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nerves

  • neuroanatomy maybe examined from the brain to the periphery or vice verse

  • here is periphery to the brain

  • nerves are the periphery of the nervous system
  • nerves are essentially bundles of axons
    • sensory bundles: carrying sensed information in
    • motor bundles: carrying actuation information out
  • there are two kinds of nerves
  • defined by where they exit from the skeleton
    • cranial nerves: neveres that exit from the cranial nerves
    • spinal nerves: nerves that exit from the spinal cord

spinal nerves

  • spinal nerves comes from each segment of the vertebral column

  • from the vertebral column out through little holes on each side

  • sacrum is the bottom of the spine
    • the location of a tail if we had one
  • over time, the holes get compressed and get smaller, squeezing the nerve openings
  • this may create unfortunate situations
    • weakness and pain
  • spinal nerves are very stereotyped
    • each level has sensory and motor components
  • the nervous system is setup very logically naturally
    • the segment connects the same level of body parts it comes out of

cranial nerves

  • the nerves that come out of the cranium are different
  • the cranium skull has holes for nerves to go out
    • there are 12 (I-XII) cranial nerves
    • some are only sensory
    • some are only motor
    • mixed type also exist
cranial nerve XI
  • spinal accessory nerve
    • has only motor bundle
  • allows shoulder movement and neck contractions
    • allows turning head
  • motor neurons of these nerves come out at the top of spinal cord
    • they go into the skull and come out of the skull
    • however, they don’t exit out of gaps of the vertebral column
cranial nerve I
  • nose sends axons through the holes in the cranium
    • in an area called cribiform plate
    • this is the cranial nerve 1
  • tiny nerves coming into the cranium through very tiny holes
    • they are unmyelinated
  • a shear force/whiplash type force can break the fragile axons in the nose
    • this causes the loss of smell
    • makes life less vibrant not being able to smell
  • in some cases these axons can grow back

CNS regional functions

  • functional neuroanatomy: functions of different parts of the neurosystem

  • key parts:

    • spinal cord
    • brain stem
    • diencephalon
    • telencephalon

sensory information from the world

spinal cord
  • one one kind of sensory information comes in:
    • touch
    • pain
    • temperature
    • pressure
    • vibration
    • blood oxygen levels
    • blood pressure
  • these are collectively called somatosensory information
    • only type of info that goes into the spinal cord
  • only body somatosensory information comes in
brain stem
  • somatosensory info from the face
    • into the pons
  • hearing and vestibular info to the hind brain
    • sound from ears, speech and other sounds
    • vestibular signal is like processed accelerometer signal
      • orientation, acceleration, standing, falling
      • all kinds of movement in space
forebrain
  • smell comes right into the cerebral cortex
    • telencephalon from the olfactory nerve holes in the cranium
  • vision comes straight into the diencephalon
    • since optical nerve pair pouched out from the diencephalon to begin with
  • the spinal cord cannot help perceive somatosensory info
    • the cerebral cortex (telencephalon) does the perception job

motor info for actuation

spinal cord
  • lower regions of the spinal cord actuate leg and foot movement
  • as the location on the spinal cord goes anterior (up), the actuation body part also goes higher
  • nerves coming higher up the spinal cord actuate the hands and arms
hind brain
  • houses important motor neurons
    • swallowing (survival)
    • talking (luxury/survival)
    • gag reflex (survival)
    • cough reflex (survival)
  • pons:
    • facial expression
    • horizontal gaze
      • moving eyes to the side
mid brain
  • vertical gaze
    • moving eyes up and down
  • depth vision convergence
forebrain
  • no motor output from the forebrain
  • it only directs the lower regions of the neurosystem to achieve motor output
    • this enables voluntary motion

autonomous actuation

sympathetic responses
  • this comes from the middle of the thoracic region of the spinal cord
  • it promotes flight or fight type responses
  • does not support relaxation activities and sleep
parasympathetic responses
  • actuation comes from the cranio sacral system
  • promotes relaxation, digestion, sleep and voiding

  • secretion activities

  • sacral regions go to colon, bladder and the sex organs

  • cranium region go to digestive, visceral and eye systems

  • the eye gets parasympathetic outflow from the mid brain
    • helps constricts the eye
    • helps falling asleep
    • damage: Miosis - set constricted pupil
  • the sympathetic outflow from the thoracic region also has a connection to the eye
    • this dilates the eye
    • injury: Mydrasis - set dilated eye

pituitary gland

  • the diencephalon came into being to control hormone release
  • hypothalamus controls all glands
    • glands release hormones into the body
  • the pituitary gland in brain is controlled by diencephalon

hemisphere function

  • primary areas of the cortex

  • front lobe
    • motor cortex strip - M1
    • sulcus - central sulcus
    • primary somatosensory cortex - S1

  • the temporal lobe is the lobe that grow to the back and comes to the front on the sides

