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Flashcards in Sensory System Deck (62):

The sensory System - what it does

-General principles

-Detects changes in the external and internal environment
General Principles:
-Specialised receptor cell converts physical or chemical signal into electrical signal
-Electrical signal travels via PNS to CNS
-signals processed by CNS
-Efferent signals from CNS elicit appropriate response


Mechanoreceptors - how they work (generally)


-stretching of cell membrane causes opening of ion channels

e.g. Pressure and vibration
-balance (equilibrium)
-muscle length and tension
-joint position and movement



-How they work (general)


-Chemicals bind to specific receptors on cell membrane
-Open channels via secondary messengers
e.g. CO2, pH, various organic and inorganic molecules

*thermoreceptors operate in similar way, but they respond to temperature



-How they work

-Default position

-Respond to light
-When stimulated, initiate chain in chemical reactions terminating in breakdown of secondary messenger molecules and closure of ion channels
-Dark is default position
-Go out in sunlight = ion channels close


Sensory Neuron axon features

-Have a peripheral axon (where signal comes in) and a central axon (where signal goes out)


Sensory Transduction

-how works if sensory receptor is specialised nerve ending

-Stimulus opens ion channels, depolarising membrane and producing receptor (generator) potential)
-Receptor (generator) potentials are graded potentials - if afferent nerve sufficiently depolarised, APs generated, propagating to CNS.


Sensory transduction

-How works if receptor cell is separate from afferent nerve

-Stimulus changes membrane potential of receptor, opening or closing Calcium channels, increasing or decreasing calcium conc in cell
-Triggers or inhibits release of chemical transmitter
-signals receptor on afferent neuron
-excitory or inhibitory potentials generated in afferent neurons
-if sufficiently depolarised, APs generated, travelling to CNS


Sensory Systems in Vertebrates (3 broad types)

1. Somatosensory system (senses external enviro)
-e.g. mechanoreceptors in skin detect touch, stretch and vibration, muscles, tendons, joints
-nociceptors in skin detect tissue-damaging mechanical, thermal or chemical stimuli
2. Visceral sensory system (sense internal environment)
-e.g. mecahnoreceptors for blood pressure; chemoreceptors and nociceptors
3. Special sensory systems (sense external environment)
-involved in structures
-photoreception, mehcanoreception and chemoreception


CNS processing of sensory information

-Conscious and unconscious signal perception

-Where processed in brain

-Some are perceived at lvl of conscious awareness(goes to cortex);
-some somatic senses (touch, temp, conscious proprioception and noxious stimuli)
-Special senses (taste, smell, vision, hearing)
-Others processed at subconscious level (goes to cerebellum);
-Some propioceptive signals (eg. muscle length and tension)
-Signals from visceral sensory system (blood, pressure, body temp)


Sensory Coding

-4 things nervous system is able to identify

-When a stimulus sensory receptor, nervous system able to identify:
-Modality (Receptor type and signal pathway)
-Location (Receptive fields)
-Duration of stimulus (receptor adaptation)


Modality Receptor Type

-how it works

-Each type of sensory receptor responds only to specific form of energy (or modality)
-i.e. eye has photoreceptors
-modality to which receptor responds best is called adequate stimulus
-modalities other than the right stimulus may activate receptor, but only at high energy levels (e.g. getting hit hard in eye)


Modality: Labelled lines

-what it is

-each form of sensory stimulus follows fixed specific neural p/way to CNS
-same pathway activated every time
-p/way for each modality terminates in specific area of brain (if occurs in cerebral cortex, modality perceived)


Stimulus Location: Receptive fields

-2 ways stimulus localisation is enhanced

-In skin, stimulus localisation enhanced by;
-Smaller receptive fields
-Greater overlap of receptive fields of different afferent nerves

*lips more sensitive than back in humans
-difference in acuity differs over body surface and between sides


How is the intensity of a stimulus worked out?

