Reflex movement Flashcards
What are the 3 types of movements we can initiate?
involuntary actions (reflexes), semi-automatic actions, voluntary actions
What parts of the CNS are involved in reflexes/involuntary actions?
spinal cord (spinal nerves) or brainstem (cranial nerves)
What type of response is initiated by reflexes?
stereotyped response (unlearned behavioural reaction)
How many synapses are often involved in a reflex arch?
one - monosynaptic circuit
What is the consequence of a monosynaptic circuit for involuntary actions?
response is rapid - short latency between stimulus onset and response
What is the common final pathway for motor pathways?
motor neurons
What are the 2 possible regions motor neuron cell bodies are located?
ventral horn of spinal cord (spinal nerves / somatic muscles) or brainstem (cranial nerves)
What can activate motor neurons?
various brain regions in the cortex or various peripheral receptors
What activates a reflex response?
various peripheral receptors
What makes up the central core of the spinal cord?
grey matter
How is grey matter in the spinal cord divided?
dorsal, intermediate and ventral horns
What part of the grey matter do sensory afferents terminate in?
dorsal root
What part of the grey matter do motor efferents exit from?
cell body in the ventral horn
What nerve contains both sensory afferents and motor efferents?
peripheral nerve
Where are the sensory endings located in the stretch reflex pathway?
muscle spindles
What are muscle spindles composed of?
sensory endings (mechanoreceptors) wrapped around intrafusal fibres. Sensory endings fuse together to give rise to group 1a afferent fibres
What class of axon is involved in reflex pathways?
Aa fibres (group Ia afferent)
In which state are muscle spindles in when they have a steady rate of AP firing?
relaxed - maintains muscle tension
Which state are muscle spindles in when no AP is generated?
Contracted as the sensory endings are relaxed
Which state are muscle spindles in when a high frequency of APs are conducted along the group Ia afferent fibre?
stretched - distorts sensory endings wrapped around the extended intrafusal fibres
Examples of stretch reflex pathways
knee-jerk, bicep jerk, ankle jerk, triceps jerk, rectus abdominis reflex
How is the stretch reflex pathway activated in the knee-jerk reflex?
mechanical stimulus is applied by tapping the patella tendon which stretches the muscle spindles in the quadriceps triggering APs
Describe the sequence of events in a stretch reflex pathway
Mechanical stimulus activates mechanoreceptors. Transduction into APs conducted along group 1a muscle afferents. Sensory afferent enters dorsal horn and continues into ventral horn where it synapses with motor efferents (Aa fibre). Transmits AP to synapse where effector is recruited and response results.
What is the effector in the knee-jerk reflex?
quadriceps (same as muscle being stretched)
Where do the sensory afferent and motor efferent synapse in reflex pathways?
in ventral horn
What features of the reflex pathway enables a rapid response?
Aa fibres have the largest diameter and thickest myelin sheath resulting in fastest conduction speed and the pathway in monosynaptic
Which neurotransmitter is released in the motor end plate of stretch reflex pathways?
ACh
How can you investigate the physiological features of the stretch reflex?
by recording an electromyogram (EMG) using surface electrodes
Which muscle is an EMG recorded for the jaw jerk reflex?
masseter
Which muscle is an EMG recorded for the ankle jerk reflex?
gastrocnemius muscle
Outline the events that take place during the jaw jerk reflex
- Hammer taps chin
- spindles of jaw elevator muscles are stretched
- AP conducted along group 1a muscle afferents to brainstem (Aa)
- monosynaptic activation of motor neurons of jaw elevator muscles
- AP travel along motor axon (Aa)
- Synapse to recruit effector
- Jaw elevator muscles contract
- Jaw jerks upwards, closing mouth
What are the jaw elevator muscles?
masseter, temporalis, medial pterygoid
Describe the sequence of events in the ankle jerk reflex
- Hammer taps Achilles’ tendon
- Stretches spindles within gastrocnemius
- AP conducted along group 1a muscle afferents to spinal cord (Aa)
- monosynaptic activation of motor neurons of gastrocnemius
- AP travel along motor axon (Aa)
- Synaptic recruitment of effector
- Gastrocnemius contracts
- foot extends (plantar extension)
Describe the appearance of an electromyogram (EMG) of electrical activity in skeletal muscle
stimulus artefact followed by latency of response and then a peak and trough conveying the amplitude of response
What is the cause of the stimulus artefact?
tapping of the tendon with hammer
What is the latency of response?
the time delay between the onset of the stimulus and the onset of the bidirectional response
Which axon type do both the sensory afferents and motor efferents have in the stretch reflex pathway?
