Section 7: Sensory System

Sensory System

Module 7

Transduction Environment

--> how external stimuli detected by sensory receptors


Environmental Stimuli

  1. Stimulus must be detected by sensory receptor
  2. Different environmental stimuli, therefore different receptors needed

Mechanical Stimuli

--> Touch, pressure, vibration, sound, proprioception (muscle sense)

Chemical Stimuli

--> Taste, pain, odours

Electromagnetic Stimuli

--> Light

Other Stimuli

--> Gravity, motion, acceleration, heal


Adequate Stimulus

  1. Some receptors detect more than one stimulus

Adequate stimulus <-- Stimulus sensory receptor most sensitive to

--> Example: Rod & cone cells in eye adequate stimuli --> Light


Receptor Potentials

  1. Once sensory receptor stimulated by environmental stimulus
  2. Cause change in ion permeability, leading to local depolarization
  3. Called "generator/receptor potential"
  4. Receptor no voltage gated channels
  5. Therefore, receptor potential must spread to area on sensory neuron that contains voltage channel
  6. Usually at first node of ranvier on axon
  7. action potential then generated & propagated along axon & in spinal cord
  8. In receptors with no axons, depolarization spread & to synapse, results in release of neurotransmitters
  9. Hair cell in ear


Shared Characteristics between EPSP's & IPSP's

  1. Generally depolarizing but can be hyper-polarizing too
  2. Caused by increasing permeability to sodium ions -> increase permeability K+ for hyper-polarizing
  3. Are local, spread like EPSP, decrease with time and distance from stimulus
  4. Proportional strength stimulus, increase stimulus, increase receptor potential, increase action potential likeliness


Somatosensory System

  1. Detects & processes sensations of touch, vibration, temperature & pain <-- most originate in skin

Cutaneous Receptors (receptors in skin)

  1. Hair follicle: Sensitive to find touch and vibration
  2. Free Nerve Endings: Pain & temperature (hot/cold)
  3. Meissners Corpuscles: Detect touch low frequency vibrations (30 & 40 cycles)
  4. Ruffinis Corpuscles: Detect touch
  5. Pacinian Corpuscles: Detect high frequency vibrations & touch (250-300 cycles)


Receptive Field

  1. Area on surface of skin where adequate stimulus will activate particular receptor to fire an action potential in neuron
  2. Any stimulus applied outside receptor field will not generate action potential


Spinothalamic (anterolateral) Tract

  1. Somatosensory pathway
  2. Transmits information dealing with basic sensations (pain, temperature, touch)
  3. Information from sensory neuron (1st order) enters spinal cord
  4. Synapses with (2nd order) neuron
  5. This neuron crosses to opposite contralateral side of spinal cord & ascends to thalamus
  6. Thalamus: relay center all sensory (except smell)
  7. Synapse occurs with neuron (3rd order) then travels
  8. Travels to somatosensory cortex
  9. Sensory information from right side of body goes to left side of the brain


Dorsal Column Medial Lemniscal

  1. Somatosensory Pathway
  2. Transmits information: fine detail touch, proprioception & vibration
  3. Sensory neuron information (1st order) enters spinal cord & immediately travels up spinal cord -> Before crossing contralateral side
  4. Upper spinal cord sensory neuron synapses with (2nd order neuron) -> Then crosses to opposite side of spinal cord
  5. Continues to thalamus, synapses again onto (3rd order)
  6. Then travels to somatosensory cortex


Primary Somatosensory Cortex

  1. Once sensory information has reached the brain, travels to primary somatosensory cortex which is located

-> In parietal lobe on postcentral gyrus behind central sulcus


Somatosensory Homuncutus

  1. Organization of PSC in topographical Representation of body
  2. It is geographically preserved
  3. Some areas on cortex like hand, tongue, & lips require more of brain to process & get more sensory information
  4. They contain many more sensory receptors than any other part


