Flashcards in Peripheral Nervous System - Afferent Div (230 #6) Deck (102):
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
out conscious interpretation of the external world as created by the brain from a pattern of nerve impulses delivered to it by sensory receptors.
change detectable by the body - modalities include heat, light, sound, pressure and chemical changes. Energy -> electrical signals = transduction
each type of receptor is specialized to respond more readily to one type of stimulus.
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.
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.
receptive to both heat and cold
sensitive to specific chemical changes - smell, taste, chemical content of digestive tract, O2 and CO2 in the blood.
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
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)
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.
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
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.
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.
convey conscious somatic sensation, consisting of discrete chains of neurons (labelled lines), synaptically interconnected in a particular sequence
1) first-order - receptor
2) second-order - spinal cord/medulla
3) third order - thalamus
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
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.
circumscribed region of the skin surface surrounding a somatosensory neuron. The small the field, the greater the discriminative ability or acuity.
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).
an unpleasant sensory and emotional experienve associated with actual or potential tissue damage, or described in terms of such damage.
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.
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
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
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.
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.
abnormal persistant sensation of pain in the absence of painful stimuli - results from damage within the pain pathways in the PNS or CNS.
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.
anterior cavity between cornea & lens - clear watery fluid that is continually formed and carries nutrients to the cornea and lens
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
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
specialized anterior derivative of the choroid layer, forms a ring around the outer edge of the lens - produces aqueous humour and contains ciliary muscle
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.
anterior clear outermost layer of eye - contributes most extensively to eye's refractive ability
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.
inner layer of nerve cells in retina - important in retinal processing of light stimulus, forms optic nerve. First sign of APs in visual pathway.
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
between aqueous humour and vitreous humour, attaches to ciliary muscle by suspensory ligaments - provides variable refractive ability during accomodation
area immediately surrounding the fovea - has high acuity because of abundance of cones
leaves each eye at optic disc (blind spot) - first part of visual pathway to the brain
anterior round opening in middle of iris - permits variable amounts of light to enter eye
innermost layer of eye - contains the photoreceptors (rods and cones)
photoreceptors in outermost layer of retina - responsible for high-sensitivity, black and white and night vision. About 100 million per retina, mostly in the periphery. Much convergence in the rod pathways, more than 100 rods may converge via bipolar cells on a single ganglion cell.
tough outer layer of eye - protective connective tissue coat, forms visible white part of eye, anteriorly specialized to form cornea
suspended between ciliary muscle and lens - important in accomodation
between lens and retina - semifluid, jelly-like substance that helps maintain spherical shape of eye.
Protective Mechanisms of Eye
1) eyelids- shutters to protect eye from environment.
2) tears - produced by lacrimal duct, drains into tiny canals in corner (to back of nasal passageway), or down face when overflowing.
3) eyelashes - trap fine, airborne debris.
aqueous humour is not drained as rapidly as it forms dues to blockage of drainage (canal of Schlemm) and it pushes the lens back into the vitreous humour, which is pushed into inner neural layer of retina (retinal/optic nerve damage)
1) individual packets of energy called photons
2) dist between two peaks is wavelength
3) amplitude is intensity
4) divergent light rays entering eye must be bent inward to be focussed back to a focal point
5) bending is called refraction
6) convex (outside of ball)
7) concave (curves inwards)
the curvature of the cornea is uneven, so light rays are unequally refracted. The cornea contributes most extensively to eye's refractive ability due to diff in difference in density at the air-cornea interface.
the ability to adjust the strength of the lens, depending on the shape, regulated by the ciliary muscle. CM is circular ring of smooth muscle attached to lens by suspensory ligaments
1) muscle is relaxed, ligaments are tight and lens is pulled into flattened weakly refractive shape - FAR vision
2) when muscle contracts, ligaments relax, lens becomes more strongly spherical (inherent elasticity) - NEAR vision
sympathetic NS causes relaxation, parasympathetic NS causes contraction.
age-related reduction in accomodative ability. Mature lens cells cannot regenerate or repair themselves, lens cells in the centre of the lens are the oldest and farthest away from aqueous humour. Central cells die and become stiff.
elastic fibres in lens that are normally transparent become opaque so that light rays cannot pass through
normal eye - a far light source is focused on the retina without any accommodation, while the strength of the lens is increased to bring a near source into focus.
the eyeball is too long or the lens is too strong - a near light source is brought into focus on the retina without accommodation, but a far light source is focused in front of the retina and is blurry - better near vision than far vision - corrected with a concave lens
either the eyeball is too short or the lense is too weak - far objects are focused on the retina only with accommodation where near objects are focused behind the retina even with accommodation and the result is blurry. Better far vision than near, and correctible with a convex lens.
leading cause of blindness in Western Hemisphere. loss of photoreceptors in the ML in association with advancing age.
undergo chemical alterations when activated by light.