-Action potential Rate and Burst duration
-Also, recruitment of additional neurons - stimuli of increasing intensity activates greater number of receptors
-may be within a single sensory unit of by stimulation of additional units


Stimulus duration: Tonic receptors

-what they are

-what suited for


-Most receptors adapt to stimulus
-w/ constant stimulus intensity, there is a decrease in magnitude of receptor potential and AP rate in afferent neuron
*tonic receptors adapt slowly -> are suited to signaling prolonged stimuli

e.g. tension receptors in tendons and stretch receptors in skin


Stimulus Duration: Phasic Receptors

-What they do
-what suited for

-Phasic receptors adapt rapidly
-Suited to detecting dynamic qualities of mechanical stimuli
-examples are Pacinian corpuscls in skin (detect high-frequency vibrations)


Sensor Receptors on Body surface

-Mechanoreceptors: detect various forms of mechanical energy, including pressure, vibration, touch and stretch
-Thermoreceptors: detect temperature
-Nociceptors: detect tissue-damaging (noxious) mechanical, therma and chemical stimuli.


Temperature Receptors

-how they work

-temperatures detected

-Free nerve endings (mainly in skin, lining or oral cavity and on surface of tongue)
-activation of receptor opens ion channels in cell membrane -> allows ions to enter cell, eliciting generator potential
*are separate receptors for cold, cool, warm and hot

In humans: perceived as thermal gadations from cold to hot (43 are tissue damaging)
-distribution of receptors not uniform
-cold receptors more superficial and in greater numbers


Thermo transient receptor protein
-what it is
-what else they can detect

Thermoreception in snakes

-what's special about it?

-Series of 6 temp.-activated ion channels called transient receptor potential (TRP)
-some also respond to chemicals
-TRPA1, TRPV1 and TRPV2 are nociceptive

In snakes;
-some snakes have highly sensitive thermoreceptors -> enables them to locate prey in darkness
-are in small pits in skin on eiyher side of head (in pit organs)


Nociceptors on Body surfaces

-what they are perceived as

-Free nerve ending receptors that respond to tissue-damaging stimuli
-in brain, signals perceived as pain
-activated by high intensity mechanical and thermal stimuli - most also w/ chemical


Proprioceptors in Muscles, tendons and Joints

-Where occurs
-where signal travels to (3)

-Mechanoreceptors in S.M., tendons and joints
-Detect changes in muscle length, changes in muscle tension and position of joints
-Signals go to cerebral cortex (conscious perception), spinal cord (generation of spinal reflexes) and unconscious area of brain (cerebellum)


Spinal Reflex

-info from somatosensory receptors travels to spinal cord (which acts as integrating centre)
-initiate reflex response w/out input from brain
-but also travel to brain where conscious perception occurs


Spinal reflexes: Knee jerk reflex

-Hammer tap stretches tendon, which, in turn, stretches sensory receptors in leg extensor muscle
-sensory neuron synapses w/ and excites motor neuron in spinal cord
-sensory neuron also excites spinal interneuron
-interneuron synapse inhibits motor neuron to flexor muscles
-Motor neuron conducts AP to synapses, causing contraction
-Flexor muscle relaxes b.c. activity of its motor neurons has been inhibited
-leg extends


Somatosensory Cortex

-how organised

-Sensory area of brain in which somatic sensations perceived
-info arising from adjacent areas of body register in adjacent areas of cortex
*size of specific area relates to sensitivity of body region

-areas close together on skin are close together in brain

-called somatrophic organisation


Chemoreceptors; Taste

-where found
-how organised

-Each taste bud contains 50-150 taste receptor cells (TRCs)
-found on dorsal surface of human tongue has ~5000 taste buds contained in 3 types of papillae
*taste buds on places other than tongue


Basic Tastes in Humans

-How we taste
-Flavours and places they are recognised

-Chemicals in food dissolve in saliva (how we taste)
-Probable that each TRC detects only 1 chemical
-signal TRC via specific receptor or ion channel
-Can taste all flavours everywhere, but some places are more sensitive


3 types of receptor cells in taste buds

-which one connects to afferent nerves

-Type 1: (support or glial cell) - cells probably detect Na+ ions via ion channel
-Type 2: (receptor) cells detect either sweet, umami or bitter tastes
-Type 3: (Presynaptic) cells detect sour taste
-only cells to synapse w/ afferent nerves, signalling via serotonin


Neural coding for Taste (Humans)

