Spinal Reflexes and the Somatosensory System Flashcards
what is a reflex?
- rapid, involuntary, stereotyped and coordinated response to a sensory stimulus
- can involve muscular contraction and glandular responses
- can be learned (Pavlovian) or unlearned (involving the somatic nervous system)
what are spinal reflexes?
- somatic reflexes involving spinal cord circuitry
- require stimulation and sensory input
- quick: a few synapses are involved
- involuntary: we are aware as they happen, but they are hard to suppress
- little input from higher centres
- stereotyped: occur in the same way each time
what is the patellar/stretch reflex?
- monosynaptic
- motor neuron stimulates extensor muscle contraction
- NOT IN RESPONSE TO PAIN
- tap on the knee stretches the thigh extensory muscle and associated tendon
- the reflex is a motion to correct the stretching
- important in maintaining body posture
- part of the proprioceptive system
what is the patellar/stretch reflex?
- monosynaptic
- motor neuron stimulates extensor muscle contraction
- NOT IN RESPONSE TO PAIN
- tap on the knee stretches the thigh extensory muscle and associated tendon
- the reflex is a motion to correct the stretching
- important in maintaining body posture
- part of the proprioceptive system
what is the process of the stretch/myotactic reflex?
- Muscle spindle senses muscle stretch and send signals to spinal cord
- monosynaptic connection to motor neuron in spinal cord fires AP which contracts the bicep
- Simultaneously, a connection to an inhibitory interneuron inhibits firing of APs in motor neurons connected to the triceps
- the antagonist muscle is relaxed
- reciprocal inhibition
what is reciprocal inhibition?
- the stretch of one muscle inhibits the activity of the opposing muscle
- Reciprocal inhibition prevents muscles from working against each other when external loads are encountered
what are muscle spindles?
- proprioceptors
- detect muscle stretch
- found in most striated muscle
- abundant in muscles involved in fine motor ocntrol
- innervated by type Ia sensory fibres
- Ia sensory fibres input to alpha motor neurons which provide a response in the muscle
do muscle spindles have muscles themselves?
yes
- muscle spindles are also innervated by gamma motor neurons
- gamma motor neurons stimulate intrafusal muscle fibres to adjust the tension in the muscle spindle while the extrafusal muscle fibres of the surrounding muscle contract
- ensures the spindle is never slackened
what is kinesthesia?
- loss of a sense of your body moving in a space
what is the Golgi Tendon Organ (GTO)?
- proprioceptor
- detects muscle tension during contraction
- GTO sensory Ib afferents being activated causes activation of inhibitory interneurons
- the interneurons inhibit alpha motor neurons that innervate the same muscle
- GTO reflex uses negative feedback that regulates muscle tension
- protects muscle from damage when large forces are generated
- prevents muscle contracting too much
what is the difference between the muscle spindle and the GTO?
Muscle spindle detects stretch in muscle
GTO detects tension in muscle
what is the flexor withdrawal reflex?
- quick contraction of flexor muscles to withdraw a limb from an injurious stimulus
- results from activation of nociceptors
- polysynaptic reflex
- activation of multiple excitatory neurons sustains the response: parallel after-discharge circuit
what is the parallel after-discharge circuit?
- stimulus input takes different periods of time to reach output neuron depending on the number of interneurons
- ensures the initial signal is sustained over an extended period
what 2 other responses are involved in the flexor withdrawal reflex?
- reciprocal inhibition
- inhibitory interneurons are activated to relax extensor muscles - crossed extensor reflex
- due to rapid withdrawal of the limb causing imbalance, flexor reflexes involve contralateral element
- crossed extensor reflex provides postural support during limb withdrawal
what is the process of the flexor withdrawal reflex combined with crossed extensor reflex?
Flexor withdrawal reflex:
- stepping on glass stimulates nociceptors in right foot
- sensory neuron activates multiple excitatory and inhibitory interneurons
- ipsilateral motor neurons to the flexor are excited
- ipsilateral flexor contract
- motor neurons to extensor are inhibited
Crossed extensor reflex:
- contralateral motor neurons to extensor are excited
- contralateral extensor (left leg) contracts to maintain balance
- motor neurons to contralateral flexor are inhibited
what are central pattern generators?