Aa axon
Which has the shorter latency between jaw jerk and ankle jerk reflexes?
jaw jerk reflex has a shorter latency
Why does the jaw jerk reflex have a shorter latency than the ankle jerk reflex?
the jaw jerk neuronal pathway is shorter (masseter -> brainstem -> masseter vs gastrocnemius -> spinal cord -> gastrocnemius)
Which section of the spinal cord is involved in the ankle jerk reflex?
sacral
Which nerve is involved in the ankle jerk reflex?
tibial nerve
Amplitude definition
measure of electrical activity in skeletal muscle
Which out of the jaw jerk and ankle jerk reflexes has the greater amplitude?
ankle jerk reflex has a greater amplitude
Why is the amplitude of the ankle jerk reflex greater than the jaw jerk reflex?
gastrocnemius muscle is larger than the masseter and therefore there is a greater recruitment of muscle fibrils in the gastrocnemius
Which receptors are involved in somatic reflexes?
proprioceptor endings, mechanoreceptor endings, nociceptor endings, chemoreceptors, photoreceptors
Which reflex is constantly happening along the vertebral column?
tendon-jerk reflexes
Function of tendon-jerk reflexes
maintain posture and muscle tone
What are the components of the tendon-jerk reflex?
proprioceptors, group 1a muscle afferents, homonymous motor neurons
Which receptors are involved in the cutaneous reflexes?
mechanoreceptor endings, nociceptor endings
What is the importance of having protection mechanisms (activated by somatic reflexes)?
- escape mechanism
- prevent muscle overloading
- prevent inadvertent ingestion of a foreign body
- digestive aid
Which reflex provides an escape mechanism?
flexion withdrawal reflex
What is the stimulus for the flexion withdrawal reflex?
noxious stimulus (recruits free nerve endings of Ad / C fibres)
Which pathways are activated by noxious stimuli?
spinothalamic pathway (to perceive pain) and flexion withdrawal reflex to move away
How many synapses are involved in the flexion withdrawal reflex?
disynaptic pathway (afferent, interneuron, efferent)
Describe the pathway of the flexion withdrawal reflex
noxious stimulus recruits Ad / C afferent axons which enter the dorsal horn and synapse with an interneuron (divides). On branch of the interneuron recruits a motor efferent in the ventral horn which results in the effector contracting.
Why does the interneuron in the flexion withdrawal reflex split?
one branch transmits the impulse to higher centres to reinforce spinothalamic pathway and allow momentary control to make a decision. the other branch projects into the ventral horn to recruit motor neurons for the withdrawal reflex
Example of the importance of the branching of the interneuron to higher centres
if you pick up a boiling pot of water, the momentary control allows you to throw the water away from you rather than immediately letting go
Why is the latency of response of the flexion withdrawal reflex slower than a monosynaptic reflex?
because there a 2 synapses (disynaptic pathway)
Which muscle structure is associated with group 1a afferents?
muscle spindles
Which muscle structure is associated with group 1b afferents?
Golgi tendon organs
Where are the receptors responsible for prevention of muscle overloading located?
in tendon (Golgi tendon organs)
What is the name of the reflex that prevents muscle overloading?
inverse myotactic reflex (inverse stretch reflex)
How does the inverse myotactic reflex work?
receptors are activated by stretch and contraction which sends signals along group 1b afferents which inhibits motor neuron activation hence preventing overloading of muscle
Which structure detects the load of the biting force as there are no Golgi tendon organs in jaw muscles?
periodontal ligament
How is the overloading of teeth prevented?
mechanoreceptors in PDL detect increasing load and send impulses along afferents which inhibit motor neurons of jaw elevator muscles
What condition may increase the bite force?
anaesthesia of molars (no impulses sent along sensory afferents that innervate PDL)
What is the gag reflex called?
pharyngeal reflex
Function of pharyngeal reflex
prevents unintentional ingestion of a foreign body
How is the pharyngeal reflex activated?
mechanical stimulus activates mechanoreceptors in posterior tongue / soft palate
Which 2 possible nerve can contain the sensory fibres in the pharyngeal reflex?
glossopharyngeal (CN IX) or trigeminal (CN V) nerve
Where does the afferent nerve (CN V or IX) terminate in the pharyngeal reflex?
in the spinal trigeminal nucleus
Where is the spinal trigeminal nucleus located?
in the lower medulla of the brainstem
Which nerve is responsible for the motor function of posterior oral and pharyngeal muscles?
vagus nerve (CN X)
What is the name of the vagus motor nucleus?
nucleus ambiguus
Where is the nucleus ambiguus located?
in the upper medulla of the brainstem
Describe the pathway of the pharyngeal reflex
mechanoreceptors detect the stimulus and recruit afferent fibres (CN V or IX) which terminate in the spinal trigeminal nucleus where it synapses with an interneuron. Interneuron projects upwards towards the nucleus ambiguus where it bilaterally activates vagus motor neurons. This results in bilateral contraction of posterior oral and pharyngeal muscles.
Why does bilateral contraction occur in the pharyngeal reflex despite only one side being stimulated?
to increase chance of expelling foreign object (interneuron stimulates bilateral vagus efferents)
Example of a reflex that involves glands as the effector organ
salivary reflex
What is the purpose of the salivary reflex?
digestive aid
What are the possible stimuli for the salivary reflex?
gustatory, visual, olfactory stimuli and chewing forces (mechanical stimuli)
What is the outcome of activation of the salivary reflex?
increased salivary flow
