Unit 2 - the CNS

explain the evolutionary trends in nervous systems

1. Nerve net (jelly fish)

2. Bilateral symmetry (anterior and posterior)

   • cephalization

3. Consolidation of peripheral nervous system to create nerves (as opposed to a loose network like with the nerve net)

   • Ventral nerve cord(s) (starts at front)

   • Dorsal nerve cord (goes back)

   • Spinal cord (both routes protrude into spinal cord)

4. Increasing role of forebrain (more power there throughout evolution)

which nerve is responsible for most parasympathetic innervation in the CNS

what do dorsal and ventral mean

dorsal = back

ventral = front

explain the development of the vertebrate CNS (how the neural fold is made)

• Neural plate Borders come together and create the neural fold

• the neural plates converge to make the neural tube (which will then become the CNS)

• neural crest cells shed off the neural tube and migrate throughout the body → contribute to many structures, including the peripheral nervous system

explain the development of the human CNS (4 weeks)

• anterior end of neural tube specialized into three regions (the forebrain, midbrain, and hindbrain)

explain the development of the human CNS (6 weeks)

neural tube differentiated into major brain regions present at birth

Hindbrain

• Medulla oblongata

• Cerebellum and pons

Forebrain

• Diencephalon

• Cerebrum

explain the development of the human CNS (11 weeks)

growth of the cerebrum much more rapid than that of other regions

explain the development of the human CNS (birth)

cerebrum covers most of other brain regions; convoluted surface due to rapid growth in confined space

what provides protection and support for the CNS

• surrounded by a bony cage → cranium (brain) and vertebrae (spinal cord)

• three layers of connective tissue → meninges (in both the brain and spinal cord)

• fluid between layers → cerebrospinal fluid (in central canal (spinal cord) and in ventricles (brain)

what are the meninges

the cura mater

arachnoid

pia mater (closest to brain, follows grooves on it, thin)

what is a subdural hematoma

when blood leaks into the subdural layer (between dural and arachnoid layer)

where is cerebral spinal fluid found

• in ventricles within brain and the hollow central canal within spinal cord

  • fluid is continuous with both

• in ventricular system and bathes external surfaces of brain, between meninges

what are ventricles

fluid-filled cavities in the brain

• two lateral ventricles, two descending ventricles that extend through in the brain stem

what are ependymal cells? how do they work

• A type of glial cell

• line the inner surfaces of the brain ventricles and the central canal of the spinal cord → create barriers between compartments

• act as a barrier between the cerebrospinal fluid (CSF) and the brain tissue, regulating what flows in and out of the brain and maintaining overall brain health

• they allow CSF to escape into the subarachnoid space so ventricles don’t expand into the brain → bathes CNS from outside as well (cleanses brain)

• specialized ones in the choroid plexus create CSF

what allows CSF to escape into the subarachnoid space so ventricles don’t expand into the brain → so they can bathe CNS from outside as well (cleanses brain)

ependymal cells

what produces CSF

the specialized ependymal cells within the choroid plexus within each ventricle (each ventricle has them)

• blood from the capillaries in the choroid plexuses is filtered through ependymal cells producing CSF (water and some ions, nutrients, and vitamins passed through)

what is interstitial fluid

surrounds neurons and glial cells

what is plasma

within cerebral blood vessels

how does CSF compare to plasma

low protein, no blood cells

• presence of RBCs or elevated protein in CSF collected via lumbar puncture (sampling of fluid from subarachnoid space between vertebrae) suggests infection

what are oligodendrocytes

form myelin sheaths within the CNS (white matter)

what are microglia

immune cell lineage - phagocytic

what are astrocytes

numerous in brain

• regulate local extracellular fluid by releasing chemicals

• lots of projections (look like a star)

• provides another layer around capillaries in the brain (a part of the the blood brain barrier)

what type of cells are ependymal cells

non-neuronal cells (type of glial cell → NOT epidermal cells)

how is CSF reabsorbed into venous blood

through arachnoid villi (finger-like extensions of the arachnoid membrane into the dural venous sinuses)