  • temporal lobe
    • primary auditory cortex in the middle (A1)
    • visual cortex in the back (V1)
  • the primary cortex in the left hemisphere actuated right-side body parts
    • right hemisphere analogous to the left hemisphere

auditory cortex

  • input for both ears
  • there might be minor predominance

visual cortex

  • left visual cortex see everything on the right
    • irrespective of the which eye the input is coming in
  • symmetrical distinction
    • analogous to the left

language

  • language only exists in the left hemisphere
  • if memory is altered by a disease of the brain,
    • the left hemisphere most likely carries the cause of the disorder

aphasia

  • language disorder of left hemisphere
    • difficulty with language
    • verbal memory is negatively affected
    • derailed semantic communication
  • this has existed and be recognized for centuries

  • if there is an aphasia, the left hemisphere is to be first examined

  • symptoms
    • lack of finding words
    • alien and incoherent word use
    • issues with syntactic rules
    • pre-posing one word on the other
  • the thought could not be expressed with words
    • due to lack of the ability to find words
    • the few words that are found are incoherent
    • grammatical capability goes away
  • the tone (prosody) and facial expression is used by the affected person to convey thoughts
    • in lieu of verbal memory
    • a more frustrating experience
  • non-fluent aphasia
    • a lesion in the left frontal cortex cause this
    • the words dont come out well
    • but speech is understood fine
  • in the best of circumstances, this can be recovered from
    • many instances where symptoms of aphasia are completely gone after recovery

language circuits

  • spoken language comes into auditory cortex from the ear on both sides
  • on the left side, there is the TPJ - temporo-parietal junction
    • sensory processing of speech sounds is worked out here in the TPJ
  • then the information goes into frontal lobe in two pathways, ventral and dorsal
    • the dorsal pathway does speech production
    • the ventral pathway does speech comprehension

working

  • sounds come in
    • they have to be analyzed in many time scales
  • then they have to be classified into different meanings
    • based on the syllables used and
    • the time scale context
  • the function of ventral pathway is lexical interface
    • semantic meaning assignment is done here
    • so speech can also be made here
  • the job of the ventral pathway is to organize and produce the interpretation of sound
    • this is then sent to the TPJ
  • in both sign language or braille, the visual cortex is used
    • used in deaf people
  • in interpretation of the meaning, however,
    • the ventral pathway’s lexical interface is utilized
    • for semantic interpretation
  • language comprehension happens in the lexical interface
    • can take input from either the auditory or the visual cortex
  • lesions in the ventral pathway affects communication output
    • like output sign language
    • like expressing thoughts
  • lesions in the temporal mid brain (TPJ specifically) cause fluent aphasia
    • speech and reading comprehension are impaired
  • more subtle forms affect understanding complicated sentences
    • subtle fluent aphasia
  • hearing and comprehension circuits are linked
  • improvement of such conditions in more a rule than an expection

flat schematic of the brain

  • a tool for locating parts in the brain

cerebral-lobes

fig: cerebral lobes; top - side view; bottom - midsagital cut cross section view

  • the temporal lobe is formed by the ram’s horn type expansion of the telenchephalon
    • if the temporal lobe is pried to the sides and the space below is observed
    • there is another lobe on the inside
    • this lobe is the insular lobe
  • frontal lobe handles motor functions
    • has the primary motor cortex
  • parietal love handles somatosensory functions
    • has the primary somatosensory cortex

CNS-block-diagram

fig: CNS - block diagram

  • cerebral cortex is only the mantle of the telencephalon
  • basal ganglia is critical to action selection
  • amygdala is the fear and feeling of fear processor

blood in brain

oxygen and cerebral blood flow

  • oxygen keeps the brain working
  • brain is 2% of body mass
    • takes 25% of oxygen intake
  • if brain loses oxygen supply, neurons begin dying within minutes
    • unlike other cell types, new neurons are not born when old neurons die
    • brain neuron death do not heal

  • heart can work with for about 30 minutes without oxygen

  • high altitudes in an example
    • the air is under much less air pressure
    • air molecules are much more dispersed
  • only a third of the oxygen molecules is available on a mountain top compared to the beach
    • the brain doesn’t function so well in mountain situations
    • this is called acute mountain sickness

brain oxygen supply

  • brain gets its oxygen supply from oxygenated blood
    • profusion: blood supply in brain
  • blood flow depends on two variables
    • pressure in arteries (arterial pressure)
      • high pressure
    • pressure in the veins (venous pressure)
      • low pressure
      • 5-10 mmHg
  • the profusion pressure in the difference between pressure in arteries and veins
    • standard for non brain organs like
      • spinal cord
      • liver
      • stomach etc

cerebral profusion pressure (CPP)