1) opsin - protein of disc membrane
2) retinene - derivative of vit A bound within interior of opsin. Rhodopsin - absorbs all visible wavelengths (in rods). In cones, Red, Blue & Green are absorbed.
converting light stim into electrical signals. Photoreceptors hyperpolarize, instead of depolarize, when excited. In bright light, activated photopigment activates transducin which in turn releases enzyme phosphodiesterase, which degrades cGMP. This closes Na+ channels, causing hyperpolarization. In the dark, cGMP is fine, so Na+ channels are open and the cell is depolarized. Photoreceptors are INHIBITED by their adequate stimulus (hyperpolarized by light) and excited in the absence of stimulation (depolarized by darkness).
go from bright sunlight to dark - can't see anything at first. Photopigments in bright light get broken down, decreases photoreceptor sensitivity. They are gradually rejuvenated.
go from dark to bright - you are dazzled. some photopigments are broken down by intense light, sensitivity of eye decreases and normal contrasts can once again be detected.
result of dietary deficiency of vit A, since retinene is a derivative of vit A and used for night vision.
the perception of the many colours of the world depend on the three cone types' various ratios of stimulation for RGB. Distinct colour vision centre in the primary visual cortex processes the inputs.
when individuals lack a particular cone type, colour vision is a product of only two types of cones.
the field of view that can be seen without moving the head.
1) the image detected on the retina at the onset of processing is upside down and backwards.
2) info tx'ed to brain from retina in not a point-to-point record of photoreceptor activation.
3) various aspects of visual info (form, colour, depth, movement) are separated and projected in parallel pathways to different regions of the cortex.
4) due to wiring, the left half of cortex rx'es info from right half of field as detected by both eyes, and vice versa.
on-centre ganglion cell
increases its rate of firing when light is most intense at the centre of it's receptive field (middle of the donut is lit up).
off-centre ganglion cell
increases its rate of firing when light is most intense at the periphery of it's receptive field (donut is lit up). Enhances difference in light level between one small area at the centre of a receptive field and the illumination immediately around it.
info is separated as optic nerves meet at optic chiasm, underneath hypothalamus. Fibres from medial half of each retina cross to the opp side, but those from lateral half stay on the same side. Reorganized bundles of fibres are OPTIC TRACTS.
fibre bundles that go to different zones in the cortex, directed by the lateral geniculate nucleus in the thalamus (first stop for visual pathway in the brain). 30% of the cortex neurons participate in processing visual input (hundreds of millions).
binocular field of vision / depth perception
information from the two eyes are not identical - slightly different vantage point. Overlapping area is known as binocular FOV - important for depth perception
double vision - when the disparate views from both eyes are seen simulaneously:
1) eyes are not both focused on same object simulaneously
2) binocular info is improperly integrated during visual processing.
visual cortical neurons
1) simple & complex - stacked on top of one another in the cortical columns of the primary visual cortex
2) hypercomplex cells are located in the higher processing area.
Some fibres terminate elsewhere for nonsight activities:
1) contribution to cortical alertness and attention
2) control of pupil size
3) control of eye movements (six external eye movements).
2) External Auditory Canal (Ear Canal)
3) Tympanic Membrane (Eardrum)
Inner Ear: Cochlea
1) Oval Window
2) Scala Vestibuli
3) Scala Tympani
4) Cochlear Duct (Scala Media)
5) Basilar Membrane
6) Organ of Corti
7) Tectorial Membrane
8) Round Window
Inner Ear: Vestibular Apparatus
1) Semicircular Canals
skin-covered flap of cartilage located on each side of head - collects sound waves and channels them down the ear canal; contributes to sound location
External Auditory Canal (Ear Canal)
tunnels from the exterior through the temporal bone to the tympanic membrane - directs sound waves to tympanic membrane; contains filtering hairs and secretes ear wax (cerumen) to trap foreign particles
Tympanic Membrane (Eardrum)
thin membrane that separates the external ear and the middle ear - vibrates in synchrony with sound waves that strike it, setting middle ear bones in motion
inside is exposed to atmospheric pressure via the 'eustachian tube' which connects middle ear to pharynx. Opening it (yawning) allows the air pressure in middle ear to equalize.