- extensors and flexors in limbs are alternately active or inhibited during walking
- activity of muscles in different limbs is coordinated so that legs alternate
- even if spinal cord connection to brain is severed, the alternation of limb movement is maintained
- local circuits are present to generate alternating pattern of flexion and extension
what is the cellular basis of pattern generators?
- different neurons exhibit distinct patterns of rhythmic firing
- there is inhibition across the midline
how is C. elegans a model organism for proprioception?
- TRP-4 stretch/mechanosensor allows DVA neuron to detect worm body bending
- the DVA neuron inhibits alternating motor neurons, allowing alternating muscle contraction
mutagenesis identified a mutant with loss of TRP-4 stretch receptor, causing the worm to have loopy swimming
what are the two inputs into body sense?
- mechanical stimuli
2. painful stimuli and temperature
what are Meissner (tactile) corpuscles?
location:
- dermal papillae of skin e.g. palms, eyelids, lips, tongue
modality:
- light touch
- texture (movement)
sensitive to 30-50Hz
rapidly adapting
what are Pacinian (lamellated) corpuscles?
location:
- dermis, joint capsules, viscera
modality:
- deep pressure
- stretch
- tickle
- vibration
sensitive to 250-350Hz
rapidly adapting
what are Ruffini corpuscles?
location:
- dermis, subcutaneous tissue, joint capsules
modality:
- heavy touch
- pressure
- skin stretch
- joint movements
a proprioceptor
slowly adapting
what are Merkel (tactile) discs?
location:
- superficial skin (epidermis)
modality:
- light touch
- texture
- edges
- shapes
slowly adapting
what are free nerve endings?
location:
- widespread in epithelia and connective tissues
modality:
- pain
- heat
- cold
what are hair follicles?
location:
- widespread in epithelia
modality:
- varied according to type
there are both rapid and slowly adapting subtypes
what are the encapsulated nerve endings?
- Meissner corpuscles
- Pacinian corpuscles
- Ruffini corpuscles
what are the unencapsulated nerve endings?
- Merkel discs
- free nerve endings
what is a cellular receptor?
- device made of cells that detects changes in the body or the environment
what is a molecular receptor?
- molecule located on surface of cell with transmembrane linkage to cytoplasm
- detects changes in molecular environment e.g. hormones, growth factors
are all sensory receptors neurons?
no:
- some specialist cell types like photoreceptors, auditory/vestibular hair cells and taste receptors synapse with primary afferent neuron
some receptor cells can be neurons however e.g. olfactory receptor cells
what are rapidly adapting/phasic receptors?
- give info about changes in the stimulus
- e.g. Pacinian corpuscle
what are slowly adapting/tonic receptors?
- continue to respond as long as stimulus is present
- gives info about persistence of stimulus
- e.g. Ruffini corpuscle
how are axons classified?
- according to the conduction velocity
- this relates to diameter: the faster the axon, the larger the diameter
how are axons from the skin classified?
A, B and C:
- A is fastest and largest
- C is slowest and smallest
A is broken down by Greek letters, alpha, beta, delta
- alpha is fastest
- delta is slowest
how are axons from the muscles classified?
Roman numerals: I, II, III, IV
- I = largest and fastest
- IV = smallest and slowest
are pain fibres slower or faster than proprioceptors?
slower
how is sensory information organised in the spinal cord dorsal horn?
Topological organisation:
- cell bodies of different sensory neurons are grouped in the DRG
- projections are organised into different layers of the dorsal horn
- e.g. info from different classes of hair follicle is represented in different layers
sensory info remains spatially organised as it is sent to the brain
what are the two main routes of somatosensory projections to the brain?
- Medial Lemniscal Tracts
2. Spinothalamic tract
what is the medial lemniscal tract?
- carries mechanoreceptive and proprioceptive signals to thalamus
- projects into spinal cord before going to brain
what is the spinothalamic tract?
- carries pain and temperature signals to the thalamus
- goes to the neuron immediately in the brain at the midline
what are the 3 neurons that sensory info travels through to reach the higher centres?
- First order neurons:
- detect stimulus and transmit to spinal cord (DRG neurons - second order neurons:
- neurons that relay signal to the thalamus - third order neurons:
- carry the signal from the thalamus to the cortex
what is special about second order neurons?
