Reflexes

Question 1

What is a reflex?

Answer 1

A specific, involuntary, unpremeditated, ‘built-in’ response to a particular stimulus.

Question 2

What is a learned (acquired) reflex?

Answer 2

Many operations (including during driving a car).

Question 3

Describe the concept of homeostasis with respect to reflexes.

Answer 3

• A stimulus-response sequence

ie

• A reflex
  
  • Sometimes aware of stimulus and / or response.
  • For many reflexes, no conscious awareness.

Question 4

What is a reflex arc?

Describe it.

Answer 4

• A reflex arc is a pathway mediating a reflex.

1. Stimulus - A detectable change in internal / external environment.
2. Receptor - Detects the change.
3. Integrating centre - Signal received (along with those from other stimulus / receptor interactions).
4. Effector response - Sometimes response gives negative feedback.

Question 5

Describe the spinal or motor reflex.

Answer 5

• Sensory component and effector component.
• Does not involve conscious thought.

1. Sensory receptor
2. Sensory nerve fibre
3. Motorneurone from spinal cord to the muscle
   
   • Alpha motor neurons
     
     • ​have axons with a very large diameter.
     • transmit impulses with a conduction velocity of between 70-120m/s (268mph).
   • They are classed as being of the Aα type.
   • These motor neurons innervate skeletal muscle (outside the CNS).

Question 6

Describe the organisation of the spinal cord for motor functions.

Answer 6

• Grey cord matter.
  
  • Integrative area for cord reflexes.
• 1 = entry point for almost all sensory signals.
  Then branches in 2:
  
  • One terminates immediately in grey matter.
  • One signals to higher level NS.
• Each spinal cord segment has several million neurons.

Question 7

What are the spinal cord segment types (in addition to sensory neurons)?

Answer 7

1. Anterior motor neurons
   
   • Several thousand neurons, 50-100% larger than most others, located in each segment of the anterior horns of the cord grey matter.
   • Give rise to nerve fibres that leave the cord via anterior roots and directly innervate skeletal muscle fibres.
     
     • These neurons are:
       
       • Alpha motor neurons
       • Gamma motor neurons

Question 8

Describe alpha motor neurons (anterior motor neurons).

Answer 8

• Give rise to large type A alpha motor nerve fibres, averaging 14µm diameter;
  
  • these fibres branch many times after entering the muscle and innervate the large skeletal muscle fibres.
• Stimulation of a single alpha nerve fibres excites 3-several hundred skeletal muscle fibres, ie in the motor unit.

Question 9

Describe gamma motor neurons (anterior motor neurons).

Answer 9

• Much smaller than alpha motor neurons.
• Gamma motor neurons are located in the spinal cord anterior horns.
  
  • Approximately 1/2 as many as alpha motor neurons.
• Transmit impulses through much smaller type A gamma motor nerve fibres, averaging 5µm in diameter.
  
  • Go to small, special skeletal muscle fibres called intrafusal fibres.
  • These constitute the middle of the muscle spindle, which helps conrol basic muscle ‘tone’.

Question 10

Describe interneurons?

Answer 10

• About 30x as numerous as anterior mns.
• Are small and highly excitable.
  
  • Often show spontaneous activity, capable of firing 1500 times per second.
• Have many interconnections with each other.
  
  • Many also synapse directly with anterior mns.

Question 11

Describe the role of muscle spindles and golgi tendon organs in muscle control.

Answer 11

• Muscle function control needs excitation of the muscle by spinal cord anterior motor neurons.
• Feedback of sensory information from each muscle to spinal cord, indicating functional status of each muscle at each instant.
  
  • Ie, what is the muscle length, what is its instantaneous tension, how rapidly is its length or tension changing?
• Information is provided by:
  
  • Muscle spindles throughout belly of muscle
    
    • Send information to NS about muscle length or rate of change of length.
  • Golgi tendon organs located in the muscle tendons and transmit information about tendon tension or rate of change of tension.

Question 12

Describe the sensory innervation of muscle spindles.

Answer 12

• Sensory fibres originate in the central portion.
• Are stimulated by stretching of midportion of the spindle.
• Receptor excitation can be via:
  
  1. Lengthening whole muscle, stretching mid-portion of spindle.
  2. Contraction of end portions of the spindle’s intrafusal fibres, stretching midportion of the spindle.
• There are 2 types of sensory endings in central receptor area:
  
  • Primary afferent: type 1a (~17µm diameter, ~70-120m/s).
  • Secondary afferent: type 1 (~8µm diameter, ~15-30m/s).

Question 13

Describe nuclear bag fibres.