→ CSF moves from arachnoid villi in the subarachnoid space into venous blood (it’s reabsorbed)

explain the circulation of CSF

what is a hydrocephalic brain and how does it compare to a normal brain

ventricles are enlarged due to CSF build-up (when it can not drain)

what do astrocyte foot processes do

secrete paracrine factors that promote tight junction formation → aids in tight junction formation → key in the brain (and blood-brain barrier)

what do tight junctions do

• prevent solute movement between endothelial cells

• determine how leaky/prone to exchange blood vessels

• VERY tight in brain, not as tight in spinal cords

what is the blood-brain barrier and what can/can not cross it

• lipid soluble molecules cross readily

• hydrophilic substances (ions, amino acids, peptides) only cross if specific transporters / carriers are present on endothelial cells of capillaries within the CNS

what determines if an antihistamine is drowsy or not

non-drowsy ones do not cross the blood-brain barrier

what causes parkinson’s disease

loss of dopaminergic neurons in regions of the brain

how is parkinsons treated

• dopamine can not readily cross the blood-brain barrier

• L-dopa (dopamine precursor) can be transported into brain because it is similar structurally to L-tyrosine (chemical that has a transporter for it)

• then is transferred into dopamine after crossing to treat parkinsons

what are 2 things the brain absolutely needs to survivr

oxygen and glucose

explain neural tissue’s oxygen requirement

neurons are obligate aerobes

• unable to switch to anaerobic metabolism

• O2 readily crosses the blood brain barrier to compensate

explain neural tissue’s glucose requirement

capillaries of CNS express high levels of glucose transporters to promote adequate levels of glucose

• brain is responsible for approx half of body’s glucose consumption

what are the sections of the spinal cord

Cervical

Thoracic

Lumbar

Sacral

what does the spinal cord do

major path for info flow between CNS and skin, joints, muscles

• contains neural networks involved in locomotion

• divided into 4 regions, each of which is divided into segments

• each segment gives rise to pair of spinal nerves

what do parasympathetic and sympathetic nerves connect to respectively within the CNS and what do they innervate

parasympathetic:

• connect to the brainstem and sacral sections of the spinal cord

• innervates

which subsection of the nervous system (parasympathetic or sympathetic) does the vagus nerve fall under

the parasympathetic NS

what are the 2 types of matter that primarily compose the CNS

white and gray matter

what are white and grey matter respectively

compose the CNS

White matter:

• type of brain tissue that consists primarily of myelinated neuronal bodies (the myelin sheaths giving them that white colour)

• typically just myelinated axons that form tracts going to/from the brain

Gray Matter:

• unmyelinated neuronal bodies (cell bodies, unmyelinated axons, dendrites)

• makes up structures within the CNS such as the dorsal and ventral horns

• typically considered an integration site within the CNS so it is involved in processing and passing information between neurons

what type of matter makes up the dorsal and ventral horns

grey

what type of neurons make up the dorsal root of the spinal cord

sensory neurons

what are the dorsal root ganglia

• clusters of neuronal cell bodies within the dorsal root of the spinal cord. The cell bodies are typically bipolar sensory neurons

  • bring in sensory information from the body.

explain the flow of afferent and efferent information in and out of the spinal cord

• The spinal cord is split up into 2 roots → the dorsal and ventral roots

• dorsal roots (or also called the afferent root) carry (primarily sensory) information from the body to the spinal cord

• dorsal root ganglia are clusters of neuronal cell bodies within the dorsal root.

  • These cell bodies are typically bipolar sensory neurons

  • bring in sensory information from the body.

• sensory info is then brought from the dorsal root ganglia to the dorsal horns → so the dorsal horns are where the terminals for these sensory neurons that protrude from the dorsal roots are.

• dorsal horns receive this incoming information from the terminals of the sensory neurons.

• signal reaches the dorsal horns for integration then goes into the ventral horns (so this is where the ventral root begins).

• The ventral root (also called the EFFERENT root) is the off-ramp for information so it carries newly integrated signals (that now typically contain motor information to go TO the body in response to the sensory information previously obtained FROM the body)

• So within the ventral root the signals are carried through the ventral horns and into the ventral roots where they will propagate out of the spinal cord

• because the newly integrated information is most commonly motor information now, the ventral root is typically composed of motor neurons instead of sensory neurons like in the dorsal root

what is the difference between bipolar neurons, DRG neurons, and somatic motor neurons

explain how spinal reflexes work

spinal reflex initiates response without input from brain

• stimulus stimulates sensory neurons that send signal to spinal cord

• interneurons in the spinal cord (grey matter) serve as the integration center for this info → often also sends signals up to brain so brain knows whats happening but that’s SEPARATE process from this one

• interneurons send a command to muscles or glands which invokes a response from the target muscle/gland

what composes ascending and descending tracts in the CNS

white matter

what are the ascending tracts in the spinal cord and what do they do overall and specifically

sensory info going up

• dorsal columns (touch/pressure and propioception) [main one we focus on]

• spinocerebellar (proprioception (posture, coordination))

