Physiology of the Spinal Cord Flashcards
What is the detection of a stimulus by a receptor?
Sensation
What is the interpretation of a sensation by the brain in alignment with existing emotions and memories?
Perception
What is a nociceptor?
A receptor that is sensitive to harmful stimuli
What do nociceptors respond to?
Chemical
Thermal &
Mechanical
Stimuli (that is harmful)
What type of receptors in joints are used for proprioception?
Mechanoreceptors
Which receptors respond to touch and pressure?
Merkell’s
Which receptors respond to deep pressure & fast vibration?
Pacinican
Which receptors respond to skin stretching?
Ruffini’s
Which receptors respond to texture and slow vibration?
Meissner’s
Which receptors respond to cold?
End bulbs of Krause
What happens which hair are moved in their follicles?
They directly stimulate underlying nerves.
Which receptor is a stack of epithelial cells enclosed in connective tissue sheath?
Meissner’s corpuscle
Which receptor is a lamellae of flattened cells? (Looks like an onion)
Pacinian
How do receptors tell the brain there is a stimulus?
They detect the stimulus - convert it into neural activity by generating an AP (all or nothing).
What is movement of a stimulus from a receptor to the brain known as?
Stimulus transduction
How do Pacinian corpuscles work?
Layers of flattened cells - deep pressure/vibration distorts the structure = AP - travels to dorsal root ganglion.
When receptors have to transmit messages further - what is their shape like?
Larger & myelinated
Which receptors are the fastest?
The ones which detect proporioception
Which receptors detect pain?
Bare nerve endings
What do mechanosensory neurons detect?
Proporioception
Vibration
Light touch
Where do the mechanosensory neurons ascend & synapse?
Ascend in dorsal columns
Synapse with 2nd order neuron in medulla.
Ascend same side as synapse.
Where do pain & temperature neurons ascend and synapse?
Synapse at level of entry into the spinal column, cross midline and then ascend in lateral spinothalamic tract (opp side to synapse).
Which part of the nervous system is responsible for:-
- Simple responses
- More complex responses
- Control of complex motor skills & movements?
Simple = spinal cord level (reflex)
More complex = Brainstem
Complex motor = Cerebrum
How can sensory input be modulated?
It can be downgraded by messages sent down from the brainstem or from within the spinal cord itself.
E.g. Pain comes in from the periphery - but the brain sends messages that inhibits the pain stimulus and reduces the sensation of pain.
How do we discriminate between sensations?
Different receptor types respond to different specific stimuli = different perception of sensations.
How do we have 2 point discrimination?
To do with the spatial distribution of receptors and how many are present in that area of skin.
If there are lots of receptors you can discriminate between two things even when close together.
If there are less receptors - the distance between the two stimuli needs to increase in order for the brain to detect two differerent stimuli.
Where is the density of mechanoreceptors the greatest?
The hands and face
How does the body detect different temperatures?
Have cold and warm receptors.
When cold - the cold receptors fire more impulses = signal of cold environment.
When warm - the warm receptors fire more impulses.
Where are the cell bodies of lower motor neurons found?
In the anterior horn of the grey matter in the spinal cord.
In which part of the spinal cord are the anterior motor neurons found?
Ventral horn
Name two types of lower motor neurons
Alpha motor neurons
Gamma motor neurons
What is a motor neuron + the muscle fibre called?
Motor unit
Why will damage to a single spinal nerve not cause complete paralysis?
Because each muscle receives motor fibres from more than one ventral root (i.e. is innervated by more than one nerve)
What are groups of cells sitting close to motor neurons called that are responsible for lateral inhibition?
Renshaw cells
What is lateral inhibition?
A way of sharpening or focussing a signal to a muscle. Renshaw cells transmit inhibitory signals to surrounding motor neurons - modifying their action.
What are the types of reflexes that we have?
Simple
Complex (polysynaptic) reflexes
What do mechanoreceptors detect?
Forces generated within the body = proprioception
Name two types of mechanoreceptors for proprioception & what they respond to.
Muscle spindles (respond to muscle length)
Golgi tendon organs (in muscle tendons - respond to tension)
Which three parts of the CNS are responsible for controlling muscles?
Spinal cord
Cerebellum
Cerebral cortex
Is integration between all of these.
How do muscle spindle receptors respond to stimulus?
Emit messages continually & are stimulated by stretch.
When stretched - the impulses they send increase.
When there is sudden stretch movement - is dynamic jump in signal frequency.
If gradual increase in stretch - the impulses increase proportionally to the stretch.
Where do messages from muscle spindle fibres go in a monosynaptic pathway?
Spindle activated - message sent through sensory nerve (cell body in dorsal root ganglion) - to grey matter of spinal cord.
Here it synapses with α motor neuron - which activates causing contraction of the muscle.
Give an example of a reflex that is a monosynaptic reflex from the muscle spindle receptors.
Knee jerk reflex
How does the knee jerk reflex work?
Dynamic stretch (caused by hitting patella tendon) - stretches quadriceps muscle = dramatic increase in messages sent from muscle spindle receptors via sensory nerve to cell body in dorsal root ganglion.
Sensory nerve synapses with lower motor neuron (in anterior horn LMN) - which sends messages back to muscle causing it to contract.
What happens if you hit the tendon again?
There is a static stretch reflex = longer but weaker reflex (protective mechanism).
What is a Golgi tendon organ?
A sensory receptor through which muscle tendon fibres pass
What do golgi tendon organs detect?
Muscle tension
Where are golgi tendon organs found?
In the ends of the muscles
How do golgi tendon organs work?
Muscle is stretched = increased tension - detected by GTO - messages sent by sensory nerve fibre (cell body in dorsal root ganglion) - into grey matter of spinal cord - stimulates an interneuron (inhibitory) - reduces the message and sends inhibitory response back through the anterior motor neuron = relaxation.
What do golgi tendon organs protect against?
Excess tension
Given an example of a golgi tendon organ reflex.
Clasp Knife reflex
How does the clasp knife reflex work?
Golgi tendon organ responds to tension in the muscle (not stretch) - sends messages via the sensory nerve to the spinal cord - activating the inhibitory interneuron - which reduces messages back to the α motor neuron = reduction in activity of that muscle
What is the clasp knife reflex sometimes called?
The inverse stretch reflex
What type of feedback / control determines
- Where we want our muscles to be
- How they are maintained in this position
Where we want them to be = higher control
Maintaining them in position = negative feedback loops
Give an example of a reflex that utilises a polysynaptic reflex pathway
Flexor reflex
What do flexor reflexes allow?
Flexion of muscle to allow withdrawal from pain
What does the flexor reflex do?
Activates flexor circuits whilst inhibiting extensor circuits to allow limb withdrawal
What type of neurons allow connection with the other side of the body?
Why is this important in flexor reflex?
Interneurons
Stabilises the body where there is a sudden response.
When the flexor reflex is activated, which other reflex is activated?
The crossed extensor reflex
How can the knee jerk reflex be used clinically?
Assess how sensitive stretch reflexes are.
How is the knee jerk reflex affected when there is
(a). lower motor neuron disorder
(b). upper motor neuron disorder
(a). Reduced
(b) Exaggerated
How are reflexes important clinically when there has been a cord transection?
Sensory fibres in feet & joints can allow leg extension (which can hold weight) - meaning there can be rhythmic stepping reflexes even though the messages are not reaching the brainstem - this happens through the spinal cord alone.
What is the clinical implication of reflexes when there has been a bone fracture?
The body detects pain - reflexes then kick in to try and keep the area still = muscle rigidity & spasm - however this in itself can cause even further pain to the patient.
