PHTY209 Lecture 20: Reflexes
Reflexes
• What is a reflex
o Reflexes are subconscious stimulus-response mechanisms. That makes them mostly
a very objective part of the neurologic exam.
o Reflexes are:
• An involuntary response to a specific sensory stimulus
• Can be protective (e.g. avoiding hazardous situations – withdrawal reflex)
Can integrate motor movements so they function in a coordinated manner
(e.g. postural adjustments to external stimuli, walking)
• Essentially "wired into" the CNS so that they are always ready to function
o Monosynaptic/Polysynaptic
• Monosynaptic
▪ All reflexes are sensorimotor arcs if reduced to their simplest level
▪ That is at a minimum they involve one synapse and one segment of
spinal cord - some type of sensory signal (afferent) and some motor
response
▪ This direct synapse between the sensory fibre and motor neuron is
known as a monosynaptic reflex.
• Polysynaptic reflexes
▪ Many reflexes however are much more complex involving interneurons,
several segments of the spinal cord and pathways ascending to and
from the cerebral hemispheres
• Even simple reflexes can have many inhibitory and facilitatory influences that
can amplify or suppress the response of the reflex when tested
• This can include intersegmental and intrasegmental connections in the spinal
cord, as well as descending influences from the upper motor neuron (e.g.
brain stem, cerebellum, basal ganglia, cerebral hemispheres).
• Outline the importance of assessing reflexes in physiotherapy clinical practice
o We assess reflexes to check the nervous system is functioning correctly.
o Testing looks at the function and interplay of both sensory and motor pathways.
o It provides important information about peripheral and spinal circuits and level of
background excitation
o Examples
• The knee jerk reflex assumes the pathways to/from the muscle are working -
so too the spinal cord at that level
• The corneal reflex you learnt about in the cranial nerve practical in week 3,
provides information about cranial nerve 3 and the functioning of the
brainstem
o Upper motor neurons
• Generally modulate reflexes like the knee jerk reflex - that is, they generally
send impulses down the spinal cord to keep the knee jerk reflex calm
• Most of the pathways that descend the spinal cord have a tonic inhibitory
influence of spinal reflexes
• In conditions such as stroke or other upper motor neuron pathology, this
down calming influence on the reflexes can be lost, and reflexes can become
increased or overactive (hyperreflexia)
o Lower motor neurons
• We may see decreased reflexes if the nerves travelling between the spinal
cord and the relevant muscle are affected (hyporeflexia/areflexia)
• This is, a lesion that damages the sensory or motor limb of a reflex arc at any
level of the sensory or motor pathway
• For example: the peripheral nerve and receptor, the dorsal root or dorsal root
ganglian, the spinal cord grey matter, the ventral root, the peripheral nerve,
the neuromuscular junction, or the muscle
o Types of reflexes
• Superficial (skin and mucous membrane) e.g. corneal, nasal, anal
• Deep tendon or myotatic reflexes (e.g. biceps, achilles, patella, brachioradialis,
triceps, jaw)
• Visceral reflexes (e.g. light, accommodation, bladder)
• Pathological reflexes (e.g. Babinski - which we will talk about later in this
lesson)
• As physiotherapists most commonly assess deep tendon reflexes (also known
as myotatic, tendon reflexes or stretch reflexes)
▪ It involves neurons at a single level of the spinal cord - a segmental
spinal reflex
▪ And involves contraction of skeletal muscle that occurs when a muscle
is stretched (or lengthened)
• Describe the components of a reflex arc
o Sensory receptor
o Afferent/sensory neuron
o Synapse
o Efferent/motor neuron
o Effector (muscle/gland)
• Describe the reflex (reaction) time and its components
o Is the time delay between the stimulus and the response
o Includes conduction time for impulse plus synaptic delay
o Is shorter for monosynaptic pathways and longer for polysynaptic reflexes
• Describe the components of a stretch reflex arc
o Involves neurons at a single level of spinal cord
o Specific pattern of muscle contraction in response to a particular stimulus
o Examples: stretch reflex, withdrawal reflex
• Patellar tendon reflex (SC level - L2-L4)
• Spinal and somatic reflex
• Patella tendon is tapped sharply just below the knee
• Activated the stretch receptors in the muscle spindles of the quadriceps
muscles which responds with a rapid extension of the knee
• Achilles tendon reflex (SC level - S1-S2)
• Spinal and somatic reflex
• Sharply tap the Achilles tendon when foot is relaxed
• Gastrocnemius contracts, causing plantarflexion
• This reflex aids balance during standing
• Withdrawal reflex (dependent on upper or lower limb)
• Crossed extensor reaction - limb on opposite side extends to support
body weight
• e.g. standing on sharp object on left foot
• Left foot lifts up, right leg tenses and extends for more balance
• Plantar reflex (SC level - L5, S1)
• Toes flex when plantar of foot is rigorously stroked
• Babinksi's sign - fanning out of toes
• The neural pathways between higher centres (in the brain) and
the peripheries aren't fully developed - hence they cannot walk
o Contraction of skeletal muscle that occurs when a muscle is stretched (lengthened)
o Important in maintaining normal muscle tone/length and posture; prevents
overstretching
o Detected by a proprioceptor in the muscle; (stretch receptor = muscle spindle)
o Components
• Sensory receptor - muscle spindle (signals a change in muscle length)
• Afferent neuron - Ia and II afferent fibres
• Integration at the segment of the spinal cord
• Efferent neuron - α motor neuron
• Effector - skeletal muscle fibres
o Monosynaptic loop
• Stretch of the muscle belly containing the muscle spindle initiates AP in the Ia
afferent (threshold dependent)
• Ia afferent synapses with the α motor neuron, if reaches threshold then an AP
results in the α motor neuron
• Integration by the α motor neuron
• α motor neuron brings about muscle contraction via the neuromuscular
junction, results in a brief muscle contraction (phasic response) - relieves the
stretch
• Reaction time between muscle stretch and response includes nerve
conduction time plus synaptic delay
Document Summary
Reflexes: what is a reflex, reflexes are subconscious stimulus-response mechanisms. That makes them mostly a very objective part of the neurologic exam: reflexes are, an involuntary response to a specific sensory stimulus, can be protective (e. g. avoiding hazardous situations withdrawal reflex) For example: the peripheral nerve and receptor, the dorsal root or dorsal root ganglian, the spinal cord grey matter, the ventral root, the peripheral nerve, the neuromuscular junction, or the muscle: types of reflexes. Is the time delay between the stimulus and the response. Includes conduction time for impulse plus synaptic delay. Is shorter for monosynaptic pathways and longer for polysynaptic reflexes: describe the components of a stretch reflex arc. Involves neurons at a single level of spinal cord: specific pattern of muscle contraction in response to a particular stimulus, examples: stretch reflex, withdrawal reflex, patellar tendon reflex (sc level - l2-l4)
