6. CNS/Sensory Flashcards
CNS =
Brian + spinal cord
afferent neurons responsible for
sensory input
3 types of sensory afferents
- cranial nerves: go straight to brain
- spinal nerves: somatic sensation
- visceral: inflammation, pain inputs
sensory afferents have their axons where?
going into CNS
efferent neurons responsible for
motor output
motor neurons have their cell bodies where?
in the CNS
types of motor efferents
- cranial nerves + spinal nerves (contain mix of afferent and efferent)
- somatic efferent: send signals which innervate skeletal muscles
- autonomic efferent: innervates interneurons and smooth&cardiac muscles
- enteric efferent: control digestive tract
spinal cord (anatomy)
meets brainstem at base of skull
brainstem consists of: (3)
- Medulla
- Pons
- Midbrain
thalamus (anatomy)
relay station, sensory pathway
corpus callosum (anatomy)
major connection in middle, containing a bunch of neurons travelling between the 2 hemispheres
cerebrum aka…
cerebral cortex
cerebrum has foldings called…
gyrus and sulcus
4 parts of cerebrum
- frontal
- parietal
- occipital
- temporal
cerebellum (anatomy)
contains many neurons, contributing to motor skills
central sulcus
crack/folding that separates primary somatosensory processing from primary motor cortex
coronal splice
cutting down through the cerebral cortex
gray matter
where all cell bodies are
white matter
where axons are
ventricles
cavities where cerebral spinal fluid flows
cervical nerves innervate…
neck, shoulders, arms and hands
thoracic nerves innervate…
shoulders, chest, upper abdominal wall
lumbar nerves innervate…
lower abdominal wall, hips and legs
sacral nerves innervate…
genitals and lower digestive track
gray matter composed of
- dorsal horn (back)
- ventral horn (stomach)
- central canal in middle
spinal segment composed of
- dorsal root
- ventral root
- dorsal root ganglion
dorsal root carries…
sensory afferents
ventral root carries…
motor efferents
dorsal root ganglion is where…
cell bodies of sensory afferents are located
ectoderm
top part which develops into CNS
mesoderm
becomes muscles, organs
endoderm
big cavity that becomes the digestive system
dura
lining of CNS
early development of nervous system in weeks 1-3
inner cell mass develops into embryonic disk
early development of nervous system in weeks 3-4
ectoderm folds into groove which will then close to form the neural tube
-> becomes CNS and part of PNS
early development of nervous system in week 4
vesicles develop with cavity in middle, forming the forebrain, midbrain and hindbrain
early development of nervous system in following 8 months
- forebrain becomes cerebral hemispheres + thalamus
- midbrain becomes midbrain
- hindbrain becomes cerebellum + pons + medulla
- cavity becomes ventricles + central canal
Cerebral Spinal Fluid (CSF) is produced where and by what?
produced in the 4 ventricles by the chloroid plexus
4 ventricles in the brain
- 2 lateral ventricles: largest ones, majorly producing CSF
- 3rd ventricle in middle of thalamus
- 4th ventricle attached to central canal
CSF composition
sterile, colorless, acellular fluid containing glucose
CSF circulates actively or passively?
passive circulation: oozing out from chloroid plexus
Cerebral Spinal Fluid (CSF) functions (3)
- support and cushion the brain: makes brain float in skull since gravity of brain and CSF are equal
- provide nourishment to the brain: glucose
- remove metabolic waste through absorption at the arachnoid villi
foramen of Monro
opening that connects lateral ventricles to 3rd ventricles
subarachnoid space
where CSF circulates around the brain
arachnoid villi
organelles that take CSF out of the subarachnoid space to empty it into venous blood
what covers the brain and spinal cord?
the membranes/meninges of CNS
3 meninges
- Dura mater
- Arachnoid membrane
- Pia mater
Dura mater
tough covering protecting the CNS + contains dural sinus
dural sinus
where arachnoid villi empty CSF to the blood
arachnoid membrane
not as tough as dura but creates subarachnoid space
trabeculae
found in arachnoid membrane, produce subarachnoid space
pia mater
thin and delicate, attaches itself to the cortex
what % of total blood does brain receive?
