nervous system L21-26 Flashcards
3 sub-divisions of the somatosensory system
cutaneous
visceral
proprioception
function of touch sense
recognition and properties of things
control of movement
communication
steps in converting stimulus energy into action potentials
- sensory transduction
- action potential generation
sensory transduction
stimulus converted into a graded electrical potential/ receptor potential
depends on stimulus strength
action potential initiation
receptor potential exceeding threshold> action potential of nerve fibres
stimulus strength coded by firing rate
cutaneous receptors
superficial (merkel’s disc/ epidermal-dermal border/ free nerve ending/ meissner’s corpuscle)
deep (pacinian corpuscle/ ruffini’s corpuscle)
skin receptors defined by
location
size and structure
rapid/slow adaptation
size of receptive field
types of nerve fibres connecting to skin receptors
A-beta large diameter myelinated fibres
A-delta small diameter myelinated
C fibres small diamter unmyelinated
receptive field
area over which stimulus activates receptor associated w single neurone/ nerve fibre
rapidly adapting receptors
firing at stimulus onset
highlights appearance of new stimuli/ stimulus change
slowly adapting receptors
nerve continues firing action potentials throughout stimulus
pacinian corpuscle mechanical property changes
capsule present> rapid adaptation of receptor potential
capsule not present> less adaptation of receptor potential
pacinian corpuscle capsule
onion-like layers w fluid in between
rapid adaptation
allows for v rapid vibration
temp receptors
warm > C fibres
cold > A-delta fibres
nociceptors
noxious/ painful stimuli
mechanical/ thermal
sharp pain A delta followed by C fibre burn following
somatosensory system in rodents
modifiable (one-to-one anatomical relationship)
accessible (easy to perform targeted functional measurement)
2 pathways transmit cutaneous signals to the brain
dorsal column pathway
spinothalamic tract pathway
dorsal root ganglion
cell bodies of primary sensory nerve fibres
fibres enter spinal chord at root
nucleus
cluster of neurones in CNS w shared properties
dorsal column pathway
first synapse in dorsal column nuclei in medulla
decussation at medulla level L to R cortex / R to L cortex
dorsal column info carried
fine-discrimination touch
stimuli moving over skin surface
recognition of objects by touch
lateral inhibition in dorsal column
^stimulus contrast and therefore edges and form of tactile objects
fibres from neighbouring receptive fields inhibit one another
spinothalamic tract functions
coarse touch
temp
nociception
flexion/ extension
flexion> contract flexors and relax extensors
extension> relax flexors and contract extensors
synergists / antagonists
synergists> muscles pulling in same direction
antagonists> muscles pulling in opposite directions
neurones innervating muscle
large neurones
alpha motoneurones
in ventral horn of spinal chord
motor unit
motoneurone and muscle fibres it innervates
large motor unit
innervates many muscle fibres
strong force and little precision
lifting/ holding weight
small motor unit
innervates only a few muscle fibres
fine control more precision
finger muscles for manipulating objects
upper motoneurones
cerebral motor cortex/ brain stem
command and control
basis of voluntary control for movement
interneurones in spinal chord
excitatory and inhibitory
important for spinal movement programmes
muscle spindle
receptor type
intrafusal fibres in parallel w extrafusal fibres
innervated by 1a afferents going to spinal chord
extrafusal muscle fibres
do work of contraction
golgi tendon organ
receptor type in series w extrafusal fibres
1a afferent fibre of muscle spindle receptor
carries output signal of receptor to spinal chord
gamma motorneurones in controlling muscle spindles
alpha-gamma coactivation
keeps spindle firing constant and in operating range even though muscle contracts
golgi tendon organs
activated by muscle tension
innervated by 1b nerve afferents
types of movement
reflex (simple movement)
voluntary (complex/ may be learned)
rythmic (initiated and terminated by conscious control, automatic pattern in spinal chord)
reflexes
one sensory and one motor neurone reflex arc
spinal/ cranial origin
monosynaptic/ polynaptic
stretch reflex
monosynaptic> maintains contraction w increased load
golgi tendon reflex
protection against excessive load for control of posture
voluntary control of movement
brain pathways control alpha motoneurones in spinal chord
cerebral cortex > planning, coordinating and initiating
basal ganglia> planning and selection of movement
cerebellum > fine-tunes movement
sound
defined by frequency/ amplitude
P
peak pressure for a particular sound
Pref
reference pressure
each 1dB step
pressure increasing by a factor of 10^1/20
dB threshold for hearing damage
90dB
outer ear
pinna
externalisation of sounds/ localisation of sounds in vertical plane/ acoustic gain due to ear canal resonance
middle ear
air filled
equal pressure either side of tympanic membrane
impedance transformer
overcomes air fluid mismatch ossicles acting as levers/ area of tympanic membrane greater than stapes footplate
endolymph vs perilymph
endo ^K+
peri ^Na+
human ear hair cells
one row of inner hair cells 3500
~3 rows of outer hair cells 12,000
stereocilia at tips > bend in response to pressure changes
separation of sound frequencies in cochlea
travelling wave from base> apex
max displacement position depends on sound frequency
frequency analysis by cochlea
upon entrance, basilar membrane vibrates, travelling up basilar from base to apex, max displacement depending on frequency, low freq max displacement at cochlea apex and high freq max displacement at base
basilar membrane displacement excites hair cells
inner hair cell excitation
transmitter release/ excitation of auditory nerve fibres