The Visual System

  1. Detects light, converts into action potential & sends to primary visual areas for processing
  2. Once processed become aware visual environment

Consists of:

Eye: Contains photoreceptors that convert light to Action Potential

Visual Pathway: Transmits action potentials

Primary Visual Area: Process incoming signals (in occipital lobe)


The Eye

  1. Light Passes through Cornea
  2. Amount of light regulated by Iris
  3. Constricts bright light or dilates low light
  4. Lens flips light (upside down & backwards) & focuses it onto retina at back of eye
  5. Retina contains photoreceptors
  6. rods and cones (point at back of head)
  7. Fovea, part of retina with highest concentration of cone cells
  8. Center of vision focused here


Rod Cells & Cone Cells

  1. Don't have axons therefore no action potentials
  2. Generate receptor potentials that release inhibitory neurotransmitters from synaptic end

Rods - Extremely sensitive light

  1. Function best low light
  2. Contain one photopigment
  3. Don't detect colours
  4. Rods located mostly region of retina outside & around fovea

Cones - Function best bright light

  1. Ideal detecting detail
  2. 3 types each sensitive 1 primary colour
  3. Found in large concentrations on fovea


Other Cells Retina

  1. Retina contains pigment layer at very back of eye that observes & absorbs excess light
  2. Other cells: bipolar, ganglion, horizontal, amacrine
  3. Other cells responsible for integration of information from rod & cones & production of action potential (bipolar)


Transduction Light to Action Potential

  1. Light hyperpolarizes these cells & shuts them off
  2. These cells release inhibitory neurotransmitters when depolarized in dark
  3. Inhibit bipolar
  4. Light strikes photoreceptors, they hyper-polarize, shut off & stop releasing inhibitory neurotransmitters
  5. Bipolar cells depolarize (independently) & become activated
  6. Depolarization of bipolar cells may lead to action potential in ganglion cells

In dark

  1. Depolarize rod/cone - release inhibitory neurotransmitters

In light

  1. Hyperpolarize rod/cone - no inhibitory released
  2. Bipolar cells depolarizes


Types of Eye Movements

  1. Saccades -> Rapid, jerky eye movements -> Used to rapidly move eye to object of interest Example: Reading on computer
  2. Smooth Pursuit -> Smooth movement of eyes made to keep moving object focused on fovea Example: Watching bird fly keeping head still
  3. Vestibular Ocular Reflex (VOR) -> When you focus your attention on object & then move head back & forth or shake up/down Example: Agreeing/disagreeing & staring at someone
  4. Vergences -> When object of interest is approaching or moving away from you -> When object moving away - eyes diverge -> When object moves closer - eyes converge Example: Staring at pencil moving away from & toward face


The Auditory System

  1. Converts sound waves from external environment to action potentials that travel to auditory system of brain
  2. Human car 20 Hz to 20,000 Hz
  3. Most accurate hearing 1,000 - 3,000 Hz

Structures:

  1. External ear contains auricle & external auditory canal
  2. Middle ear: eardrum (tympanic membrane), ear ossicles (made up 3 bones - malleus, incus & stapes) & Eustachian tube
  3. Inner ear consists of vestibular apparatus (balance) & cochlea (processing sound)


Cochlea

(Shape of shell of snail)

  1. Hollow area inside "shell" divided in 3 components
  2. Upper Scala Vestibuli (Vestibular duct)
  3. Middle cohclear duct
  4. Lower scala tympani
  5. Separating Cochlear duct & tympanic duct is basilar membrane
  6. Basilar Membrane contains organ of corti
  7. Organ of corti: Where sound converted to action potentials by special hair cells
  8. Hair cells embedded in tectorial membrane
  9. Sound waves cause basilar membrane vibrate, causes hair cell vibration