• spinothalamic (pain, temp)

these tracts are organize the info throughout the spinal cord and pile on as they get closer to the brain

what are the descending tracts in the spinal cord and what do they do overall and specifically

descending motor info

the corticospinal tracts (voluntary movement)

what is the oldest and most primative part oft he brain

the brainstem

what is the brainstem organized a lot like? explain how

the spinal cord

• most cranial nerves originate from there

  • most cranial nerves originate from here

    • carry sensory and motor info for head/neck

    • contain cranial nerves (notably cranial nerve X - vagus)

• contains nuclei associated with reticular formation

what is reticular formation and what contains nuclei associated with it

reticular formation

• network of neurons involved in processes such as arousal/sleep, muscle tone, coordination of breathing, etc

• brainstem contains nuclei associated with reticular (network) formation

what are the main structures that make up the brainstem and what do they do

• midbrain

  • eye movement coordination, visual and auditory reflexes

• pons

  • relay station between cerebrum and cerebellum

  • works with medulla to regulate breathing

• medulla

  • gray matter involved in controlling of many involuntary functions (eg. blood pressure, breathing, swallowing, vomiting, etc)

  • white matter is involved in ascending somatosensory tracts and descending corticospinal tracts

    • site of decussation (crossing over)

Notably pons and medulla

what is the site of decussation (crossing over) for most neurons in the corticospinal tract

the medulla

what is cranial nerve X and what does it do?

vagus nerve - provides sensory info to a lot of places

what composes the cerebrum

1. the cortex (has lobes, outside of brain)

   • each cerebral hemisphere is divided into 4 lobes

     • frontal, occipital parietal, temporal

2. subcortical structures (structures beneath cortex)

what is the cerebellum

second largest structure in the brain, coordinates movement

where is the diencephalon and what 4 structures composes it? explain what its structures do

in the middle of the brain, between the brainstem and cerebrum

• thalamus

  • relays and integrates sensory info from lower parts of the CNS

• hypothalamus

  • major centre for homeostasis

    • influences autonomic responses, endocrine systems

    • contains centers that drive behaviour related to hunger, thirst, etc)

  • pituitary gland

    • regulated by hypothalamus

• pineal gland

  • secretes the hormone melatonin → involved in circadian and seasonal rythums

what does the cerebrum do

• site of “higher” brain functions

  • largest and most distinctive part of brain in higher primates

t/f: degree of folding related to capacity for higher processing in a species

f → not completely determined by folding

what is a sulcus and gyrus

sulcus - forrow or groove

gyrus - convolution (ridges on surface, bumps)

what are the 3 regions of cerebral gray matter and what do they do

1. basil gangliali (basil nuclei since in CNS)

   • coordination of movement

2. limbic system

   • linking emotion/fear with higher cognitive functiuons

3. cerebral cortex

t/f: behavioural state and cognition effect motor system output in the brain

true → internal feelings and cognitive functioning influence reactions, reflexes and each other

what do sensory areas in the cerebral cortex do

translate sensory input into perception (awareness)

what do motor areas in the cerebral cortex of the brain control

skeletal muscles

what do association areas in the brain do

• integrate info from sensory and motor areas → many areas involved with coordinating somatic movement and sensory info

what do the gustatory cortex and olfactory cortex interpret respectively

taste and smell respectively

what information does the parietal lobe and frontal lobe process specifically

parietal - contains somatic sensory cortex → processes sensory info from skin, musculoskeletal system, viscera

frontal - contains primary motor cortex, involved in skeletal muscle movement

what do the somatotopic maps in motor and somatosensory cortices show

• Motor: hands and face have a higher SA on the motor cortex → more control

• Somatosensory: lips and hands are very big

where do ascending and descending tracts go and what type of signals are they respectively

ascending → to brain, sensory information

descending → from brain, motor information

explain the flow of motor information from the brain to right tunk muscles and right limb muscles respectively

trunk muscles:

• motor signals sent from primary motor cortex of left cerebral hemisphere

• signals go down nerves in the anterior corticospinal tract

• crosses over to the right side of the body lower down the spinal cord

• synapses with somatic motor neurons to skeletal muscles on the right side of the trunk

limb muscles:

• motor signals sent from primary motor cortex of left cerebral hemisphere propogate down nerves

• signals go down nerves in the lateral corticospinal tract

• crosses over to the right side of the body higher up in the brainstem (specifically the medulla)

• synapses with somatic motor neurons to skeletal muscles on the right limbs

trunk muscles go straight down anterior corticospinal tract, cross low,

limb muscles go down the anterior corticospinal tract, cross high (in medulla), go down lateral corticospinal tract

where do most fibers from the motor cortex cross over

the base of the brain (medulla)

how does the spinal cord act as an integrating centre with the brain