15%
what substrate(s) is/are metabolised by the brain?
glucose + very little glycogen
brain requires continuous supply of
glucose and oxygen
how is glucose transported in the brain
automatically goes into neurons, no need for insulin
what carries blood to the brain
common carotid artery and vertebral artery
what carries blood to the rest of the body
aorta (85% of blood)
internal carotid artery
supplies base of the brain
external carotid artery
supplies outside of the head
basilar artery
the 2 vertebral arteries joined together
Circle of Willis
internal carotid + basilar artery form a loop, allowing for continuous supply of blood even if one of the carotids gets blocked
CSF circulation summary
chloroid plexus -> subarachnoid space -> dural sinus -> venous system
blood circulation summary
heart -> vertebral/carotid arteries -> Circle of Willis -> brain -> venous system
blood brain barrier
capillary wall with tight junctions between endothelial cells allows only a few things to leave the blood
what does the blood-brain barrier let through
- lipid soluble substances: water, O2, CO2
- small ions: Na+, K+, Cl-
- glucose through active transport
what isn’t let through the brain barrier
- plasma proteins
- large organic molecules
glia
non-neuronal cells in brain which support neuron by regulating extracellular conditions
astrocytes
type of glia cells
astrocytes functions (3)
- phagocytosis of debris
- providing structural support
- inducing tight junctions
sensation
awareness of sensory stimulation
perception
understanding of a sensation’s meaning
how do we perceive sensation
we perceive the neural activity/pattern produced by the energy of sensory stimulation
Law of Specific Nerve Energies
regardless off how a sensory receptor is activated, the sensation felt corresponds to that of which the receptor is specialised
Law of Specific Nerve Energies example
rubbing eyes creates a pressure which stimulates light to be perceived
Law of Projection
regardless of where in the brain you stimulate a sensory pathway, the sensation is always felt at the sensory receptors location
Law of Projection example
Phantom limb pain after amputation
modality
general class of a stimulus
summary of Laws of Perception/Sensation
the brain knows the modality and location of every sensory afferent
stimulus reception steps
- stimulus energy activates afferents
- receptor membrane/cell contain ion channels, which respond only to adequate stimulus
- transduction: stimulus activates ion channels at receptor membrane/cell
- action potential sent to the brain
- neurotransmitter release
what affects neurotransmitter release?
variations in stimulus energy strength
adaptation of afferent response
signals changes in stimulus energy, allowing us to be sensitive to changes in sensory input
non-adapting encodes…
stimulus intensity and slow changes
slowly adapting encodes…
some stimulus intensity and moderate stimulus change
rapidly adapting encodes…
fast stimulus changes
Receptive Field (RF)
region in space that activates a sensory receptor or neuron
where is the receptive field strongest?
at its centre
population code
overlapping receptive fields
acuity
ability to differentiate one stimulus from another
small RF =
high acuity, ie. lips
-> can tell location go stimulus more precisely
large RF =
low acuity, ie. back
bottom up mechanism
lateral inhibition, reducing activity of neighbouring neurons
–> have no control over
top down mechanism
use of background knowledge to interpret what we see
–> can be controlled
sensory information sharpened by…
bottom up and top down mechanisms
somatic senses
touch, pain, proprioception, temperature
somatosensory system stimulus energy
mechanical, thermal, chemical
somatosensory system receptor class
mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors
touch receptors are called
mechanoreceptors
mechanoreceptors (touch)
specialised end organs that surround the nerve terminal, allowing only elective mechanical information to activate the nerve terminal
superficial layers of touch mechanoreceptors
- Meissner’s corpuscle
- Merkel disk
–> closest to skin surface so most sensitive
Meissner’s corpuscle key points
- fluid filled structure enclosing the nerve terminal
- rapidly adapting
- sensitive to light stroking and fluttering
Merkel disk keys points
- small epithelial cells surrounding the nerve terminal
- slowly adapting
- sensitive to pressure and texture
deep layers of touch mechanoreceptors
- Pacinian corpuscle
- Ruffini endings
–> less sensitive: require more energy to be activated
Pacinian corpuscle key points
- large concentric capsules of connective tissue surrounding the nerve terminal
- rapidly adapting
- sensitive to strong vibrations
Ruffini endings key points
- nerve endings wrapped around spindle-like structure
- slowly adapting
- sensitive to stretching and bending of skin: can detect shape of object
proprioception somatosensation anatomy
muscle spindles provide sense of static position and movement of limbs and body: motor control
skin mechanoreceptor activation
- mechanical deformation of skin
- deformation of cell membrane of afferent neuron
- cytoskeletal strands stretched, pulling ion channels open
- receptor potential signalled
temperature receptors are called
thermoreceptors
thermoreceptors
free nerve endings containing ion channels that respond to different temperature ranges
cold afferents thermoreceptors
- 0-35 degrees
- activated by menthol
warm afferents thermoreceptors
- 30-50 degrees
- activated by capsaicin and ethanol
what do extreme temperatures activate?
