Sensory Systems pt 1 Flashcards
sensory pathway
stimulus –> receptor transduces stimulus into intracellular signal –> action potential (APs) travel along afferent neuron –> information reaches subcortical integrating/relay centers eg thalamus/cerebellum/medulla –> information reaches appropriate areas of cortex
true/false - all sensory receptors are the same
no, different level of complexity. some have free nerve endings, sometimes the nerve ending is inside of connective tissue, sometimes there’s a special sense receptor that the neuron will synapse with. sometimes myelinated, sometimes no myeline sheath.
what are the different types of receptors?
chemoreceptors - sense oxygen, pH, various organic molecules
mechanoreceptors - pressure, cell stretch, vibratin, acceleration,s ound
photoreceptors - light
thermoreceptors - heat
how does signal transduction happen?
via graded potentials
stimulus will open and close ion channels in the receptor membrane, either directly or via secondary messenger systems (mostly Na+ or Ca2+ flowing in, sometimes K+ flowing out)
the change in membrane potential = receptor potential
convergence
receptive fields for 1o neurons often overlap, where several converge into a single 2o neuron; this allows for the processing of larger receptive fields, but also less accurate fields
convergence accuracy
lowered accuracy as a result of lower two-point discrimination
as two signals fall within the same receptive field, they’re perceived as just one signal by the brain
would a smaller receptive field provide greater or lowered accuracy?
greater - one signal to one neuron
where is sensory information integrated? give examples.
CNS lol 1o somatosensory auditory olfactory visual
where is sensory information processed?
processed in the thalamus (hearing, vision, taste, somatic senses)
direct to brain = olfactory bulb –> olfactory cortex
cerebellum (equilibrium)
where is visceral sensory information processed?
integrated in brain and spinal cord
does not usually reach consciousness eg blood pressure
SOMETIMES somewhat conscious eg pressure (fullness) or pain
grey matter composition
sensory and motor nuclei
dorsal horn: somatic and visceral sensory (afferent)
lateral horn: autonomic efferent nuclei
ventral horn: somatic motor nuclei (efferent)
white matter composition
tracts - ascending and descending
ascending tracts
dorsal column: pain/touch/proprioception
spinocerebellar: proprioception
spinothalamic: pain/temperature
descending tracts
lateral corticospinal: voluntary movement
ventromedial/anterio corticospinal: fine voluntary movement + motor control of trunk muscles
how are different sensations distinguished
all AP are identical aka…CNS decodes it by:
- modality
- location
- intensity
- duration
how is sensory modality determined?
type of neuron activated + where the pathway terminates in the brain
known as “labelled line coding”
eg - touch receptors from a certain body part will project to a specific location in the somatosensory cortex
how does the brain translate special sense APs?
hair cells in ear respond to different frequencies, but no receptive fields relating to sound source location (brain figures location out on its own lol)
lateral inhibition
enhances contract, improvides stimulus localization
1o neuron will respond to stimulus in proportion w stimulus strength, but IMPORTANTLY: the pathway closest to the neighbours will inhibit neighbours, which will increase perception of stimulus
how is stimulus intensity determined?
- # of receptors activated (diff receptors have diff thresholds for stimulation - more receptors = higher threshold has been reached)
- higher frequency of APs = higher intensity, up to max.
tonic vs phasic receptors
tonic - slowly adapting, respond throughout stimulus
phasic - rapidly adapt, on and off
somatic senses: modalities
touch, temperature, nociception (pain/itch), proprioception
where do somatosensory pathways synapse
thalamus
which somatosensory pathways cross high? which cross low?
pain, temperature, coarse (crude) touch, cross midline of spinal cord
fine touch, vibration, and proprioception cross high - midline medulla
what are the different cutaneous receptors?
merkel's disk meissner's corpuscle ruffini corpuscle pacinian corpuscle free nerve endings
describe location / receptive field / adaptation / function of merkel’s disk
superficial / small / slow / sustained touch + pressure + texture
describe location / receptive field / adaptation / function of meissner’s corpuscle
superficial / small / fast / beginning and end of fine touch + pressure
describe location / receptive field / adaptation / function of ruffini’s corpuscle
deep / large / slow / sustained gross touch + vibration + stretch
describe location / receptive field / adaptation / function of pacinian corpuscle
deep / large/ fast / beginning and end of gross touch + vibration
describe location / receptive field / adaptation / function of free nerve ending
variable / variable / variable / pain + temperature + hair movement
nociception? what mediates it?
pain - sensation, not just stimulus
mediated by free nerve endings expressing ion channels that respond to a lot of strong stimuli (chemical/mechanical/thermal)
controlled by TRP (transient receptor potential) channels
what mediates pain? what is its relation to nociception?
pain is mediated via release of local chemicals - K+, histamine, prostaglandins, serotonin, substance P
will either activate nociceptors or sensitize them
TRP channels? where are they expressed? what do they mediate?
transient receptor potential channels
expressed on membrane of many diff cell types
mediate pain, heat, cold, some tastes, pressure, vision, osmotic pressure, stretch
TRPV
trp vanilloid - responds to temperature, pepper, garlic, cloves, thyme, oregano, wasabi, menthol, peppermint
TRPV1
heat, capsaicin, mustard, wasabi, H+
if somatic pain info is sent to cerebral cortex, where else will it be sent for info processing?
limbic system and hypothalamus for emotion reactions and autonomic responses
list the relative conductional velocities of various fibre types
alpha motor neuron = muscle spindle = golgi tendon organ > cutaneous receptors (fine touch) > gamma motor neurons > fast pain > slow pain
visceral pain - describe its localization
poorly localised, often perceived to be on surface of body rather than visceral organ
how are neural reflexes classified?
