Physical Functions

Vision

Hearing

Movement

General

Receptor cell: neural activity in response to physical event

Neurons code for / represent sensory information;
Don't physically represent stimuli

Structures & function

  • Iris: coloured
  • Pupil: aperture in iris to let more/less light in
  • Cornea: focuses light, not adjustable, 80% refractive power, in front of iris
  • Lens: focuses - focuses light, accommodates (changes shape), 20% refractive power, behind iris
  • Retina: lined with photoreceptors (image inverted)
  • Fovea: tightly packed receptors, 70% input to brain, no blood vessels/ganglion cells -> acute, detail
  • Blind spot: no receptors, where optic nerve & blood vessels exit at back of eye -> no vision

Retina

  • Light -> R&C -> H&B -> A&G -> ganglion axons form optic nerve
  • Rods & cones have photopigment - release energy when struck by light
  • Cones: detail - 90% input
    ** Fovea, 1 cone -> 1B/G => detail; g
    ** Good in bright light
    ** High wavelength sensitive (low = blue, high = red) - good colour
  • Rods: movement - more rods (20:1)
    ** Periphery, many rods -> 1B/G => poor detail
    ** Low wavelength sensitivity (500nm) - poor colour/spatial locaiton
    ** Good in dim light
  • Horizontal: connects R&C, lateral inhibition
  • Amacrine: connects G(/A/B), refine input to G, (some for colour vs light vs mvt vs shape)
  • Ganglion cells: sends summary info from many rods to brain

Wavelength vs colour

  • Blue = 350nm
  • Green = 500nm
  • Red = 700nm

Theories

Trichromatic: relative activity of 3 cones (more sensitive to diff wavelengths)

Opponent-process: more active in response to 1 wl, less in response to opposite; red/green, yellow/blue, black/white - afterimage

Retinex

  • Retinex: retina + cortex - cortex compares input from diff parts of retina to determine brightness/colour
  • Context changes colour perception -> colour constancy

Visual pathways to brain
Eye
-> nasal->contralateral, temporal-ipsilateral at optic chiasm
-> LGN in thalamus
-> primary visual cortex/V1/striate cortex
-> dorsal or ventral stream

  • Magnocellular cells in LGN: large cell bodies & receptive fields (like rods)
  • Dorsal pathway = why - through parietal cortex
  • Action - visually guided movements, depth/movement/patterns;
    damage = bump, can't reach, can't post letter - visuomotor
  • Complex cells in cortex: bar/shape, moving in direction within large area (field), e.g. bar moving L to R
  • Middle temporal cortex/M5: motion processing - direction & speed;
    damage = motion blindness (can see object but no mvt)
  • Posterior parietal cortex: movement perception

Receptive fields

Visual field: visual pattern that most excites neurons (for each neuron)

  • Depends on location, wavelength, movement, orientation, size, colour
  • Receptor (small) -> B cell -> ganglion cell (large)

Gratings

  • Square-wave: blocked stripes
  • Sine-wave: graded stripes

Spatial frequency: sine wave grating corresponding to width of simple cell

  • Low frequency = wide = large objects = overall shape
  • High frequency = narrow = small objects = detail
  • -> simple cells respond to particular spatial frequency, not feature

Disorders

Visual object agnosia

  • Can't recognise objects
  • Inferior temporal cortex damage

Prosopagnosia

  • Can't recognise faces
  • Damage to fusiform gyrus of temporal cortex

Dimensions

  • Pitch = frequency = waves/sec
  • Loudness = amplitude (in general) = height
    (15-20k Hz; low = low sound)
  • Location: arrives at ear at diff times
  • Timbre: wave shape
  • Prosody: conveying info by tone of voice

Structures & function

  • Sound -> strike tympanic membrane
  • -> vibrates bones in middle ear (hammer, anvil, stirrup) - amplifies vibrations
  • -> stirrup vibrates oval window of cochlea
  • -> moves fluid in cochlea
  • -> vibration displaces hair cells between basilar membrane & tectorial membrane in organ of Corti (cochlear duct)
  • -> opens K+ channels in hair cell membrane - mechanical
  • -> stimulates auditory nerve cells

Auditory pathways to brain
Ear
-> cochlear nucleus: cross in midbrain to
-> superior olive: compare sounds to tell location
-> inferior colliculus: orienting (bring attention to sound)
-> medical geniculate nuclei: sensitive to complex sounds
-> cortex: tonotopic (higher at back, complex on outside)

Theories

Place

  • 1 place in basilar membrane = 1 frequency
  • But membrane wound too tightly, neurons can't respond quickly enough

Frequency

  • Basilar membrane vibrates -> AP at same frequency
  • Frequency = pitch, no. cells firing = loudness
  • But limited by refractory period to 100Hz

Muscles

Categories

  • Skeletal/striated: body & limbs, involuntary & voluntary*
    Smooth: internal organs, involuntary (pupils, blood vessels, intestines)
  • Cardiac: heart, fibres fused so contract together
  • Fast-twitch: contract fast, sprint, fatigue fast, don't use O2
  • Slow-twitch: contract slow, long-distance running, don't fatigue, use O2
  • Neuromuscular junction: synapse between alpha motor neuron & muscle fibre
  • Motor unit: 1 motor neuron + >=1 muscle cells it innervates (more force - more motor units)
  • Acetylcholine excites -> so muscles only contract (no message for relax or opp. direction)
  • Antagonistic muscles: to move joints in 2 directions (one contracts, other relaxes)
  • Flexor, e.g. hand to shoulder
  • Extensor, e.g. straighten arm