pain receptors
nociceptors
free nerve endings containing ion channels that open in response to intense mechanical deformation, excessive temperature of chemicals
pain receptors are called
nociceptors
visceral pain receptors are activated by…
inflammation inside internal organs
nociceptor activation
- skin poked with knife: nociceptors activated
- action potential sent by afferents
- substance P released in spinal cord, activating 2nd order neurons
- pain experienced
Hyperalgesia (def)
bottom up mechanism which increases the threshold for pain to tell the body to let it heal without using it
Hyperalgesia (steps)
- follows nociceptor activation
- enhancement of surrounding nociceptors by injured tissue + mast cells release histamine
- substance P causes dilation of nearby blood vessels
Touch and Proprioception pathway
- AP enters through spinal nerve
- goes up dorsal root ganglion and dorsal columns ipsilaterally
- medulla: midline crossed
- thalamus –> somatosensory cortex (contralaterally)
An injury to the dorsal root leads to loss of somatosensations at which level
at the level of the lesion only
Temperature and Pain pathway
- AP enters through spinal nerve
- goes through dorsal root ganglion and dorsal horn ipsilaterally
- cross midline at central canal
- anterolateral columns (contralaterally)
- branches into reticular formation –> thalamus –> somatosensory cortex
an injury to the anterolateral columns leads to loss of pain and temperature at which level?
at the level of lesion and below
where do all somatic senses arrive in the brain?
somatosensory cortex
somatotopic map
illustrates higher acuity in some places due to more neurons and therefore smaller receptive fields
referred pain
visceral and somatic pain afferents synapse on the same 2nd order neuron so the brain doesn’t know which afferent was responsible for the AP signal
-> skin assumed
what do descending pathways regulate?
nociceptive information
Analgesia
reduction of pain through top down mechanism (controlled)
analgesia pathway
- neurons come down spinal cord through midbrain and reticular formation (medulla) and dorsolateral funiculus
- opiate neurotransmitter released
- substance P transmission inhibited
visual sensory system stimulus energy
light
visual sensory system receptor class
photoreceptors
outer eye anatomy
- sclera
- cornea
- pupil
- iris
cornea
clear section of sclera
pupil
opens/closes to let more/less photons in
iris
controls the pupil
inner eye anatomy
- lens
- vitreous humor
- retina
- retinal pigment epithelium
- fovea centralis
- optic disk
lens
focusing light on retina
vitreous humour
clear jelly containing blood vessels which block photoreceptors
retina
contains neurons and photoreceptors
retinal pigment epithelium
lining behind the retina, contributing to transduction
fovea centralis
highest visual acuity spot, centre of vision containing cones photoreceptors
optic disk
blind spot where optic nerve leaves from, containing no photoreceptors
light refraction
lens bends/refracts light to a single point
what part of the eye refracts light?
cornea and lens but mostly cornea
point where photons are refracted
retina
how does the image appear at the retina
focused and inverted
accomodation for near vision
lens changes shape to adapt to changes in object location
what happens if object becomes closer to eye
- focus point is behind the retina
- lens accommodates by contacting ciliary muscles
–> bends more light so focal point can be brought back on retina
nearsightedness
- eye is myopic
- focal point appears before retina: too much refraction
cause of nearsightedness
eyeball is too long
farsightedness cause
eyeball is too short
farsightedness
- eye is hyperopic
- focal point is behind the retina
astigmatism
lens or cornea are not spherical
presbyopia
lens gets stiff and is unable to accomodate for near vision
cataract
change in lens color (opaque), blocking photons from reaching retina
what can be found at the back of the retina
cones and rods
convergence in retina
shift from many photoreceptors to way less photoreceptors in ganglion cells
what forms the optic nerve
axons from ganglion cells
phototransduction process
- light activates opsin molecule
- opsin changes conformation, causing chromophore to come off
- G-protein cascade triggered: cGMP converted to GMP
- sodium channels close since they are only activated in presence of cGMP
- photoreceptors become hyperpolarised, causing a reduction of neurotransmitter release
opsin
proteins that capture photons
chromophore
attached to opsin, needed for vitamin A
how many different opsin molecules?
