Reflexes Flashcards
define reflex
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
what is the pathway that media’s reflexes
- A detectable change in internal/ external environment (stimulus)
- Detects the change (receptor)
- Signal received (along with those from other stimulus/receptor interactions)(integrating centre)
- Sometimes response gives negative feedback (effector, response, negative feedback
The spinal or motor reflex
- 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.
sensory nerve fibres
• 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.
what are the two differing features of axons
- Diameter
2. Myelination
Motoneurone from spinal cord to 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 neurones innervate skeletal muscle ie outside CNS
sensory signals in grey matter
1= entry point for almost all sensory signals. Then branches in 2;
– One terminates immediately in grey matter
– One signals to higher level NS
anterior motor neurones
- 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.
two types of anterior motor neurone
- alpha motor neurons
* gamma motor neurons.
alpha motor neurons
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.
gamma motor neurons.
• 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”.
Interneurones
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
What are the two things 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
Where is the information for muscle function control provided from
- musclespindlesthroughoutbellyofmuscle
– send information to NS about muscle length or rate of change of length - Golgitendonorganslocatedinthemuscletendonsandtransmit information about tendon tension or rate of change of tension.
muscle spindles by sensory innervation
• 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.
what are the 2 types of sensory ending in the central receptor area
– primary afferent: Type Ia (17um diameter, 70-120m/s)
– secondary afferent: Type II (8um diameter, )
2 types of muscle spindle IF fibres
(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
excitation rules
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.
Static response
Response of both 1° and 2° endings to the length of the receptor.
Dynamic response
Response of 1°ending (but NOT 2°) to rate of change of receptor length.
Gamma control of motor nerves
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.
Muscle stretch reflex
ie basic circuit of the muscle spindle stretch reflex
- 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.
what does the monosynaptic pathway allow
allows a reflex signal to return with shortest possible delay back to muscle after excitation of the spindle.
what does dynamic stretch reflex oppose
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.
what does static stretch reflex cause
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.
The stretch reflex
eg the knee-jerk reflex
- Maintains balance and posture.
* Monosynaptic reflex.
Golgi tendon organ (GTO)
- 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.
GTO reflex and prevention of excessive
tension
• 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.
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.
what can be used to make postural adjustments to preserve balance
Interpretation of movement within images on the retina
Baroreceptor reflex (1)
curs via the medullary cardiovascular centre.
Arterial baroreceptors respond to changes in pressure.
Increase firing rate in response to increased pressure, and vice versa.