Answer 13

• A type of muscle spindle intrafusal fibres.
• 1-3 in each spindle.
• Several muscle fibre nuclei are congregated in expanded ‘bags’ in the central portion of the receptor area.
• Primary sensory nerve ending is excited by both nuclear bag intrafusal fibres and the nuclear chain fibres. BUT:
• Secondary is usually excited only by nuclear chain fibres.

Question 14

Describe nuclear chain fibres.

Answer 14

• A type of muscle spindle intrafusal fibre.
• 3-9 in each spindle.
• 1/2 diameter, 1/2 as long as the nuclear bag fibres.
• Have nuclei aligned in a chain throughout the receptor area.
• Primary sensory nerve ending is excited by both nuclear bag intrafusal fibres and the nuclear chain fibres. BUT:
• Secondary is usually excited only by nuclear chain fibres.

Question 15

Describe the control of intensity of static and dynamic responses by the gamma motor neurons.

Answer 15

• Static response
  
  • ​Response of both primary and secondary endings to the length of the receptor.
• Dynamic response
  
  • ​Response of primary ending (BUT NOT SECONDARY) to rate of change of receptor length.
• Gamma motor nerves to muscle spindle are: gamma-dynamic (gamma-d) and gamma-static (gamma-s).
• Gamma-d excite mainly the nuclear bag intrafusal fibres.
  
  • Dynamic response of the muscle spindle is enhanced, but static response is hardly affected.
• Gamma-s excited mainly the nuclear chain intrafusal fibres.
  
  • Enhances static response but has little influence on the dynamic response.

Question 16

Describe the muscle stretch reflex.

Answer 16

• Ie, the basic circuit of the muscle spindle stretch reflex:
  
  • Type 1a proprioceptor nerve fibre.
  • A branch of fibre goes directly to the anterior horn of the cord grey matter.
  • Synapses with anterior motor neurons that send motor nerve fibres back to the same muscle from which the muscle spindle fibre originated.
• Ie, monosynaptic pathway allows a reflex signal to return with shortest possible delay back to muscle after excitation of the spindle.

Question 17

Describe the dynamic component of the stretch reflex.

Answer 17

• Dynamic stretch reflex opposes sudden changes in muscle length.
• It is elicited by dynamic signals from the primary sensory endings of the muscle spindles, caused by rapid stretch or unstretch.
• Ie, a muscle is suddenly stretched / unstretched - strong signal is transmitted to the spinal cord - instantaneous strong reflex contraction (or decrease in contraction) of the same muscle from which the signal originated.
• Ends within fraction of second after the muscle has been stretched (or unstretched) to its new length.

Question 18

Describe the static component of the stretch reflex.

Answer 18

• Static stretch reflex causes degree of muscle contraction to remain reasonably constant.
  
  • Except when person’s NS specifically wills otherwise.
• Follows and continues for a prolonged period.
• Is elicited by continuous static receptor signals transmitted by primary and secondary endings.

Question 19

What is the function of the knee-jerk reflex (a type of stretch reflex)?

Answer 19

• Maintains balance and posture.
• Monosynaptic reflex.

Question 20

What is the role of the golgi tendon organ (GTO)?

Answer 20

• Helps control muscle tension.
• Is an encapsulated sensory receptor with muscle tendon fibres passing through.
• ~10-15 muscle fibres are usually connected to each GTO.
• GTO is stimulated when this small bundle of muscle fibres is ‘tensed’ by contracting or stretching the muscle.

Question 21

Describe the golgi tendon reflex and prevention of excessive tension.

Answer 21

• GTO stimulation by increased tension in connecting muscle => signals are transmitted to the spinal cord to cause reflex effects in the respective muscle.
• This reflex is entirely inhibitory . ̇. Gives negative feedback mechanism to prevent the development of too much tension on the muscle.
• When tension on the muscle ( . ̇. on the tendon) becomes extreme, inhibitory effect from GTO can be so great that it leads to a sudden reaction in the spinal cord that causes instantaneous relaxation of the entire muscle, ie lengthening reaction.
  
  • is probably a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone.

Question 22

Give examples of reflexes involving special senses.

Answer 22

• Rotatory nystagmus: eye movements driven by moving visual images.
• Post Rotatory nystagmus: eye movements driven by the movement of fluid in the semi-circular canals of the inner ear.
• Interpretation of movement within images on the retina – used to make postural adjustments to preserve balance.

Question 23

What is the baroreceptor reflex?

Answer 23

• Occurs via the medullary cardiovascular centre.
• Arterial baroreceptors respond to changes in pressure.
• Increase firing rate in response to increased pressure, and vice versa.

Question 24

Describe the classification of nerves based on conduction velocity.

Answer 24

Question 25

Describe the classification of nerves based on sensory function.

Answer 25