stereocilia bend toward longest, K+ entry and depolarisation, Ca2+ entry and transmitter release> nerve fibre activation
stereocilia bend toward shortest, no K+ entry> hyperpolarisation no transmitter
cochlea filters by frequency in 2 ways:
- neurone responds best to one frequency
- each location along cochlea amplifies one frequency
outer hair cells
change length w stereocilia bending in response to sound stimulation
prestin (motor protein) driven
inject energy and amplify basilar membrane movement
*ototoxic drug damage
basilar membrane motion amplification
elongation and shortening of outer hair cells ^basilar membrane displacement > enhanced hearing sensitivity and frequency selectivity
coding sound frequency steps
- place code (tonotopic organisation)
- time code (phase locking for low frequency sounds)
tonotopic organisation
auditory system tracking where info originated on basilar membrane within processing centres
phase locking
synchronisation of firing to peaks in wave-form of low frequency
1/period
frequency
auditory nerve
connects cochlea to brain
first synapse = cochlear nucleus
sound localisation
vertical plane > interactions of sound on pinna
horizontal plane >interaural differences in sound wave timing and frequency
interaural sound cues for sound localisation
all sounds > difference in sound onset time between the two ears
long, continuous sounds> difference in phase of sound between 2 ears
interaural time difference location
path difference
low f sound <15000Hz
wave takes longer to reach far ear> phase difference
high f sound >15000Hz
ambiguous phase difference
localizing high frequencies by interaural intensity differences
head casts sound shadow for high f
brain detects difference in intensities
long wavelength relative to obstacle diffracted
sound localization in elevation
interference between sound waves striking pinna and going straight into auditory canal
detected in brain auditory pathway
cochlear implant
electrical activation of cochlear nerve fibres by electrode array inserted into cochlea
cornea
refracts light to bring into focus in the retina
lens
accommodation and adjusting refractive property of eye
more refraction if closer to bring into focus
iris
pigment detects eye color
circular/ radial muscles controlled by autonomic ns adjusts pupil diameter according to light levels/ emotional signals
pupil
opening allows light entry
retina
layer at back of eye w photoreceptors/ horizontal cells/ bipolar cells/ amacrine cells/ ganglion cells/ nerve fibres
temporal > near temple
nasal> toward nose
visual field of eye
covers 150 degrees
ciliary muscles role in accomodation
lens flattened for distant vision/ rounded for near vision
contraction allows zonulas of zin to slacken, lens expands and becomes more thickened
weakens w age
emmetropia
normal focussing
myopia
short-sighted
too much focal power for length > light rays converge in front of retina
hyperopia
long-sighted
too little focal power for length
light rays converge behind retina
foveal pit
fovea cells in upper layer of retina
pushed aside to allow light to photoreceptors, regions of high acuity
fovea
pit of centre of macula
central part of visual field
max acuity area
highest density of cone receptors, colour vision
blind spot/ optic disc
region where nerve fibres/ blood vessels leave the eye
retina organisation
pigment epithelium#
2 types of photoreceptors
pigment epithelium
cells at retina back with photoreceptors embedded
contain melanin black pigment
absorbs light preventing scattering stray light
2 photoreceptor types
rod (scotopic vision/ low light levels)
rod (photopic region/ high light levels
2 photoreceptor types
rod (scotopic vision/ low light levels/ 20*more than cones)
cones (photopic region/ high light levels)
3 types of cone photoreceptors
sensitive to long (red)/ medium (green)/ short (blue) wavelengths
highest acuity
enable colour vision
threshold
1/ sensitivity
dark adaptation
moving from bright to low light, cones not sensitive and rods are bleached
rod recovery w visual sensitivity
light sensitive photopigments in rods and cones
rods> rhodopsin
cones> 3 different wavelength sensitive pigments
rhodopsin
opsin protein + 11-CIS retinal
bound together in dark
conformational chage upon light exposure
> bleaching
rod channels in dark
cyclic GMP levels high, keep Na+ channel open
Na+ current in> K+ current out
rod depolarised to -40mV and transmitter released
rid channels in light
rhodopsin light breaching activates G protein/ transducin
cGMP production decrease so Na+ channels close
hyperpolarisation to -70mV
less transmitter release
visual pathway
retina> lateral geniculate body> visual cortex> dorsal/ ventral stream
photoreceptors function
releases glutamate in dark, exciting/ inhibitting bipolar cells
bipolar cells function
excite/ inhibit retinal ganglion cells
ganglion cells
form optic nerve fibres
centre surround organisation due to horizontal connections> contrats at edges of visual objects
receptive field
retinal patch where light excites to fire impulses
cone ganglion
centre surround organisation fed by different cone types:
red> green
blue> yellow
white light> weak firing
nasal vs temporal fibre routes
nasal fibres cross at optic chiasm
temporal fibres remain uncrossed, stay on same side
L eye temporal/ R eye nasal go to left LGN and cortex
R eye temporal and L eye nasal got to tight LGN and cortex
lateral geniculate body
per visual field site w contralateral/ ipsilateral layers keeping projections from each eye separate
3 cell types for 3 layer types
topographic mapping of visual world
visual cortex
5 areas V1 (primary) to V5 (MT)
forward projections from V1 to higher cortical areas
primary visual cortex
topographic mapping
cortical= centre of vision
selective responses of neurones in eye
simple cell receptive field> overlapping inputs from LGN neurones
complex neurones in cortex sensitive to specific orientation of stimulus