- they cross the midline so are commissural (ipsilateral to contralateral)
how are axons of the Medial Lemniscal system topologically organised?
Upper body: 1st order axons follow lateral pathway and synapse to 2nd order neurons in cuneate nucleus
Lower body: `1st order axons follow medial pathway and synapse to neurons in gracile nucleus
2nd order axons cross midline and ascend in medial lemniscus
- topology is reversed so that lower body axons are lateral when reaching thalamus
3rd order axons reverse topology so lower body axons synapse to medial cortical neurons, while upper body axons go to lateral cortex
how are axons of the Medial Lemniscal system topologically organised?
Upper body: 1st order axons follow lateral pathway and synapse to 2nd order neurons in cuneate nucleus
Lower body: `1st order axons follow medial pathway and synapse to neurons in gracile nucleus
2nd order axons cross midline and ascend in medial lemniscus
- topology is reversed so that lower body axons are lateral when reaching thalamus
3rd order axons reverse topology so lower body axons synapse to medial cortical neurons, while upper body axons go to lateral cortex
what are the dorsal column nuclei?
- cuneate nucleus
- for upper body axons in lateral pathway synapsing to 2nd order neurons - gracile nucleus
- for lower body axons in medial pathway synapsing to 2nd order neurons
what is topological organisation?
spatial arrangement of objects relative to one another
where is the somatosensory cortex is the brain?
- sits posteriorly to the central sulcus
- postcentral gyrus
what is the role of the somatosensory cortex?
- produces a topological map of the body in the cortex
- the map is fine
- reflects the fact that each DRG innervates a dermatome (a specific domain of the body)
what is a dermatome?
- each sensory ganglion innervating a specific region of the skin
- regions arise due to the dermis of each region being formed from a somite
what is a somite and how does it relate to the specific innervation of sensory ganglions?
- somites form musculature and skeleton
- in the embryo each DRG (sensory ganglion) is associated with a specific somite, so innervates the tissues that arise from that somite
topological organisation is preserved in the spinal cord and the somatosensory projections
what do topological and topographical mean?
topological = the way in which constituent parts are interrelated/arranged
topographical = representation of distribution of parts/features on the surface of/within the organ or organism
where else are there topological maps in the nervous system
- in the thalamus, which acts as a way station between 2nd and 3rd order neurons
- in the dorsal column nuclei
is the topographic map representative of the physical size of the body parts it is relating to?
no
- the area occupied by different regions in the cortex is not proportional to their physical size
- e.g. area for hands is much larger than area for legs
what does the receptive field of a neuron depend on?
- where the neuron is in the body, as some regions have denser innervation than others
how can the size of a receptive field be measured?
Two point discrimination:
- assess the ability of the subject to discriminate two sharp points set apart at different distances
- if the subject feels two pin points, then the distance between the points is larger than the receptive field
allows determination of sizes of receptive fields in different parts of the body
how does receptive field size vary?
- where receptive fields are large, discrimination is low e.g. in arms and legs
- where receptive fields are small, discrimination is high e.g. fingers
how is the cortex involved in receptive fields?
- there is more cortex dedicated to regions where receptive fields are small
- in small regions, number of nerve endings from different neurons is higher, so there is more sensory input
- this results in larger number of axons per unit area being represented in the cortex
what does the number of sensory neurons innervating a particular area depend on?
the behavioural significance of that area
what does behavioural significance determine in the brain?
- cortical representation of certain body parts
- e.g. the representation of the hand is large in the human cortex, whereas in rodents, whiskers have a larger representation in the cortex
where is the somatotopic map preserved?
- in the coronal plane of the postcentral gyrus
where are sensory modalities located in the postcentral gyrus?
- along the sagittal axis
what are Brodmann areas?
- different regions that receive inputs from different types of sensory receptors
how is the cortical map plastic?
- neurons which are used more are strengthened
- neurons which are used less are eliminated
what experiment proves cortical map plasticity?
- digits of monkey are mapped onto different areas of the cortex
- digit 3 was removed
- over months, the areas dedicated to digit 2 and 4 spread to where the neurons for digit 3 were
- if the stimulus increased to the digits, the area in the cortex that represents those digits increases
