Reflexes and the somatosensory system Flashcards
What does monosynaptic mean?
Only one synapse is involved
What do extensor and flexor muscles do?
Extensor- straighten/extend limbs
Flexor- bend/reduce angle at joints of limbs
What is the myotactic/stretch reflex?
Tapping on the knee stretches the thigh extensor muscle and associated tendon
Sets in motion process to correct the stretching
Important in maintaining posture and is part of the proprioceptive system
Steps in the myotactic reflex
Sensory fibres sense muscle stretch and send signals to SC
Direct monosynaptic connection to motor neuron fires AP which contracts bicep muscle
Simultaneously an inhibitory interneuron ihibits firing of motor neurons connected to the antagonistic muscle to relax it
What is reciprocal inhibition?
The relaxing of the antagonistic muscle to the one that is being contracted to accommodate the contraction
What are muscle spindles?
Special sensory receptor that detects muscle stretch
One of the body’s many proprioceptors
Found in striated muscle, most abundant in areas of fine motor control
What are muscle spindles innervated by? What do they provide feedback to?
Ia sensory fibres
Alpha motor neurons (MNs innervating the muscles around the spindle)
What are gamma motor neurons?
Stimulate intrafusal muscle fibres to adjust tension in the muscle spindle as the extrafusal muscle fibres of surrounding muscle contract so that the spindle is never slack
What is a golgi tendon organ (GTO)?
Another kind propioceptor that detects muscle tension due to muscle contraction, not muscle stretch (which is detected by muscle spindles)
What happens when GTO sensory afferents are activated?
Activation of inhibitory interneurons which inhibit motor neurons that innervate the same muscle
So GTO reflex is a negative feedback circuit that regulates muscle tension to protect it from damage when large force is applied
What is the flexor or withdrawal reflex?
Quick contraction of flexor muscles to withdraw a limb from an injurious stimulus (e.g. heat or cut)
Arises from stimulation of nociceptors
Polysynaptic despite speed of response- parallel after discharge circuit
What is a parallel after discharge circuit? Why is this useful?
When a sensory neuron outputs to several other neurons that converge back onto a singular neuron
Initial signal is sustained over an extended period
What is the crossed extensor reflex?
Provides postural support during withdrawal reflex as rapid withdrawal of a limb would cause imbalance
What happens during the crossed extensor reflex?
Stimulus > sensory neuron activates inhibitory and excitatory neurons
Ipsilateral neurons to flexor are excited and cause ipsilateral flexor to contract
Contralateral neurons to extensor are excited and cause contralateral extensor to contract
What is the difference between cranial and spinal reflexes?
Cranial reflexes are processed/controlled by the brain, happen in the head region and use cranial nerves
Spinal reflexes are processed/controlled by the spinal cord, happen in the rest of the body and use spinal nerves
Example of a spinal monosynaptic reflex and a spinal polysynaptic reflex
Mono- Myotactic/stretch reflex
Poly- Flexor/withdrawl reflex
What are central pattern generators?
Local circuits that can generate the pattern of alternating flexion and extension that we see in locomotion
Rhythmic neuron firing
Severing spinal cord connections to the brain doesn’t stop alternation
Where are messiner/tactile corpuscles located? What modality do they detect?
Dermal papillae of skin (palms, eyelids, lips, tongue)
Light touch and texture
Where are pacinian/lamellated corpuscles located? What modality do they detect?
Dermis, joint capsules and viscera
Deep pressure, stretch, tickle, vibration
Where are ruffini corpuscles located? What modality do they detect?
Dermis, subcutaneous tissue, joint capsules
Heavy touch, pressure, skin stretch, joint movements
What sensory receptors are encapsulated nerve endings?
Messiner/tactile corpuscles
Pacinian/lamellated corpuscles
Ruffini corpuscles
Where are merkel/tactile discs located? What modality do they detect?
Superficial skin (epidermis) Light touch. textures, edges, shapes
Where are free nerve endings located? What modality do they detect?
Widespread in epithelia and connective tissues
Pain, heat, cold
Where are hair follicles located? What modality do they detect?
Widespread in epithelia
Varied according to type
What sensory receptors are unencapsulated nerve endings?
Merkel/tactile discs and free nerve endings
Why are some sensory receptors encapsulated?
Capsules are involved in filtering to give specific frequencies
What sensory receptors are phasic?
Messiner corpuscles
Pacinian corpuscles
Some hair folicle subtypes
What sensory receptors are tonic?
Ruffini corpuscles
Merkel discs
Some subtypes of hair follicles
What are the sub types of primary afferent axons from the skin in order from fastest to slowest?
What are they called from the muscles?
A (split into alpha, beta, gamma), B, C
I, II, III, IV
How is sensory information organised in the spinal cord?
Cell bodies of different sensory neurons are grouped in the dorsal root ganglion
Their projections are organised into different layers of the dorsal horn
Spatially organised
What are the medial lemniscal tracts?
Carry mechanoreceptive and proprioceptive signals to the hypothalamus
What is the spinothalamic tract?
Carries pain and temperature signals to the thalamus
What does it mean when a neuron is commissural?
Crosses the midline (2nd order)
What do first order neurons do?
Detect the stimulus and transmit it to the spinal cord
What do second order neurons do?
Relay the signal to the thalamus as it is the gateway to the cortex
In both medial lemniscal and spinothalamic tracts they cross the midline
What do third order neurons do?
Carry the signal from the thalamus to the cortex
What is topological organisation?
Spatial arrangement of the objects relative to one another
In the ML system, what pathway do 1st order neurons from the upper body follow? Where do they synapse onto 2nd order neurons?
Lateral pathway
Cuneate nucleus
In the ML system, what pathway do 1st order neurons from the lower body (below T6) follow? Where do they synapse onto 2nd order neurons?
