Reflexes Flashcards
What is a reflex?
A specific, involuntary, unpremeditated, ‘built in’ response to an involuntary stimulus
What are the components of a spinal or motor reflex?
There is a sensory and an effector component
What is the sensory component in a spinal or motor reflex?
The sensory receptor
What is a monosynaptic reflex?
A reflex arc with one synapse, e.g. spinal or motor reflex such as the knee jerk reflex
What are the principle differences between axons?
Myelination and diameter
Describe the sensory component of the spinal/motor reflex
- Axons are routed to the CNS inside nerves
* Single nerves can carry both sensory and motor axons and information can travel in 2 directions
Where do most sensory signals enter the spinal cord?
At the sensory route: dorsal/posterior root
What happens once the sensory signal has entered the spinal cord?
• Branches into 2:
- One terminates immediately in the grey matter
- One signals to a higher level
Describe the anterior motor neurones
- Several thousand
- 50-100% larger than most of the other neurones
- Located in each segment of the anterior horns of the cord grey matter
- Give rise to nerve fibres that leave the cord via the anterior roots and directly innervate the muscle fibres
What muscle fibres do the anterior motor neurones give rise to?
- Alpha motor neurones
* Gamma motor neurones
Alpha motor neurones
- Give rise to type A alpha motor nerve fibers averaging 14μm in diameter which branch after entering the muscle and innervating large skeletal muscle fibers
- Stimulation of a single alpha nerve fibre can excite many skeletal muscle fibres e.g. motor unit
Gamma motor neurones
- Much smaller than alpha motor neurones
- Located in the spinal cord anterior horns
- Transmit impulses through A gamma motor nerve fibres, averaging 5μm in diameter which go to intrafusal fibres and constitute the middle of the muscle spindle which helps to control basic muscle tone
- There are approximately half the number of gamma as there are alpha
What are intrafusal fibres?
Small, specialised skeletal muscle fibres
Which type of motor neurone send impulses to the intrafusal fibres?
Gamma motor neurones
What are interneurones?
- Small and highly excitable neurones which transmit impulses between neurones
- 30 times as numerous as anterior motor neurones
- Have many connections with each other but may also synapse directly with anterior motor neurones
Describe the activity of interneurones
• Often show spontaneous activity, capable of firing 1500 times per second
What provides sensory information from the muscles to the spinal cord?
- Muscle spindles
* Golgi tendon organs
Describe how the muscle spindles feedback to the spinal cord
- Sends information to the nervous system about muscle length or rate of change of length
- Located throughout the muscle belly
Describe how the Golgi tendon organ feedbacks sensory info to the spinal cord
- Transmits information about tendon tension or the rate of change of tension
- Located in the muscle tendons
In muscle spindles, where do the sensory fibres originate?
In the central portion
What causes receptor excitation in the muscle spindle
- Lengthening the whole muscles, stretching the mid portion of the spindle
- Contraction of the end portions of the spindle’s intrafusal fibres, stretching the mid-portion of the spindle
What are the 2 types of sensory endings in the central receptor area in a muscle spindle?
- Primary afferent: Type IA
* Secondary afferent: Type II
What are the muscle fibre types in order of their conduction velocity/diameter from high to low? (conduction velocity classification)
- A alpha
- A beta
- A gamma
- A delta
- B
- C
What are the muscle fibre types and their origin? (sensory function classification)
- IA- muscle spindles
- IB - Golgi tendon organs
- II - muscle spindles; touch and pressure receptors
- III - Pain and temperature receptors
- IV - Pain and other receptors
What is the origin of fibre type IA
Muscle spindles
What is the origin of fibre type IB
Golgi tendon organ
What is the origin of fibre type II
Muscle spindles; touch and pressure receptors
What is the origin of fibre type III
Pain and temperature receptors
What is the origin of fibre type IV
Pain and other receptors
What are the 2 types of muscle spindle intrafusal fibres?
- Nuclear bag
* Nuclear chain
Describe nuclear bag fibres
- 1-3 in each spindle
- Several muscle fibre nuclei are congregated in expanded ‘bags’ in the central portion of the receptor area
- Excites primary sensory nerve endings (Type IA)
Describe nuclear chain fibres
- 3-9 in each spindle
- 1/2 the diameter of nuclear bag fibres
- 1/2 the diameter of nuclear bag fibres
- nuclei aligned in a chain throughout the receptor area
- Excites both primary (IA) and secondary (II) sensory nerve endings
Describe the response of sensory nerve fibres in a static response
- Both the primary (IA) and secondary (II) endings
* Respond to the length of the receptor
Describe the response of sensory nerve fibres in a dynamic response
- Response of a primary ending (NOT SECONDARY)
* Respond to the rate of change of the receptor length
What are the gamma motor nerves to muscle spindle?
- Gamma-dynamic
* Gamma- static
Gamma-d
- Excite mainly the nuclear bag intrafusal fibres
* Dynamic response of the muscle spindle is enhanced but the static response is hardly affected
Gamma-s
- Excites mainly the nuclear chain intrafusal fibres
* Enhances the static response but has little influence on the dynamic response
What is the muscle stretch reflex?
- Basic circuit of the muscle spindle stretch reflex
* Monosynaptic pathway
In terms of the muscle stretch reflex, what is the type IA proprioceptor nerve fibre?
- A branch of fibre goes directly to the anterior horn of the cord grey matter
- It then synapses with anterior motor neurones that send motor nerve fibres back to the same muscle from which the muscle spindle fibre originated
What is the benefit of a monosynaptic reflex?
• Allows a reflex signal to return with the shortest possible delay back to the muscle after excitation of the spindle
What is the role of the knee jerk reflex?
To maintain balance and posture
What does a dynamic stretch reflex do?
• Opposes sudden changes in muscle length
Which sensory endings are involved in the dynamic stretch reflex?
Primary (Type IA) sensory endings of the muscle spindles
Describe a dynamic stretch reflex
- Muscle is suddenly stretched or unstretched
- Strong signal is transmitted to the spinal cord which causes and instantaneous strong reflex contraction of the same muscle from which the signal originated
- Ends within a fraction of a second after the muscle has been stretched or unstretched to its new length
Describe the static stretch reflex
- Causes a degree of muscle contraction to remain reasonably constant
- Elicited by continuous static receptor signals transmitted from the primary (Type IA) and secondary (Type II) endings
- Follows and continues for prolonged periods
What is the Golgi tendon organ?
An encapsulated sensory organ with muscle tendon fibres passing through
• Has 10-15 muscle fibres attached
When does a Golgi tendon organ become stimulated/
When the small bundle of fibres connected to it is tensed by contracting or stretching the muscle
Describe the Golgi Tendon Organ reflex
- GTO stimulated but the increased tension in the connecting muscle
- Signals are transmitted to the spinal cord
- Causes reflex effects in the respective muscle
What is the purpose of a Golgi Tendon Organ reflex?
• Prevents the development of too much tension on the muscle
What happens when tension on a muscle becomes extreme?
• When tension on the muscle (and therefore tendon) becomes extreme, the 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
Rotatory nystagmus
eye movements driven by moving visual images
Post Rotatory nystagmus
Eye movements driven by the moment of fluid in the semi circular canals of the inner ear
What is the interpretation of movement within images on the retina used for?
To make postural adjustments to preserve balance
Describe the baroreceptor reflex
- Occurs via the medullary cardiovascular centre
- Arterial baroreceptors respond to changes in pressure
- Increased firing rate in response to increased pressure
