tendon Reflex is used:

Answer 27

Example: pick up delicate object

1. fingers touch object
2. ⇒ increase in muscle tension activates Golgi tendon reflex
3. ⇒ inhibits α-motor neurons
   
   • inhibits grasp so you don’t crush object

• Also prevents overly forceful movements and too much muscle tension, prevents muscle injury

Question 28

Mechanisms that modulate spinal reflexes:
Inhibitory interneurons help coordinate reflex actions

Answer 28

• mediate reciprocal innervation
• receive many convergent inputs from muscle afferents and descending pathways

1. Stretch reflex: Ia interneurons
   
   • inhibit antagonist muscles
2. Golgi tendon reflex: Ib inhibitory interneurons receive input from:
   
   • Golgi tendon organs, muscle spindles, joint and cutaneous receptors and descending pathways,
   • inhibit agonist muscles

Question 29

Mechanisms that modulate spinal reflexes:

Descending control

Answer 29

• Regulate the strength of reflex by:
  
  • changing the tonic level of activity
• Force of a reflex can vary even though sensory stimulus stays constant
• Sites for modulation in spinal cord:
  
  1. α-motor neurons
  2. interneurons
  3. presynaptic terminals of afferent fibers
• Reflexes are not fixed but can be modified
• Reflex strength depends on: stimulus intensity, context, posture, task requirements, control from supraspinal sites