Medial pathway
Gracile nucleus
What are the dorsal column nuclei?
Cuneate nucleus and gracile nucleus
In the ML system where do 2nd order neurons cross the mid line? What does this mean in terms of their topology?
Medial lemniscus
Reversed- upper body neurons are medial and lower body neurons are lateral upon entering the thalamus
In the ML system, where do 3rd order neurons from the upper body synapse onto?
Lateral part of somatosensory cortex
Topology reversed again
In the ML system, where do 3rd order neurons from the lower body synapse onto?
Medial cortical neurons
Topology reversed again
Why are projections topologically organised like this?
Forms a map of the body in the somatosensory cortex due to the DRG being organised into dermatomes
What is a dermatome? How do these arise?
Specific region of skin innervated by a sensory ganglion
The dermis of each region is derived from its somite
What is a somite? How do these develop into dermatomes?
Iterated embryonic structures that give rise to the underlying musculature and skeleton
In the embryo, each sensory ganglion (DRG) is associated with a specific somite and subsequently innervates the tissues arising from that somite
How many pairs of dorsal roots are there?
31
How can you measure the size of a receptive field?
By assessing the ability to discriminate two sharp points set apart at different distances
Why is more cortex dedicated to areas where receptive fields are smaller?
Where receptive fields are large, discrimination is low (legs and arms), whereas where receptive fields are small, discrimination is high (fingers)
The number of sensory neurons innervating an area depend on the behavioral importance of that area
How are sensory modalities organised in the postcentral gyrus?
Different sensory modalities are localised along the sagittal axis
What is the specificity theory?
Pain is a distinct sensation, detected and transmitted by specific receptors and pathways to specific pain regions of the brain
What is convergence theory?
Pain is an integrated, plastic state represented by a pattern of convergent somatosensory activity within a distributed network
What are the different types of nociceptors? What do they detect?
A delta fibres- fast, lightly myelinated, mechanosensitive and mechanothermal sensitive
C fiibres- slow, unmyelinated, polymodal (mechanical, thermal, chemical)
What kind of receptor are nociceptors?
Afferents with free nerve endings
How does the pain detected by A and C fibres differ?
A- fast, immediate pain ‘first pain’
C- slow, more diffuse, longer lasting, ‘second pain’
What is TRPV1 and when is it activated?
Capsaicin receptor for A and C fibres
activated when temp is 45 degrees or above and by capsaicin- vanilloid which is the active component in chilies
What are the two pathways that carry pain information to the brain?
Sensory discriminative (spinothalamic tract) Affective motivational
What information does the sensory discriminative pathway carry?
Location, intensity and type of stimulus
What information does the affective motivational pathway carry?
Signals unpleasantness and enables autonomic activation, fight or flight response
What is the difference between the two pain pathways in terms of organistion?
SD- topologically preserved
AM- no topographic mapping
What parts of the brain are used in the AM pathway?
Parabrachial nucleus- responds to painful stimuli anywhere on the body’s surface
Cingulate cortex- strong correlation of unpleasant experience
Limbic system
What is hyperalgesia? What is the mechanism for this?
Increased response to a painful stimulus
Result of lowered nociceptor thresholds which heightens pain repsonse
What is allodynia? What is the mechanism for this?
Painful response to a normally innocuous stimulus
Activity dependent local release of substances like prostaglandins
Relay neurons become sensitive to non-painful stimuli
What is hyperpathia?
Variant of hyperalgesia and allodynia
Fibre/axonal loss or damage resulting in a higher detection threshold
So when threshold is reached excitability is much greater and pain is explosive
What is central sensitisation?
When central pathways themselves are damaged
Diabetes, shingles, multiple scerosis, stroke
What phenomena don’t fit into specificity theory?
Neuropathic pain
Referred pain
Central and external modulation of pain
Placebo effect
What are upper motor neurons? Where are they found?
Neurons found in the brain that control motor function
Primary motor cortex, precentral gyrus
How is the motor cortex somatotopically mapped?
Lower body is medial, upper boddy is lateral
Proportions reflect density of innervation and behavioral significance
What do axial muscles control?
Trunk movement
What do proximal muscles control?
Movement of shoulders, elbows, pelvis, knees
What do distal muscles control?
Movement of hands, feet and digits
What is a motor unit?
A motor neuron and all the muscle fibres it innervates
How are motor pools organised in the spinal cord?
The motor pool for a single muscle are grouped in rod shaped clusters extending over several vertebral segments
Somatotopically organised- medio-lateral position indicates whether it innervates a more proximal or distal muscle
What is a motor pool?
All the motor neurons that innervate a single muscle
What descending tract is key for voluntary movement?
Corticospinal tract
Where do the axons of the CST originate in the brain?
Large pyramidal cells in layer V of the motor cortex
What layer of the motor cortex do main inputs come into?
Stellate cells in layer IV
What is the difference between motor cortex upper neurons and brain stem upper neurons?
Motor cortex- primarily concerned with fine voluntary control of distal structures
Brain stem- project to medial motor pools primarily concerned with postural movement
Where do axons of the CST cross the midline? Where does that mean they project?
Pyramidal decussation in the medulla
Where do axons of the lateral CST synapse onto?
Laterally located lower motor neuron circuits that control distal muscles
Where are upper motor neurons from the brain stem found?
Ipsilaterally in the vestibulospinal tract and the reticulospinal tact
How do neurons in the vestibulospinal and reticulospinal tracts project? Where do they synapse onto?
Project medially in the spinal cord
Synapse on medially located motor neuron circuits that control axial muscles
What are the ventromedial pathways?
Vestibulospinal tract
Reticulospinal tract
What do neurons in the vestibulospinal tract control?
Head balance and turning
