Reflexes

Question 1

define reflex

Answer 1

A specific, involuntary, unpremeditated, “built-in” response to a particular stimulus.
there are also learned reflexes which you acquire whilst for example driving a car

Question 2

what is the pathway that media’s reflexes

Answer 2

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

Question 3

The spinal or motor reflex

Answer 3

• Has a sensory component and an effector component.
• Does not involve conscious thought.
• The sensory component is a sensory receptor and the effector component is the motor outflow from the spinal cord to the appropriate muscles.

Question 4

sensory nerve fibres

Answer 4

• Axons are routed to the CNS inside ‘nerves’
– typically there are many axons from many neurons inside one
nerve.
• Single nerves can carry both sensory and motor axons
– ie information can travel in two directions in a nerve.

Question 5

what are the two differing features of axons

Answer 5

1. Diameter

2. Myelination

Question 6

Motoneurone from spinal cord to muscle

Answer 6

• 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 neurones innervate skeletal muscle ie outside CNS

Question 7

sensory signals in grey matter

Answer 7

1= entry point for almost all sensory signals. Then branches in 2;
– One terminates immediately in grey matter
– One signals to higher level NS

Question 8

anterior motor neurones

Answer 8

• Several thousand neurones, 50-100% larger, than most others, located in each segment of the anterior horns of the cord gray matter.
• Give rise to nerve fibres that leave cord via anterior roots and directly innervate skeletal muscle fibres.

Question 9

two types of anterior motor neurone

Answer 9

• alpha motor neurons

* gamma motor neurons.

Question 10

alpha motor neurons

Answer 10

Give rise to large type A alpha (Aα) motor nerve fibres, averaging 14 um in diameter;
– These fibers branch many times after entering the muscle and innervate the large skeletal muscle fibres.
• Stimulation of a single alpha nerve fibre excites 3- several hundred skeletal muscle fibres ie the motor unit.

Question 11

gamma motor neurons.

Answer 11

• Are much smaller than α motor neurones.
• Gamma motor neurones are located in the spinal
cord anterior horns.
– Approx 1⁄2 as many as α motor neurones.
• Transmit impulses through much smaller type A gamma (Aγ) motor nerve fibres, averaging 5 um in diameter
– go to small, special skeletal muscle fibres called intrafusal fibres.
– These constitute the middle of the muscle spindle, which helps control basic muscle “tone”.

Question 12

Interneurones

Answer 12

Are 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 13

What are the two things muscle function control needs

Answer 13

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

Question 14

Where is the information for muscle function control provided from

Answer 14

1. musclespindlesthroughoutbellyofmuscle
   – send information to NS about muscle length or rate of change of length
2. Golgitendonorganslocatedinthemuscletendonsandtransmit information about tendon tension or rate of change of tension.

Question 15

muscle spindles by sensory innervation

Answer 15

• Sensory fibers originate in 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 fibers, stretching midportion of spindle.

Question 16

what are the 2 types of sensory ending in the central receptor area

Answer 16

– primary afferent: Type Ia (17um diameter, 70-120m/s)

– secondary afferent: Type II (8um diameter, )

Question 17

2 types of muscle spindle IF fibres

Answer 17

(1) nuclear bag
• 1-3 in each spindle
• several muscle fiber nuclei are congregated in expanded “bags” in the central portion of the receptor area
(2) nuclear chain
• 3-9 in each spindle
• 1⁄2 diameter, 1⁄2 as long as the nuclear bag fibers
• have nuclei aligned in a chain throughout the receptor area

Question 18

excitation rules

Answer 18

1° sensory nerve ending is excited by both the nuclear bag intrafusal fibers and the nuclear chain fibers.
BUT
2° is usually excited only by nuclear chain fibers.

Question 19

Static response

Answer 19

Response of both 1° and 2° endings to the length of the receptor.

Question 20

Dynamic response

Answer 20

Response of 1°ending (but NOT 2°) to rate of change of receptor length.

Question 21

Gamma control of motor nerves

Answer 21

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 excites mainly the nuclear chain intrafusal fibres.
=> Enhances static response but has little influence on the dynamic response.

Question 22

Muscle stretch reflex

ie basic circuit of the muscle spindle stretch reflex

Answer 22

• Type Ia proprioceptor nerve fiber.
• A branch of fibre goes directly to the anterior horn of the cord gray matter
• Synapses with anterior motor neurons that send motor nerve fibres back to the same muscle from which the muscle spindle fibre originated.

Question 23

what does the monosynaptic pathway allow

Answer 23

allows a reflex signal to return with shortest possible delay back to muscle after excitation of the spindle.

Question 24

what does dynamic stretch reflex oppose

Answer 24

sudden changes n muscle length

• is elicited by dynamic signals from the 1° sensory endings of the muscle spindles
• caused by rapid stretch or unstretch.
• Ends within fraction of second after the muscle has been stretched (or unstretched) to its new length.

Question 25

what does static stretch reflex cause

Answer 25

degree of muscle contraction to remain reasonably constant

• Follows and continues for prolonged period.
• Is elicited by continuous static receptor signals transmitted by 1° and 2°endings.

Question 26

The stretch reflex

eg the knee-jerk reflex

Answer 26

• Maintains balance and posture.

* Monosynaptic reflex.

Question 27

Golgi tendon organ (GTO)

Answer 27

• 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 28

GTO reflex and prevention of excessive

tension

Answer 28

• 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 -ve feedback mechanism to prevents 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 29

Rotatory nystagmus

Answer 29

Eye movements driven by moving visual images.

Question 30

Post Rotatory nystagmus

Answer 30

Eye movements driven by the movement of fluid in the semi-circular canals of the inner ear.

Question 31

what can be used to make postural adjustments to preserve balance

Answer 31

Interpretation of movement within images on the retina

Question 32

Baroreceptor reflex (1)

Answer 32

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