Reflexes Flashcards
What is the vestibuloocular reflex?
mechanism by which head movement is compensated in order to maintain stability of the visual scene on the retina
- stabilises rge position of the retina in space, during movements of the head, by rotating the eyeball in the opposite direction
What type of reflex is the vestibuloocular reflex?
An open reflex - there is no time for feed-back connection that would compensate for innacuracies in the execution of the reflex
How is the vestibuloocular reflex initiated?
stimulation of the receptors (hair cells) in the vestibular system through acceleration signals
In what circumstances does the vestibuloocular reflex occur?
In both the light and the dark as rge vestibular organs are sensitive to head acceleration, not visual cues
What is the output of the vestibuloocular reflex?
A change in eye position in the orbit
What is a reflex?
relatively predictable, involuntary and stereotyped response to an iliciting stimulus
What is a spinal reflex?
A reflex mediated by a nerve pathway with a sensory afferent to the spinal cord, linkages within the cord and an efferent to muscle from the cord
What are the basic components of a reflex pathway?
circuit = reflex arc
- afferent limb (sensory receptors and axons) that carries information to the CNS
- a central component (synapses and interneurons within the CNS)
- an efferent limb (motor neurons) that causes the motor response
What are the events in a tendon knee jerk?
- stimulus (tendon tap) stretches the muscle
- activates muscle spindle receptors
- conduction along afferent fibers (Ia) into the lumbar spinal cord
- Transmission at synapses (glutamate release) between Ia afferent and motorneuron
- conduction along efferent fiber
- neuromuscular transmission (ACh release)
- excitation contraction coupling = shortening of muscle fibers
- twitch contraction of skeletal muscle = mechanical movement of the lower limb
Which muscle is tested in the knee jerk test?
quadriceps femoris
What muscle is tested in the ankle jerk test
the gastrocnemius
What muscle is tested in the jaw-jerk test?
The masseter muscle
What tendon reflexes can be found in the arms?
- biceps reflex
- triceps reflex
- suppinator reflex
What is the semental nerve root level of the jaw jerk reflex?
CN V
What is the semental nerve root level of the biceps reflex?
C5, 6
What is the semental nerve root level of the triceps reflex?
C6, 7, 8
What is the semental nerve root level of the brachioradialis reflex?
C5, 6
What is the semental nerve root level of the knee jerk reflex?
L3, 4
What is the semental nerve root level of the ankle jerk reflex?
S1, 2
Which reflexes are monosynaptic?
- achilles
- knee jerk
What is the plantar response (the Babinski reflex) used for?
testing the integrity of the pyramidal tract
How can the plantar response (the Babinski reflex) be elcited?
A firm stroke to the outer edge of the sole of the foot, stroking down from the heel forwards with a blunt instrument
What is the response of the Babinski reflex in a normal patient?
Downward curling of the toes (plantar flexion)
Also known as babinski negative
What is a Babinski positive response a sign of?
Neonate = normal
Adult = a lesion of the cortico-spinal projection
What occurs in a Babinski positive response?
Withdrawal of the foor and fanning of the toes
What are the different types of peripheral neuropathy
- Polyneuropathy (multiple nerves affected)
- mononeuropathy (only one nerve or nerve group is affected)
What is mononeuropathy usually a response of?
damage to a single nerve or nerve group by:
- trauma
- injury
- local compression
- prolonged pressure
- inflammation
What pattern is characteristic of polyneuropathies?
A stocking-glove pattern of sensory loss
These are ‘dying back’ neuropathies = distal terminals of longest fibers affected first and progress proximally
in what way are you able to reinforce reflexes?
asking the patient to:
- clench their teeth
- pull opposing clasped hands
What is Jendrassik’s manoeuvre?
asking a patient to pull on opposed fingers, while you try to elicit a reflex in the lower limb
How does Jendrassik’s manoeuvre work?
-voluntary UMN innervation of the arm ‘overflows’ to increase the excitability of the LMN pool of the lower extremities
It can also help prevent conscious inhibition of the reflex
What abnormalities will result from damage to the stretch reflexes
- hypertonia (spasticity)
- hypotonia (flaccid muscles)
What is a sensory unit?
a single sensory nerve fiber and the receptors with which it is connected
How is sensory information coded?
In terms of the channel along which nerve impulses are travelling
How are receptors for a specific sensation distributed in the body?
In a punctuate fashion in the skin, with intervening areas of skin which are insensitive to that particular modality
What is two-point discrimination?
A method frequently used to test tactile disctrimination
i.e. that the density of cutaneous sensory innervation varies for different regions of skin
What is the ASIA sensory test?
a comaprison of sensation in all dermatomes to ‘normal sensation’ where normal is the sensation elicited when the stimulus is applied to the cheek
Explain the rationale behind the ASIA sensory test
- the cheek is innervated by cranial nerve V, and sensation is therefore unaffected bu spinal cord injury
How is two-point discrimination tested?
two needles are pressed lightly against the skin at the same time, and the person determines whether one
point or two points of stimulus is/are felt.
What are the normal outcome of a two-point discrimination test on the fingers?
On the tips of the fingers, a person can normally distinguish two separate points even when the needles are as
close together as 1 to 2 millimeters.
What are the normal outcome of a two-point discrimination test on the back?
on the person’s back, the needles usually must be as far apart as 30 to 70 millimeters before two
separate points can be detected.
Why is there a difference in response to the two-point discrimination in different areas?
Different areas of the body have receptive fields of different sizes, giving some better
resolution in two-point discrimination.
The tongue and finger pads have very high resolution, while the back has very low.
This is illustrated as the distance where the two points can be felt as separate.
They have smaller receptive fields and are located closer together
Explain the mechanism by which the dorsal column pathway transmits two-point discriminatory information
Peripheral endings of sensory receptors branch to form a receptive field
The brain recognises two separate stimuli if these stimuli land in two different receptive fields, as 2 separate sets of sensory neurons are activated
What is the result of a spinal cord injury at C4?
quadriplegia
What is the result of a spinal cord injury at T6
upper limb reflexes unaffected
lower limb initially presents with areflexia (typically bilateral in trauma injuries)
Describe the return of reflexes after a spinal cord injury
Hyperreflexia
- reflexes return and become exaggerated
- results in stiff limbs and spasticity