-Where on tongue tastes most sensitive

-Relation to brain

-All regions of tongue respond to 5 basic tastes (sweet, sour, bitter, umami
-Some regions more sensitive to certain tastes
-Tip of tongue most sensitive to salty, sweet and umami
-salty also on side of tongue
-bitter at back of tongue
-sour on sides
-Specific regions of cerebral cortex respond to specific tastes


Chemoreception: Smell (olfaction)

-How it works
-State of chemicals
-where travel
-What they are
-How they occur

-Chemoreceptor response depends on mechanisms similar to those involved in tastes
-chemicals must dissolve in mucus in nasal passage -> bind to specific chemoreceptors on afferent neurons
-signals travel to olfactory cortex where registered as various odours
-Olfactory receptor cells are neurons
-Receptor neuron cilia extend into layer of mucus lining nasal cavity
-Receptor neurons travel to olfactory bulb through series of small holes in holes in skull to synapses in olfactory bulb


Olfactory Neural Pathways

-Where signal goes

-From nose, signals travel to olfactory cortex where recognition of odour occurs
-olfactory signals also travel to parts of brain that register whether odour is pleasant or unpleasant
-also travel to parts of brain involved w/ emotion, memory and sex drive
-Establishes links between smell, memory, emotion and sexual behaviour


Species differences in olfactory Ability

-what greater smell in reflected by

-Vomeronasal Organ, what it is

-Domestic animals have much better olfactory ability than humans
-greater surface area of nasal cavity lining and in size of olfactory region of brain

Vomeronasal Organ: Accessory olfactory organ; detection of pheromones


Photoreception; Different types for different animals

-Single cell animals: retinal plate
-Flatworms: eyecup
-Higher vertebrates: camera eye (us included)
-Arthropods: Compound eye


Outermost layer of eye (2 parts)

-Sclera: Tough CT coat over majority of outside of eye (makes up "white")
-Cornea: Transparent structure in front of eye -> allows light to enter


Middle layer of eye (4 parts)

-Choroid: Vascular, pigmented layer under sclera
-provides blood to retina and stops reflection of light that reaches back of eye
-Lens: focuses light on the retina
-Ciliary Body: Contains ciliary muscles, which attach to lens by zonular fibres
-change shape of lens to focus light
-Iris: Located in front of lens; regulates amount of light entering eye by adjusting diameter of pupil


Inner Layer of eye (Retina)

-Fovea and Optic Disk

-2 types of photoreceptors

-Fovea: Where light from centre of visual field strikes retina; area of greatest visual acuity
-Optic Disk (papilla): Where optic nerve and blood vessels supplying eye pass through retina
-No photoreceptor cells
-also called "blind spot"

-2 types of photoreceptors: rods and cones


Inner layer of eye in Animals

-what animals (but pig have)
-what it is and what it does

-All but pig have dorsal reflective tapetum lucidum
-avascular layer that lacks pigment - cells contain crystalline rods
-is a nocturnal adaptation
-Reflects light, increasing stimulation of overlying retinal cells


Internal Chambers of eye (2)

-what they're called and their major purpose

-lens and ciliary body separate eye into 2 chambers
-Anterior (front) chamber: contains clear, watery fluid (aqueous humour)
-supplies nutrients to cornea and lens
-Posterior (rear) chamber: contains firm, jelly-like materia (vitreous humour)
-maintains spherical structure of eye


Refraction of Light waves by eye

-How Retina changes to look close and further away
-Control of what system

-Convex lens causes light waves entering eye to converge onto retina
-given point in visual field comes to focus on single point on retina
-For near vision (accomodation): Ciliary muscles contract, causing ciliary fibres to relax and lens to round up
-occurs under parasympathetic control
-For distant vision: Ciliary muscles relax and suspensory ligaments pull lens to a flatter shape
-occurs in absence of parasympathetic control


Structure of Photoreceptors

-what they do

-Outer and inner segment

*Change light signal to electrical signal
-Phototransduction carried out by rods and cones
-have same basic structural components
-Outer segment has disks that contain photopigment
-Inner segment contains nucleus
-Synaptic terminal contains stores for the neurotransmitter used for communicating with nerves


