Peripheral Nervous System - Afferent Div (230 #6) Flashcards
sensory afferent
1) somatic sensation arising from the body surface (including somesthetic sensation from skin and proprioception from muscles, joints, skin, inner ear)
2) special senses - vision, hearing, taste and smell
perception
out conscious interpretation of the external world as created by the brain from a pattern of nerve impulses delivered to it by sensory receptors.
stimulus
change detectable by the body - modalities include heat, light, sound, pressure and chemical changes. Energy -> electrical signals = transduction
adequate stimulus
each type of receptor is specialized to respond more readily to one type of stimulus.
photoreceptors
responsive to visible wavelengths of light. 1) Rods - more sensitive, don’t detect color, adapted for low light.
2) Cones - adapted to detect color and work well in bright light.
Made of three parts:
1) outer segment - detects light stim, closest to eye’s exterior, facing choroid. Stacks of flat membranous discs with photopigment molecules.
2) inner segment - metabolic machinery of cell
3) synaptic terminal - facing bipolar cells, closest to eye’s interior. Tx’s the signal to bipolars.
mechanoreceptors
sensitive to mechanical energy - stretching muscle fibres, bending hair cells, blood-pressure monitoring baroreceptors. Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs and Ruffini corpuscles.
thermoreceptors
receptive to both heat and cold
chemoreceptors
sensitive to specific chemical changes - smell, taste, chemical content of digestive tract, O2 and CO2 in the blood.
osmoreceptors
changes in concentration of solutes in the body fluids and resultant changes in osmotic activity
nocioreceptors (pain receptors)
sensitive to pressure and tissue damage, such as pinching or burning or to distortion of tissue.
Uses for Afferent Receptor Info
1) controlling efferent output - maintaining homeostasis, regulating motor behavior, etc
2) processing of sensory activity by reticular activating system for cortical arousal and consciousness
3) perception of the world around us
4) stored for future reference
5) profound impact on emotions
receptor potentials
a graded potential whose amplitude and duration can vary based on the strength and the rate of application of removal of the stimulus. No refractory period.
Receptors can be:
1) a specialized ending of the afferent neuron (generator potential - opens voltage-gated Na+ channels)
2) a separate cell closely associated with the peripheral ending of the neuron (receptor potential - cell sends chemical messenger to open chemically-gated Na+ channels)
Action Potentials are initiated at the peripheral end of an afferent nerve fibre (not axon hillock)
adaptation
receptors diminish the extent of their depolarization despite sustained stimulus strength - freq of AP in the afferent neuron decreases. receptor no longer responds to it to the same degree. Not the same as habituation! Adaptation is receptor adjustment in PNS, habituation is change in synaptic effectiveness in CNS.
tonic receptors
do not adapt at all, or adapt slowly. In situations where there is value to maintain info about a stimulus - muscle stretch, joint proprioceptors, etc
phasic receptors
rapidly adapting receptors, no longer respond to a maintained stimulus. Once the stim is removed, the receptor typically responds with a slight depolarization call the ‘off response’. Include tactile receptors in skin.
Pacinian Corpuscle
rapidly adapting skin receptor that detects pressure and vibration. Consists of concentric layers of connective tissue around peripheral terminal of afferent neuron (like an onion). The terminal responds to the stimulus, but as it continues, the pressure energy is dissipated because it causes the receptor layers to slip - filters out steady component of applied pressure, receptor no longer responds. Also, Na+ channels are slowly inactivated, reducing inward flow that caused depolarizing receptor potential.
somatosensory pathways
convey conscious somatic sensation, consisting of discrete chains of neurons (labelled lines), synaptically interconnected in a particular sequence
labelled lines
Sensory neurons:
1) first-order - receptor
2) second-order - spinal cord/medulla
3) third order - thalamus
etc…
different types of incoming information are kept separate within specific ‘labelled lines’.
stimulus modality - type of receptor activated + specific pathway tx’ed to cerebral cortex
stimulus location - activated receptor field + specific pathway to somatosensory cortex
stimulus strength - freq of APs in each aff neuron + # of receptors activated
phantom pain
pain perceived as originating in the fot by person whose leg has been amputated. Activation of a sensory pathway at any point gives rise to the same sensation that would be produced by stimulation of the receptors in the body part itself.
receptive field
circumscribed region of the skin surface surrounding a somatosensory neuron. The small the field, the greater the discriminative ability or acuity.
lateral inhibition
occurs via inhibitory interneurons that pass laterally between ascending fibres serving neighbouring receptive fields - blockage of weaker inputs increases the contrast between wanted and unwanted info so info can be precisely localized (no extra info from adjacent receptive fields).
pain
an unpleasant sensory and emotional experienve associated with actual or potential tissue damage, or described in terms of such damage.