  • in the cranium, there is another pressure variable
    • it is the intracranial pressure (ICP)
    • ICP is a little higher than venous pressure
      • 15 mmHg
    • this is only in the solid skull
  • the pressure differential the drives blood flow in the brain is the difference between the arterial and ICP
    • so the differential pressure is lesser
  • cerebral profusion pressure: difference between arterial pressure and ICP
    • the operational differential has to be about 60-70 mmHg
    • if not maintained, brain quits,
    • when brain quits, subject faints - termed syncope
  • in a syncope situation
    • consciousness is lost - no communication
    • no opposition to gravity - subject falls
    • not enough oxygen supplied to brain due to lack of CPP
  • the brain is more vulnerable to drops in arterial pressure
    • may cause the loss pressure that drives profusion of the brain
    • affects neuron working
    • caused by usually by dehydration
  • another scenario is the ICP is driven up because of maybe a tumor
    • in such a case the pressure differential is lower as well
    • so blood profusion is lower in the brain
    • affects neuron working

brain blood supply

  • blood brings in oxygen into brain
    • lungs oxygenate blood
  • a complex network of blood vasculature distribute blood in the brain

brain-vessels

fig: blood vasculature extracted from the brain

  • the blood supply comes into the brain from the
    • two arteries in the middle center
      • called internal carotids
      • responsible for anterior cerebral hemisphere blood circulation
        • including thalamus and hypo thalamus
    • two arteries in the bottom center
      • called vertebral arteries
      • responsible for posterior brain circulation
        • including brain stem
  • each of the internal carotids branches of into two
    • the inner branch of each are connected by a communicating artery
  • the two inputs of the vertebral artery merge to one
    • this thicker merged branch splits into two branches
    • one goes horizontally left
    • the other, horizontally right
  • the posterior and anterior circulation is connected by two communicating arteries
    • one on the left and another on the right
    • these are the posterior communicating artery
  • the three communicating arteries and the think branch system form a circulation path for blood flow
    • this helps in case there are blockages
    • helps recover from blockages
    • this circle of willis
    • it is a backup plan to bring back blood flow into the brain

brain-vessels

fig: circle of willis in blood vasculature extracted from the brain

  • the two vertebral arteries come up from the foramen magnum
  • the internal carotids come in from the base of the cranium
  • so, the entire source of blood comes up from the base of the brain

strokes

  • strokes are common neurological events that send people to the hospital
    • common neurological event that causes death
  • while stokes are physically cardiovascular events
    • the effect is fully neurological
    • a left hemisphere stroke the person may lose control on the right side of the body and become asphasic

types of strokes

ischemic
  • no oxygen supply causes this type of stroke
  • usually caused by blockages
hemorrhagic
  • blood vessels open up and bleed
  • blood tissue does not like blood and cannot survive in a sea of blood
  • usually a very sudden event

effects

  • depends on where in the blood supply the stroke hits

  • the entire region of the brain that does not receive oxygenated blood due to the broken blood vessel gets affected by the stroke

  • so strokes can affect very small regions and be mild or even barely recognizable if blood vessels at the periphery are affected
    • localized peripheral blockages and
    • small ruptures that close quickly with blood clotting the holes
  • in contrast to a central artery being affected by the stroke, which can turn off oxygen supply for large parts of the brain
    • causing damage to a large portion of the brain
  • causes of blockages
    • plaque in blood
      • arthro-sclorotic plaques
    • arteriovenous malformation
      • condition where the vessel walls are compromised
      • leads to hemorrhagic stroke
  • blockages are usually tried to be cleared out by blood thinners
  • but if a hemorrhagic stroke is treated with blood thinners, blood does not clot
    • so blood will not heal the burst blood vessel
    • so it worsens the situation
treatment
  • it is super crucial to discriminate between an ischemic stroke and a hemorrhagic stroke to administer appropriate treatment
    • as treatment for makes the other condition worse
    • clinical imaging is always relied upon (as of 2016) to discriminate between the two
      • to ensure accurate diagnosis of the source of the stroke
  • if neurons are left without oxygen or sit bathed in blood for a long time, they die
  • the neurons surrounding the stroke area are at risk of dying in this way
    • the surrounding area is called penumbra
    • treatments are aimed at this region

tumor

  • tumors beginning anywhere may eat into brain space and make it bereft of neurons
    • a tumor from skin around the brain can grow into the brain
  • their are usually gradual, but they may cross some threshold at any point
    • it is hard to estimate where the threshold lies

bleeds

  • bruises within the cranium can cause bleeds
  • consequences if bleeds in the cranium are serious
    • because of densely packed brain cells

  • skull - dura - arachanoid - pia - parenchyma (brain)

  • no space between the dura and skull, dura and arachnoid
    • blood vessels are coming in from the outside
    • into the brain
    • through the protective layers
  • underneath the arachnoid is the cerebrospinal fluid (CSF)
    • protects the brain from crashing into the skull
      • until very high acceleration
    • gives positionally stability

  • bleed in the parenchyma is a hemorrhagic stroke

  • if there is a bleed between these dura and skull, it is termed epidural bleed
    • more specifically epidural hematoma
    • if not treated quickly, it is fatal
    • this may put the person in a lucid interval
  • potential space between the dura - arachnoid created by bleeds
    • bleed here is subdural hematoma
    • consequence vary from asymptomatic to lethal
    • older people have some small subdural hematomas that show no consequence
  • bleeds in the sub arachnoid CSF space
    • humans are very sensitive to injuries in meninges
      • they offset the inability to feel pain within the brain
    • blood in this space causes the worst headache in a person’s life
    • they are lethal

references