Characteristics of Rods and Cones

-Type of vision
-Sensitivity to light
-Visual acuity

Rods: Ability to see black and white in low light; high sensitivity; 100 million per retina; low visual acuity
Cones: Provides ability to see colour in bright light; low sensitivity; 3 million per retina; High visual acuity



-How light affects light-sensitive pigment (rhodopsin)
-need to also explain what it is bound to

-Dark and light

-Light sensitive pigment (rhodopsin) located in disks in outer segment
-rhodopsin comprises protein called opsin - is bound to vitamin A derivative called retinal
-On exposure to light, retinal dissociates from opsin, initiating sequence of reactions that decrease lvls of cBMP inside cells

*signalling increases in dark (become more depolarised); in light channels begin to close (become hyperpolarized)


Neural pathways for vision


-decussate -> what it is and what it means

-From ganglion cells, signals travel in optic nerve
-optic nrve exit eye at optic disc
-2 optic nerves combine at optic chiasm
-variable proportion of nerve fibres cross over (decussate) to enter opp side of brain
-amount related to position of eyes in head (more if eyes more on side)
-Info from right and left sides of visual field processed in left and right sides, respectively


Regulation of Light entering eye

-pupillary constriction and dilation
-how it occurs and why

-when else pupils also dilate

-Contraction or relaxation of inner circular muscle of iris smooth muscle regulates how much light enters eye
-in bright light, parasympathetic stimulation causes pupillary constriction (decreasing light entering)
-in low light, lack of parasympathetic stimulation allows inner circular muscle to relax (pupillary dilation)
-Pupils also dilate in fight or flight (response to fear/excitement)
-this is mediated by sympathetic nervous system


Pupillary Light Reflex (PLR)

-what is it
-direct and indirect reflex

-Relationship to decussation

-Light shone in one eye cases pupillary constriction in both eyes
-Direct reflex in stimulated eye and indirect (CONSENSUAL) reflex in unstimulated eye

*Strength of consensual reflex decreases as percent of decussation increases


Mechanoreception: Detection of sound

-Sound transmission through ear

-all structures it hits

-when and where AP initiated

-Sound waves strike tympanic membrane -> vibrates
-Vibrations transferred through malleus, incus and stapes - which amplifies signal
-Vibrations transferred from stapes to oval window -> setting up fluid waves within cochlea
-these waves push on flexible membranes of chochlea duct, bending haris cells in organ of Corti
-Neurotransmitter released from receptor cells creates APs in axons of cochlear division of vestibulocochlear nerve -> travels to CNS
-Energy in fluid waves transferred to tympanic duct; is dissipated back into the middle ear by flexible round window


Anatomy of Cochlea

-What the sensory organ for sound is

-what 3 parts it has - more detail on hair cells

-Sensory organ for sound = organ of Corti
-is on basilar membrane
-contains hair cells, supporting cells and overlying tectorial membrane
-Hair cells: are the receptor cells; have hair-like projections called sterocilia w/ tips embedded in tectorial membrane


Sound Transduction by hair cells

-how they are involve in sound transduction and generation of APs

-Oscillations of cochlear membranes cause hair cell sterocilia to bend
-Sterociliar are different lengths
*either bend towards or away from tallest sterocilium
-if towards longest - K+ comes in (usu. is lower conc outside) - causes depolarisation, opens voltage-gated Ca channels - increases neurotransmitter release, depolarising and generating APs
-if away from longest = become hyperpolarised and little action potentials (opposite to above)
*signals from hair cells transmitted to brain via cochlear nerve


How Amplitude and Frequency are determined

-What regions are tuned for high/low frequencies?

