Sensory physiology: principles Flashcards
efferent nerves/ descending pathways: define
- somatic motor neurons/ autonomic moto neurons
- relay signals form CNS towards an effector (organ, muscle, gland)
afferent nerves/ ascending pathways: define
- sensory neurons
- relay signals from sensory receptors in PNS towards the CNS
general visceral afferents:
- provide input to brain about viscera
- processed mostly subconsciously for homeostasis
special visceral afferents:
- nerves that develop in association w GIT and carry info from ‘special senses’ of smell and taste via CN I (olfactory), VII (facial), IX (glossopharyngeal), V (trigeminal), X (Vagus)
general somatic afferents:
- transmit sensation arising from body surface (touch, pressure)
- proprioceptive info from mm, joints, skin, inner ear
special somatic afferents:
- nerves carry info from special senses vision, hearing, balance via CN II (optic) and VIII (vestibulocochlear)
sensory transduction: is
- conversion of physical/ chemical stimuli from int/ext environment into neural signals that can effect physiological/ behavioural response
sensory transduction: features
- sensory receptors respond to specific modality (energy form)
- transduce stimulus into form used by nervous system
- can be simple (mechanical opening of ion channel)
- transduction process can have amplification stage, esp if transduction is mediated by metabotropic/ G protein coupled receptor (eg. olfactory receptors)
sensory transduction: pathway
stimulus - receptor - transduction - transmission - perception
adequate stimulus: modality define
- form of energy particular receptor can respond to
adequate stimulus: define
- modality to which particular receptor responds best
exteroceptors: are
respond to stimuli orginating outside body
exteroceptors: list
- mechanoreceptors
- photoreceptors
- chemoreceptors
- thermoreceptors
- magnetoreceptors
- electroreceptors
interoceptors: are
- respond to stimuli originating inside body
interoceptors: list
- chemoreceptors (blood O2, CO2, pH)
- baroreceptors (BP)
- osmoreceptors (osmotic pressure - fluid balance)
proprioceptors: are
provide info about posture and position of body
proprioceptors: list
mechanoreceptors
- proprioception (joint/ limb position)
- vestibular system (balance/ motion)
sensory receptor type 1:
- some sensory receptors formed from specialised ending on primary afferent sensory neuron
- stimulus cause membrane ion channel to open/ close
- graded potential (generator) -> can initiate AP in same cell
- depolarising potentials will increase rate of AP generation
sensory receptor type 2:
- some sensory receptors are specialised cells separate from primary afferent neuron
- graded potential generate in receptor cell (non-spiking)
- voltage gated Ca channels open/ close and increase/ decrease rate of NT release
- depolarising/ hyperpolarising graded potential is induced in primary afferent neuron -> increase/ decrease rate of firing AP
trigger zones:
- multipolar neurons: AP at axon hillock/ initial segment where axon joins soma
- primary sensory afferent typically psuedounipolar cells -> dendrites (peripheral sensory) and axon terminal (central synaptic end)
- trigger zone close to receptors/ dendrites (peripheral end)
trigger zones: necessary why
graded potentials would not be able to travel all the way to soma (in dorsal root ganglion)
receptor adaptation: features
- receptors transduce sensory stimuli and respond w graded potentials proportional to stimulus strength and duration
- some receptors respond w depolarisation of same magnitude for as long as stimulus is applied
- most receptors reduce magnitude if sustained stimulus
receptor adaptation: types
- tonic (slow adapting)
- phasic (rapid adapting)
tonic receptors: and eg
- provide continuous updates for long lasting stimulus
- muscle stretch (prevent over stretch)
- merkel’s (pressure)
- nociceptors
phasic receptors: and eg
- signal changes in stimulus intensity
- many respond to cessation of stimulus w off response
- olfactory receptors
- touch (pacinian)
stimulus intensity: rate (frequency) coding
- larger stimulus - higher magnitude of potential - higher frequency of NT release
stimulus intensity: population coding
- stronger stimulus activates more receptors (recruitment)
- more receptors of same afferent sensory neuron stimulated simultaneously
- receptors on additional afferent sensory neurons that converge on same area of brain simultaneously
stimulus localisation:
- sensory systems organised spatiotopically (touch, vision)
acuity will depend on
- size of receptive field/ unit
- density of sensory units (spatial coverage)
- degree of overlap in receptive fields of adjacent units
- lateral inhibition
define receptive field:
- physical area within which receptors of single sensory unit can detect stimuli
overlap of receptive field:
- stimulus falling on overlapping portion of RF of two sensory receptors will stimulate them both neurons simultaneously (population coding)
lateral inhibition: features
- increases signal contrast for easy comparison
- enhance transmission of focal signals, suppress transmission of diffuse signals
- eg. of presynaptic inhibition
lateral inhibition: centre surround RF
- creates functional RF that has:
- excitatory centre
- inhibitory surround
eg. tactile acuity:
- ability to perceive two points pressed in skin
- depends on density, RF size, overlap of tactile receptors (mechanoreceptors)
- meissner’s corpuscles, merkel’s discs involved in fine touch perception and discrimination (small RF)
two point discrimination threshold:
- min distance btw points required for tactile discrimination
sensory afferent neuronal pathways have two routes:
- part of reflex arc
- relayed upward via ascending pathways -> further processing and possible conscious awareness
sensory association areas of cortex:
- identification and interpretation sensory info provided by primary cortical area
- integration, association (memory) and behaviour
define perception:
- awareness and interpretation of sensory info
perception: labelled lines
- sensory pathways that transmit info regarding particular modality are specific to that modality
Muller’s law (law of specific nerve energies):
- sensation invoked by signals generated in receptor is determined by part of brain that they ultimately activate
law of projections:
- regardless of where a particular sensory pathway is stimulated along its course
- conscious sensation generation is referred to location of receptor
law of projections: eg.
- phantom limb pain
- L arm and chest during heart attack
- pain signals from visceral pain receptors converge on same spinal neurons as somatic pain receptors
inadequate stimulus of sufficient strength may stimulate receptors to respond to inappropriate modalities:
- excessive pressure (mechanical energy) on eye stimulate retinal photoreceptors that normally detect light
- visual phenomenon (seeing stars)
consequences of Muller’s law 2: synaesthesia
- stimulation of one sensory modality eg. vision, evokes perceptual experiences in another, eg. hearing
- cross talk btw sensory pathways in brain (hearing colours, seeing or smelling sounds, seeing letters as colours)