Malleus, Incus, Stapes (ossicles)
movable chain of bones that extends across the middle ear cavity; malleus attaches to the tympanic membrane and stapes attaches to the oval window - oscillate in synchrony with tympanic membrane vibrations and set up wave-like movements in the cochlear perilymph at the same frequency. At > 70dB, tiny muscles tighten to restrict ossicular movement to diminish tx of loud sounds.
thing membrane at the entrance to the cochleal separates the middle ear from the scala vestibuli - vibrates in unison with movment of stapes, to which it is attached; oval window movement sets cochlear perilymph in motion
upper compartment of the cochlea, a snail-shaped tubular system that lies deep within the temporal bone - contains perilymph that is set in motion by oval window movement driven by oscillation of the middle ear bones
lower compartment of the cochlea - contains perilymph that is continuous with the scala vestibuli
Cochlear Duct (Scala Media)
middle compartment of the cochlea; a blind-ended tubular compartment that tunnels through the centre of the cochlea - contains endolymph; houses the basilar membrane
forms the floor of the cochlear duct - vibrates in unison with perilymph movements, bears the organ of Corti, the sense organ for hearing
Organ of Corti
rests on top of the basilar membrane throughout its length - contains hair cells, the receptors for sound; inner hair cells undergo receptor potentials when their hairs are bent as a result of fluid movement in cochlea.
stationary membrane that overhangs the organ of Corti and contacts the surface hairs of the receptor hair cells - serves as the stationary site against which the hairs of the receptor cells are bent and undergo receptor potentials as the vibrating basilar membrane moves in relation to this overhanging membrane
thin membrane that separates the scala tympani from the middle ear - vibrates in unison with fluid movements in perilymph to dissipate pressure in cochlea; does not contribute to sound reception
three semicircular canals arranged 3D in planes at right angles to each other near the cochlea - detects rotational or angular acceleration or deceleration
sac-like structure in a bony chamber between the cochlea and semicircular canals - detects::
1) changes in head position away from vertical
2) horizontally directed linear acceleration and deceleration
lies next to the utricle - detects:
1) changes in head position away from horizontal
2) vertically directed linear acceleration and deceleration
the faintest sound that can be heard:
1) freq = 20-20000 cyc/sec, but best 1000-4000
2) loudness = 0-150dB
16000 hair cells in inner cochlea are in 4 II rows, 1 inner, 3 outer. From the surface of each hair cell are 100 hairs called stereocilia, which contact tectorial membrane. Hair cells generate neural signals when surface hairs are mechanically deformed.
Inner Hair cells communicate via a chemical synapse with the terminals of afferent nerve fibres making up the auditory (cochlear) nerve.
Outer hair cells actively and rapidly change length (electromotility) to accentuate movement of basilar membrane.
Mechanism of taste
Each taste bud has a small opening called a taste pore
Taste receptor cells are modified epithelial cells with many surface folds or microvilli that protrude through the pore,
Plasma membrane of cell contains receptor sites that bind to tastants to produce depolarization
Taste receptor cell
Synapse with afferent nerve fibre
Brain stem -> hypothalamus & limbic system for behavioral stuff
Cortical gustatory area (somatosensory cortex)
5 Primary Tastes
1) salty – chemical salts, Na+ produces receptor depolarization
2) sour – acids with H+, blocks K+ produces receptor depolarization
3) sweet – glucose, activates G protein, turns on cAMP 2nd messenger
4) bitter – alkaloids, poison, activates G protein (gustducin), turns on 2nd messenger
5) umami – amino acids (glutamate), activates G protein 2nd messenger
3 cm2 patch of mucosa in ceiling of the nasal cavity:
1) olfactory receptor cells – afferent neuron whose receptor portion lies in the OM and the axon transverses into the brain.
2) basal cells (precursors for new receptor cells)
3) supporting cells (secrete mucus)
receptor portion is an enlarged knob with several long cilia like a tassel to the surface, containing binding sites for odourants (activates 2nd messenger)
5 million olfactory receptors (1000 different types)
Afferent fibres (olfactory verve) -> tiny holes in flat bone plate
Synapse in Olfactory Bulb (size of grape, one on each side) with mitral cells inside of the ball-like neural junctions called glomeruli (smell files)
1) subcortical route to limbic (primary olfactory cortex). Includes hypothalamic involvement
2) route through thalamus to cortex – conscious perception and fine discrimination of smell.
Quickly adaptive by OLFACTORY RECEPTORS
Enzymes clear smells away after binding with receptors.