- effector (skeletal muscle = somatic motor neurons, or smooth/cardiac muscle/glands/adipose tissue = autonomic motor neurons)
- integrating centre (spinal reflexes vs cranial reflexes)
- innate vs learned
- # of neurons in pathway (monosynaptic - only afferent and efferent neurons for somatic motor reflexes only - or polysynaptic - involving interneurons, autonomic reflexes)
are all reflexes integrated in spinal cord? can spinal cord reflexes be affected by the brain?
no, cranial reflexes exist
spinal reflexes can be unlearned by signalling from higher centres
can emotional stimuli be converted to a physical response?
yeah, visceral responses
what do skeletal muscle reflexes monitor?
proprioception - positions of limbs in space, relative positions of body parts
effort exerted in lifting, holding objects
what is the integrating centre of skeletal muscle reflex?
CNS, via excitatory and inhibitory neurons
can be integrated in spine and/or higher brain regions
what is the efferent pathways of skeletal muscle reflex?
somatic motor neurons = ALPHA motor neurons
what is the effector of skeletal muscle reflex?
skeletal muscle fibres = EXTRAfusal muscle fibre
what are the different proprioceptors?
muscle spindle fibres - monitor muscle stretch
golgi tendon organ - monitor muscle tension
joint receptors - mechanical distortion as bones are positioned
what are proprioceptors?
receptors that sense a change in joint movements, muscle length, muscle tension and send info to CNS
what is the CNS response to proprioceptors
activate motor neurons or inhibit interneurons
where is the golgi tendon organ?
connecting the muscle and tendon
what do muscle spindles consist of?
3-12 intrafusal muscle fibres
arranged parallely to extrafusal muscle fibres
what are muscle spindles sensitive to?
stretch –> increased muscle length
are muscle spindles tonic or phasic?
tonic
what innervates extrafusal muscle fibres
alpha motor neuron
what innervates intrafusal muscle spindle fibres
efferent gamma motor neurons, tonically active sensory neurons (1a afferent)
what innervates the golgi tendon organ?
1b afferent sensory neurons
what do muscle spindles mediate?
stretch reflex, it’ll induce contraction when it’s stretched bc we want to maintain constant length for the muscle
what is alpha-gamma coactivation? when is it seen?
maintains the tension in muscle spindles - when alpha motor neurons will contract extrafusal fibres, the gamma motor neurons will also fire and conract the intrafusal fibres
what is the steps of the stretch reflex
- muscle stretched
- increased firing of 1a sensory neuron associated w spindle
- increased firing of ALPHA motor neuron to biceps
- bicep contracts
- increased firing of inhibitory interneuron
- decreased firing of ALPHA motor neuron to triceps
- antagonistic muscle relaxes
what are isotonic and isometric?
isometric - contration of muscle without change length
isotonic - contraction of muscle and changes length without changing tension
what stimulates golgi tendon organ to stretch?
CONTRACTION of muscle (isotonic or isometric - but isometric is more sensitive)
but golgi tendon organ is insensitive to muscle stretch
what does golgi tendon organ monitor
tension –> force of contraction
what are the steps of golgi tendon organ’s involvement in monitoring muscle tension
- increased muscle tension
- firing of 1b afferent neurons associated w GTO
- increased firing of INHIBITORY interneuron
- decreased firing of alpha motor neuron
- decreased muscle contraction (therefore lowered tension)
what are the purposes of the skeletal muscle reflexes?
stretch - maintenance of posture, positional info to CNS, incl. reciprocal inhibition
withdrawal - get away from painful stimulus while maintaining posture, incl. reciprocal inhibition and crossed extensor reflex
what is crossed extensor reflex?
inhibiting and conotracting antagonist muscles in order to maintain posture
how is overall position and posture coordinated
muscles can’t communicate w each other - all must communicate w CNS
what is muscle reflex?
mostly driven by external stimuli, inherent, handled at spinal cord or brain stem w modulation by higher centres
can voluntary movement be subconscious?
yes, over time
which brain structures have major roles in integration of muscle control?
cerebellum and basal ganglia
what are the three regions of ceberal grey matter
basal ganglia - coordination of movement
limbic system - linking emotion/fear w higher cognitive functions
cerebral cortex
parkinson’s disease
death of dopamine-secreting neurons in basal ganglia
motor symptoms = tremor at rest, slowness, rigidity
main treatment = L-DOPA