-Sounds that vary in amplitude cause stereocilia to bend further in either direction (increasing or decreasing no. of AP generated)
-Cochlea "tuned" to frequency
-Different frequencies cause different regions of basilar membrane to deflect
-cochlear base (close to oval and round windows) "tuned" to high frequencies
-Cochlear apex "tuned" for low frequencies


Neural Pathways for sound

-what part in brain it goes to

-Afferent signals from hair cells travel in vestibulocochlea nerve -> terminate in cochlear nuclei
-from cochlear nuclei, nerves carry signals to thalamus, where more nerves transmit signals to auditory cortex in brain
*in auditory cortex, organisation is tonotopic (frequency "map")


Sound Localisation

-2 things used

-Brain uses subtle differences in timing and level of sound (amplitude)
-the head also deflects sound, leading to lower amplitude at ear facing away from source


Mechanoreception: Detection of Motion and Body position

-In invertebrates (lobsters)


-Statocyst: simple form of gravity receptor
-in lobsters, is a chamber lined w/ hairs at base of 2 antennae
-each statocyst contains statolith (comprise of grains of sand held together by mucus
-Gravity moves statoliths w/in statocyst - gives animal info about orientation


Lateral Line Neuromasts in Aquatic animals

-what they are and what they do

-How they work -> cupula

-Aquatic animals have lateral line that runs just below level of skin on either side of body
-contains mechanoreceptors called "neuromasts"
-detect motion in water
*similar concept to hearing (as fish moves, fluid pusses moves in lateral line and pushes against cupula; cupula contains hair cells whose stereocilia are embedded in gelatinous materia -> sterociliar on hair cells in cupula bend that creates signal)


Motion and position detectors in vertebrates: Vestibular System

-Vestibular labyrinth

-What output from brain affects

*is the only structure to detect motion
-Mechanoreceptors in a set of interconnected chambers of ear (vestibular labyrinth)
-info from vestibular receptors travels to brain
Output of brain plays major role in:
-control of posture and movement
-orientation of head
-Stabilisation of gaze
-Maintaining sense of spatial orientation of body


Mammalian vestibular apparatus

-parts (3 different)

-Comprised of; 3 semicircular canals (fluid-filled at right angles to each other), 1 utricle, 1 saccule
-canals contain endolymph
-at base of each = ampulla (jug)
-W/in each ampulla is cupula (cap)


Structure of Ampullae


-how info is transmitted to brain

-Each has ridge that extends into lumen of ampulla
-mechanoreceptor hair cells extend out of crista into gelatinous cupula
-cupula bridges width of ampulla
*forms bridges width of ampulla & mobile barrier through which endolymph can't circulate

-hair cells transmit info to vestibulocochlear nerve


Semicircular canals

-what they enable
-Function of cupula in head rotation

-Semicircular canals: enables detection of rotational movements of head in 3 planes
-when head is rotated in plane of canal, inertia of endolymph creates force that displaces cupula, causing bending of hair cells (stereocilia)


Transduction by rotation

-what occurs within semiciruclar canal and w/in ampulla

-Movement of stereocilia towards or away from kinocilium causes K channels to open or close (to = open)
-causes depolarisation or hyperpolarisation of hair cells - causing changes in calcium concentration

*what happens on one side will be different to what happens on the other (brain can decipher)


Utricle and Saccle

-what they do

-macula and where orientated in each

-Detect displacements and linear accelerations of head (e.g. tilting)
-Both contain macula (has sterocilia and associated structures)
-Utricles has them horizontally; saccular has them vertically


Structure of utricles and saccule

-how the structures contribute to movement


-otoliths sit on top of gelatinous layer
-stereocilia on top of hair cells (under gelatinous layer)

-otoliths pulls gel, which moves hair
Vertigo: otoliths break off and block channel


Transduction of Linear acceleration (utricle and saccule)

-part of brain involved

-Utricle: detects backward and forward accelaration (also in detecting position of head relative to gravity
-Saccule: Detects up and down linear acceleration (e.g. riding in elevator)

*cerebellum is involved in balance and equilibrium


Electroreception in aquatic animals

-Ampullae of lorenzini
-structure and what they do

-skin has electroreceptors that aids in navigation and locating prey
-modified neuromasts called ampullae of Lorenzini
-contain salty gel-like material that detects small electrical currents in enviro

-may also be sensitive to salinity and temperature
-In sharks, electroreceptors clustered around head


Active Electroreception -> in fish (freshwater)

-what they can do and how it works

-Some freshwater fish are able to receive feedback from electrical signals produced by fish itself
-fish produces weak electrical field from organ in tail and receives signals from ampullae along its body
-Important for navigation and prey location