Reflexes Flashcards
(32 cards)
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.