Frequency - Number of waves (cycles) per unit time

Intensity - (loudness) expressed height (amplitude) soundwave scala Vestibuli

Transfer & Amplification Sound

  1. Airways of sound travel through air & reach outer ear
  2. Waves funnelled into external auditory canal
  3. Strike tympanic membrane causing flex back and forth
  4. Levering action ear ossicles amplifies, pressure waves that strike tympanic membrane
  5. Ear ossicles cause oval window to vibrate
  6. Oval window underneath stapes
  7. Waves amplified 15-20 times original amount
  8. Cause oval window much smaller than tympanic membrane
  9. Fluid inside cochlea (perilymph) transmits waves to hair cells embedded in basilar membrane
  10. Basilar membrane detect vibrations & turn them into action potentials in auditory nerve
  11. Near different frequencies cause vibration basilar membrane
  12. Pressure waves in fluid created by vibrations of oval window produce travelling wave on basement membrane
  13. Reaches peak at different regions of membrane
  14. Happens cause membrane not consistent along its length


Basilar Membrane

  1. Tension varies along length
  2. Tight at base, loose at top
  3. Depending part of membrane thats vibrating only certain hair cells activated by certain sounds
  4. can also differ frequencies by length and stiffness of hair cell

Low Frequencies

  1. Stimulate hair cells at apex (top) of cochlea

High Frequencies

  1. Stimulate hair cells on membrane near oval window


Sound

  1. When basilar membrane vibrates hair cells are bent causing ion channels to open & depolarization of cells
  2. Depolarization causes release of neurotransmitters from hair cells
  3. Excites neurons of auditory nerve, which then fire action potentials
  4. Louder sound, stronger vibration of basilar membrane the more bent the hair cells, more neurotransmitters & higher frequency of action potentials produced
  5. Signals flow to auditory cortex located in temporal lobe of brain


Vestibular System

(Also VOR eye movement)

  1. Located inner ear next to cochlea
  2. Responsible for maintaining balance, equilibrium & reflexes
  3. Does this by detecting linear & rotational motion & position of head relative to rest of body
  4. Has two primary structures in apparatus
  5. Semicircular canals
  6. Otolith organs


Semicircular Canals

(Detect rotational/angular accelerations of head)

  1. Three canals, one for each plane of motion
  2. Filled with fluid called "Endolymph"
  3. End each canal swelling -> Ampula
  4. Inside ampula sensory region -> Crista Ampullaris
  5. Contains sensory hair cells
  6. Fixed at base, cilia embedded in gelatinous material - cupula
  7. When head bends endolymph lags behind & moves to the right (opposite side head tilt)
  8. Endolymph hits cupula & bends hair cells inside
  9. When hair cells bent certain way they depolarize & fire AP to brain
  10. When bent opposite direction hyper-polarize
  11. No signals to brain


Otolith Organ

  1. Detect linear accelerations & decelerations & position of head when its tilted
  2. Are two otolith organs in each vestibular apparatus

Utiricle - Detects horizontal accelerations & decelerations (in car)

Saccule - Detects vertical accelerations & decelerations (elevator)

  1. Each otolith organ contains many hair cells that are anchored at base & have their cilia embedded in gelatinous membrane
  2. Gelatinous membrane has otolith crystals embedded to give it weight & inertia during movements
  3. Utiricle and Saccule act together to detect head tilts


Body Accelerations

  1. Crystals lag & move opposite direction increase frequency of action potentials in nerve

Body Constant Velocity

  1. Hair cells resting state & frequency of action potential

Decelerate

  1. Hair cells bend in other direction which causes frequency of action potentials to go down from resting
  2. More rapid decelerate down AP frequency


Hair cell

  1. When at rest release small resting leveIs neurotransmitters from base to sensory nerve (fires action potentials)

Acceleration - Smaller sterocilia bend toward larger kinocilium hair cell releases more neurotransmitters causing more action potentials on sensory nerve

Deceleration - Stereocilia bend away kinocilium hair cell less neurotransmitters = fewer action potentials


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