Proprioception

Muscle spindle organ: if stretch -> contraction

  • Parallel (inside) muscle fibres
  • Muscle stretches -> muscle spindle stretches & messages motor neuron -> messages muscle -> contracts (negative feedback)
  • Before Golgi tendon
  • e.g. walk on uneven ground, prevent too much stretch

Golgi tendon organ: if tension/contract -> inhibit contraction

  • Inside tendon (between muscle & bone)
  • Muscle contracts -> Golgi tendon stretches & messages motor interneuron -> inhibits motor neuron (stop further contraction)
  • e.g. prevent injury to muscle

Reflexes

  • Consistent, involuntary, single response to stimuli
  • In spinal cord & brain stem (not brain)
  • e.g. pupil, pain, postural, infant, muscle spindle/Golgi tendon

Central pattern generators

  • Involuntary, repetitive, rhythmic sequence of behaviour
  • In spinal cord & brain stem
  • Stimulus starts it, but doesn't control frequency
  • e.g. bird flapping wing, wet dog shake

Motor programs

  • Fixed sequence of movements (not just repeated mvt)
  • Innate: yawn, smile, frown, animal self-wash
  • Learnt: riding a bike, speaking, playing an instrument
  • In SMA
  • Disrupted when think about it

Brain - cortex = complex actions

  • Larger cortical area = higher precision
  • Posterior parietal cortex: monitors body/object position - planning/intention to move -> premotor
  • Premotor cortex: prepares mvt -> primary
  • Prefrontal cortex: prepares mvt, store sensory info, consider outcomes
  • Primary motor cortex: orders outcome (more neurons for hands/face/tongue/swallowing)
  • Primary somatosensory cortex: feels body
  • Supplementary motor cortex: prepares well-learned mvt (internally cued, prevents habitual errors)
    Lateral premotor cortex: externally cued, habitual errors

Brain - subcortical = involuntary actions

  • Coughing, laughing, crying

Brain - other

  • Cerebellum: aim/timing, coordination, balance
    ** Sends corrections to motor cortex (compares expected/actual mvt)
    ** No. Purkinje cells stimulated (via parallel fibres) = passage of time
  • Basal ganglia: well-learned, internally cued (cues SMA)
    * Direct - enhances* mvt; SN on -> putamen on I -> GP off -> thalamus on -> SMA on
    * Indirect - inhibits* competing mvt; SN off -> putamen off -> GP I -> thalamus off -> SMA off
    ** Increases vigour of mvt
    ** Stores sensory info

Disorders

Cerebellum

  • Balance & coordination (like intoxication)
  • Timing/aim/error correction
  • Attention shifting

Parkinson's

  • Symptoms: can't initiate voluntary mvt (rigidity), can't inhibit inappropriate mvt (tremor), slow/weak/inaccurate, cognitive deficit, depression
  • Cause: death of dopamine-releasing neurons in SN (stuck in off); genetic + environmental (toxins)
  • Treatment: L-dopa (precursor to dopamine) + electrically stimulate GP, cell transplant

Huntington's

  • Symptoms: deterioration of motor control (jerk -> writhe)
  • Cause: damage to CN, P, GP & cerebral cortex (less I of thalamus -> involuntary/jerky mvt); hereditary (gene alters structure of huntingtin protein)
  • Treatment: tetrabenzine (reduces dopamine, serotonin, norepinephrine)

Brain - spinal cord

  • Corticospinal tracts: from brain to spinal cord
  • Lateral: precise, discrete, detailed - peripheral (hands/fingers/feet)
    ** Primary motor cortex & surrounding areas
    ** -> midbrain (red nucleus) - connects to cerebellum
    ** -> upper medulla (pyramids) - cross to contralateral
    * -> contralateral side of spinal cord -> move sides independently
  • Medial: postural, bilateral (standing/bending/walking/turning)
    ** Many areas
    ** -> midbrain (tectum, reticular formation)
    ** -> medulla & cerebellum (vestibular nucleus) - balance
    * -> both sides of spinal cord

Sensory transduction: convert physical energy to electrochemical (neural) energy

  • Parvocellular cells in LGN/fovea: small cell bodies & receptive fields (like cones)
  • Ventral pathway = what - through temporal cortex
  • Perception - recognise objects/patterns/shape, colour, fine detail;
    damage = can't ID objects, can't line up letter to post - perceptual orientation
  • Simple cells in cortex: bar/shape, in one location (small receptive field)
    => feature detection (colour/orientation/width) - e.g. red vertical bar
    => spatial frequency detection
  • Inferior temporal cortex: object ID/shape analysis, despite change in position/size;
    damage = visual object agnosia
  • Posterior inferior temporal cortex: mixed parvo/magno - colour & brightness

Lateral inhibition: stimulation in retina suppresses neighbours' response -> enhance contrast of boundaries (e.g. for brightness contrast)

  • Receptors stimulate B & H -> H inhibits B and B to the side
  • Retina & LGN: doughnut shaped
  • Cortex: edge/feature or motion detectors? spatial frequency

Primary auditory cortex/A1

  • In superior temporal cortex
    ** Respond best to particular frequency/complex tones
  • Anterior temporal cortex: ID sounds
  • Posterior temporal & parietal cortex: locate sound

Compatible theories

  • Low frequency (0-100Hz) = frequency theory
  • Intermediate (100-4kHz) = volley principle (staggered)
  • Highest (>4kHz) = place theory
    ** High: vibrate cells near base - at oval window (stiff)
    ** Low: vibrate cells at apex (floppy)

Functions

  • Movement
  • Secrete fluid (salivary/sweat glands)
  • Regulate body (blood pressure, digestion)
  • Heart pumping
  • Proprioception: sense that tracks where body parts are in relation to each other
  • Proprioceptors: receptor that detects position or mvt of body
    e.g. touch receptors: respond to squeezing/stretching

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