4
which photoreceptor has high sensitivity and night vision
rods
which photoreceptor has low sensitivity and day vision
cones
which photoreceptor contains more rhodopsin
rods to capture more light
which photoreceptor has high amplification?
rods
which photoreceptor has a faster response time?
cones
which photoreceptor is more sensitive to scattered light?
rods
which photoreceptor is more sensitive to direct axial rays?
cones
photoreceptor system with high acuity
cone system: less convergent
–> concentrated in the fovea
photoreceptor system with high convergence
rod system: many rods drive same ganglion cell
–> low acuity
which photoreceptor system contains multiple types of opsin?
cones: 3 types of opsin
-> chromatic: color perception
which photoreceptor system is achromatic?
rods: only contain 1 type of opsin
which photoreceptor is active in bright light? why?
cones active, rods inactivated
–> rods so sensitive that all opsin molecules broken down and they have no chromophore attached
which photoreceptor is active in dark? why?
rods active, cones inactive
–> not enough photons to activate cones
dark adaptation: bright light –> dark
temporary blindness:
- takes time for rods to re-activate
- cones no longer working due to absence of photons
light adaptation: dark –> bright light
temporary blindness:
- rods initially saturated: too much opsin so all rods activated at once = very bright input until opsin depleted
- cones take over due to presence of many photons
what bond does light break
bond between opsin and chromophore
what does retina report
the relative intensity of light: brightness depends on surroundings
2 types of receptive fields on retinal ganglion cells
- excitatory centre (+), inhibitory surround (-)
- inhibitory centre (-), excitatory surround (+)
when do excitatory (+) centre RF in retinal ganglion cells fire more AP?
when bright centre and dark surround
when do inhibitory (-) centre RF in retinal ganglion cells fire more AP?
when dark center and bright surround
what do retinal ganglion cells signal?
the relative differences of light (ie contrast) across their receptive fields
types of cones
- blue cones
- red cones
- green cones
- black cones
what are photoreceptors sensitive to?
the wavelength of photons –> color they carry
colorblindness
- missing a specific opsin molecule
- more common in men since opsin molecule found on X chromosome
what do retinal ganglion cells have that is specific to the fovea only?
color-opponent receptive fields
what does the output of retina encode?
relative values of brightness and color
flow of visual information to the brain (steps)
- information leaves through optic nerve: contains information from one eye with both visual fields
- the 2 optic nerves come together at the optic chiasm, where nerves on nasal side of retina cross
- visual fields divide into optic tract: contains information from both eyes with contralateral visual field
- visual information reaches the thalamus
- optic radiations travel from thalamus to visual cortex in occipital lobe
lesion between eye and optic chiasm causes
loss of vision in ipsilateral eye
lesion between optic chiasm and thalamus causes
loss of vision in contralateral visual field
lesion at optic chiasm causes
bilateral loss of temporal visual hemifields: only see inner visual field
lesion in visual cortex causes
loss of vision in contralateral visual field
the ‘where’ visual stream
parietal visual stream
primary visual cortex information received
- small RFs
- simple image features: oriented line segments
- divides into 2 pathways
parietal visual stream ‘where’
- large RFs: spatial features + motion
- polymodal: visual combined with other sensory modalities
the ‘what’ visual stream
temporal visual stream
temporal visual stream
- large RFs: complex image features
- object recognition: faces
pupillary reflex
shining light in one eye causes both pupils to contract
-> if brain bleeding: region containing optic nerve compressed so only 1 pupil constricts
auditory system stimulus energy
sound
auditory system receptor class
mechanoreceptors
sound amplitude
difference in pressure wave that form around the head
ie. the loudness
sound frequency
number of cycles per second
ie. pitch
ability to hear depends on…
amplitude and frequency
decibel (dB)
unit for measuring relative loudness of sounds
-> increase in 1dB = 10fold magnitude increase
dB =
20log(relative pressure)
(log base 10)
damage threshold
below pain threshold, can damage hearing above this point
hearing threshold
smallest amplitude that can reliably be detected
presbycusis
loss of sensitivity to hearing as you get older
ear anatomy
- tympanic membrane
- malleus, incus and stapes
- inner ear
- cochlea
tympanic membrane (eardrum)
vibrates as pressure waves change
malleus, incus and stapes
3 smallest bones in body, link tympanic membrane to inner ear
cochlea
contains neurons, where transduction occurs
flow of sound energy
- air pressure force causes oval window to go back and forth, creating a pressure wave
- fluid behind oval window amplify sound waves, ie. cochlear duct
- scala muscles contract when loud sounds occur to reduce movement of tympanic membrane
cochlear duct
middle fluid compartment of inner ear
basilar membrane
lines cochlear duct, moves up/down cochlear duct depending on sound frequency
sound frequency increases so local vibrations move…
closer to sound output
organ of corti
where basilar membrane motion converted into neuronal activity
hair cells in organ of corti
- outer hair cells
- inner hair cells
outer hair cells (organ of corti)
- receive more efferents which command them to contract
- actively shaping the motion of basilar membrane: electromotility
inner hair cells (organ of corti)
have many afferents for transduction
tip links
molecular strings connecting each stereocilia, mechanically gating ion channels
auditory transduction (steps)
- hair cells contain stereocilia which is affected by basilar membrane movement
- stereocilia moves, creating tension on tip links
- causes ion channels to be pulled open
- potassium enters hair cell: depolarisation
- calcium flows into hair cell, activating afferents
- afferent neurons produce AP
why is depolarisation different in the cochlear duct?