Nocioreceptors:
1) mechanical - crushing, cutting, pinching
2) thermal - temp extremes (esp heat)
3) polymodal - all types, including irritating chemical from injured tissues.
Do not adapt! Sensitized (receptor threshold lowered) by prostglandins (fatty acid released from plasma membrane of damaged tissues that act locally). Aspirin inhibits prostglandin synthesis.
fast pain
1) occurs on stim of mech and thermal nocioreceptors
2) carried by small myelinated A-delta fibres
3) produces shart prickling sensation
4) easily localized
5) occurs first
slow pain
1) occurs on stim of polymodal nocioreceptors
2) carried by small unmyelinated C fibres
3) produces dull aching burning sensation
4) poorly localized
5) occurs second - persists for longer time, more unpleasant
bradykinin
normally inactive substance that is activated by enzymes released into ECF from damaged tissue. Provoke slow pain (by stim polymodal), and contribute to inflammatory response.
capsaicin
peripheral receptors of C fibres are activated by it - binds with pain receptors and thermal receptors. Local application can actually reduce clinical pain since it overstimulates and damages the nocioreceptors with which it binds.
Substance P + Glutamate
after primary afferent pain fibres synapse with second-order interneurons in the dorsal horn of the spinal cord.
Substance P activates pathways that tx nocioceptive signals to higher levels.
1) cortex - localize the pain, deliberation about the incident
2) thalamus - pain is perceived without cortex, also interconnects with hypothalamus and limbic system for behaviour/emotion
3) reticular formation - increases level of alertness with the noxious encounter
Glutamate is excitatory, binds with AMPA to tx, binds with NDMA to increase Ca2+ entry and make the neuron more excitable. Partly contributes to sensitivity of injured areas.
neuropathic/chronic pain
abnormal persistant sensation of pain in the absence of painful stimuli - results from damage within the pain pathways in the PNS or CNS.
analgesic system
suppresses tx inthe pain pathways as they enter the spinal cord by blocking Substance P from pain fibre terminals.
1) periaqueductal grey matter (around cerebral aqueduct)
2) stimulation of the reticular formation in the brain stem.
Depends on opiate receptors. Morphine is synthetic, but like endorphins, enkephalins and dynorphins, which are part of the natural body’s analgesic system. Released by the descending analgesic system and bind with opiate receptors - suppress release of Substance P by presynaptic inhibition.
aqueous humour
anterior cavity between cornea & lens - clear watery fluid that is continually formed and carries nutrients to the cornea and lens
bipolar cells
middle layer of nerve cells in retina - important in retinal processing of light stimulus. They are inhibited by neurotransmitter release from photoreceptor synaptic terminal. Removal of the inhibition has the same effect as direct excitation of the bipolar cells. Graded potentials.
blind spot / optic disc
point slightly off centre on retina where optic nerve exists, devoid of photoreceptors - route for passage of optic nerve and blood vessels
choroid
middle layer of eye - pigmented to prevent scattering of light rays in eye, contain blood vessels that nourish retina, anteriorly specialized to form ciliary body and iris
ciliary body
specialized anterior derivative of the choroid layer, forms a ring around the outer edge of the lens - produces aqueous humour and contains ciliary muscle
cones
photoreceptors in outermost layer of retina - responsible for high acuity, colour and day vision. Light-sensitive ends face the choroid (away from incoming light). About 3 million per retina - most abundant in macula lutea. Little convergence of neurons in the retinal pathway for cone output - each one generally has a ‘private line’ to a ganglion cell.
cornea
anterior clear outermost layer of eye - contributes most extensively to eye’s refractive ability
fovea
exact centre of retina - region with greatest acuity. Pinhead-sized depression, bipolar and ganglion cells layers are pulled aside so that the light strikes the photoreceptors directly - only cones here.
ganglion cells
inner layer of nerve cells in retina - important in retinal processing of light stimulus, forms optic nerve. First sign of APs in visual pathway.
iris
visible pigmented ring of muscle within aqueous humour - varies size of pupil by variable contraction, responsible for eye colour.
1) circular or constrictor muscle contraction occurs in bright light to block light
2) radial or dialator muscle shortens in dim light to let more light in
lens
between aqueous humour and vitreous humour, attaches to ciliary muscle by suspensory ligaments - provides variable refractive ability during accomodation