cochlear duct has different ionic composition fo K+ becomes the depolarising ion
auditory transduction key points
- sound waves are low energy
- fast: direct channel activation
- no amplification of transduction
visual transduction key points
- photons are high energy but hard to catch
- slow: G-protein cascade
- amplification: 1 photon closes many ion channels
central auditory pathway
- afferents send information through cranial nerve
- reach medulla: half cross the midline
- on both sides: goes to midbrain, then thalamus then primary auditory cortex
central auditory pathway is…
bilateral: auditory input used to localise sound
vestibular system stimulus energy
gravity, acceleration
vestibular system receptor class
mechanoreceptors
vestibular organs found in
the inner ear
vestibular organs
- semicircular canals
- utricle
- saccule
angular acceleration involves which vestibular organ(s)?
semicircular canals: head rotations
linear acceleration involves which vestibular organ(s)?
- utricle: horizontal movements
- saccule: vertical movements
vestibular occular reflex
when the head rotates, the eyes rotate in the opposite direction so the gaze stays constant
how angular acceleration leads to transduction
- head rotation causes fluid to move, creating pressure and causing cupula to bend
- hair cells dont move but stereocilia bends
- transduction with tip links
how linear acceleration leads to transduction
- head rotation causes stereocilia to bend but otoliths have inertia so lag behind
- transduction with tip links
gustatory system stimulus energy
chemical
gustatory system receptor class
chemoreceptors
gustatory organ
tongue
taste pore
where substances bind to chemoreceptors
taste bud
- line the papillae pores on tongue
- contain taste cells which all correspond to 1 of the 5 tastes
saliva role
dissolve molecules
salty taste transduction
sodium ions from food flow through ion channels causing transduction
sour taste transduction
high acidity so high protons, which interact with ion channels leading to transduction
bitter taste transduction
- bitter molecules block channels
- or bitter molecules trigger G-protein cascade
bitter
body’s way of telling a substance is harmful
sweet taste transduction
sweet molecules (glucose) bind to receptors encoding sweet: activates G-protein cascade, leading to transduction
umami taste transduction
glutamate receptors activate G-protein cascade, leading to transduciton
central taste pathway
- doesn’t cross the midline: only ipsilateral
- cranial nerves -> medulla -> thalamus -> gustatory cortex
olfactory system stimulus energy
chemical
olfactory system receptor class
chemoreceptors
olfactory receptor cells
specific to different types of molecules + contain cilia
what forms the olfactory nerve
short axons from olfactory receptor cells
where do olfactory neurons synapse?
at the olfactory bulb
cilia
line mucus membrane
where do molecules bind in the olfactory system?
in the cilia of olfactory receptor cells
olfactory signal transduction
- molecules enter through nasal cavity
- molecules dissolved in olfactory epithelium
- odorant binds to odorant receptor cells in cilia in olfactory receptor cells
- G-protein cascade activated
- ion channel opening causes olfactory receptor cell to send AP to olfactory bulb
central olfactory pathway
projects from olfactory bulb directly to different parts of the brain, mostly the Limbic system
how is the central olfactory pathway notably different from other sensory pathways?
it doesn’t involve the thalamus
