4 Nervous System I

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Nervous System I


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Nervous System Helps maintain homeostasis by

Integration and control


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Nervous System The nervous system is one of the two

organ systems that tell other tissues

what to do

(the other is the endocrine system)


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CNS & PNS Central Nervous System (CNS)

Brain and spinal cord

Completely encased in bone

Peripheral Nervous System (PNS)

Everything else

12 pairs of cranial nerves

31 pairs of spinal nerves


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CNS & PNS

CNS: clusters of cell bodies are

called nuclei

PNS: clusters of cell bodies are

called ganglia


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Integration &C

ontrol Detect changes in homeostasis

Issue an order, usually in the form

of a chemical messenger, to

target cells, tissues, organs, etc.

Watch for response

Usually negative feedback, sometimes

positive feedback


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Integration &C

ontrol

Nervous System Endocrine System

Fast Expensive

Neurotransmitter

Very specific,target cells on the

other side of synapse

Slow Inexpensive

Hormone

Any target cell withthe right kind of

receptor


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Nervous Tissue Neurons

Sensory neurons (afferent neurons)

Interneurons (association neurons) Motor neurons (efferent neurons)

Neuroglia Oligodendrocytes

Astrocytes Ependymal cells

Microglia

Schwann cells & satellite cells


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Neurons Communication requires:

Excitability (irritability)

Conductivity

Secretion of neurotransmitter


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Typical Neuron

Soma orcell body

Dark staining regions of RER called

Nissl bodies Dendrites (from 1 to 1000s per neuron)

Axon (0 or 1 per neuron)

Axon hillockAxon collaterals

Axon terminals (synaptic knobs)


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StructuralC

lassification Multipolarneurons

Unipolarneurons

Bipolarneurons

Anaxonicneurons

[afferent neuron, association neuron,efferent neuron is a functionalclassification]


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Axonal Transport

E.g., in a motor neuron the cell body

and the end of the axon may be

separated by > 1 meter

Have to be able to move things from the

cell body all the way down

Fast axonal transport (20-400 mm/day)

Slow axonal transport (0.5-10 mm/day)


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Neuroglia Oligodendrocytes

Occur in CNS

Form myelin sheath, insulation that

speeds up signal conduction


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Neuroglia Astrocytes

Occur in CNS, very abundant

Part of the blood-brain barrier, i.e.,

nutrients, wastes and so on have to go

through the astrocyte to cross between

the blood and the neurons


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Neuroglia Ependymal cells

Occur in CNS

Resemble cuboidal epithelium, but no

basement membrane and not derived from

the embryonic epithelial tissue

Line the canals and ventricles, producecerebrospinal fluidorCSF


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Neuroglia Microglia

Occur in CNS

Small macrophages (develop from

white blood cells), phagocytize dead

cells, pathogens, debris, etc.


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Neuroglia Schwann cells (neurilemmocytes)

Occur in PNS

Form myelin sheath much like that formed

by the oligodendrocytes in the CNS


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Neuroglia Satellite cells

Occur in PNS

Found next to neuron cell bodies in

ganglia, uncertain function


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Myelin

Oligodendrocytes in CNS, Schwann

cells in PNS, many layers of plasma

membrane around axon

Segmented: gaps = nodes of Ranvier

orneurofibril nodes

Saltatory conduction


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Myelin

Outer layer of glial cell = neurilemma,

present even in unmyelinated nerves

Wraps around once = unmyelinated

Wraps around many times = myelinated


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Gray & White

Matters

Gray matter: cell bodies and

unmyelinated axons

White matter: mostly myelinated axons


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Conduction

Larger diameter axon = fasterconduction

Myelin = faster conduction

A fibers (large, myelinated) up to

140 m/sec C fibers (small, unmyelinated) around1 m/sec


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Regeneration

In the PNS, a damaged nerve can

regenerate if the cell body and

at least a little neurilemma survive

Doesnt work the same way in the CNS

(at least, not yet)


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Synapse Electrical synapse: gap junctions

connect cells

Found in smooth muscle and cardiac

muscle, rare in nervous tissues

Chemical synapse: neurotransmitter

released into synaptic cleft


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Chemical Synapse

Neurotransmitter stored in synaptic

vesicles withinpresynaptic cell,

released in response to Ca+2 ions fromextracellular fluid

Receptors onpostsynaptic cell,

generally chemically-gated ion channels


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EPSP

EPSP = Excitatory postsynaptic potential

Temporarily depolarizes membrane of

postsynaptic cell Neurotransmitter usually acetyl choline (ACh)

May need 10+ EPSPs to get postsynaptic cell to

reach threshold

Limited duration, acetylcholinesterase (AChE)degrades ACh within synaptic cleft


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IPSP Inhibitory postsynaptic potential

Temporarily hyperpolarizes postsynaptic

membrane Neurotransmitter often GABA (gamma

amino butyric acid)

Opens K+ and Cl- channels,

hyperpolarizes membrane Makes it harder for postsynaptic cell to

reach threshold


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Summation Temporal summation

Many EPSPs or IPSPs arriving in a

short period of time (might all be comingfrom the same presynaptic neuron)

Spatial summation

Many EPSPs and IPSPs arriving frommany different neurons (but at about the

same time)


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Neurotransmitters Biogenic amines

Epinephrine (adrenaline)

Norepinephrine (noradrenaline)

Dopamine

Serotonin

Histamine


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Other Neurotransmitters

Excitatory amino acids

Glutamate

Aspartate

Inhibitory amino acids

GABA

Glycine


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Other Neurotransmitters

Neuropeptides

Endorphins, enkaphalins, dynorphins

(opiates target some of these receptors)

Dissolved gasses

CO (carbon monoxide)

NO (nitric oxide)

Others


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Poisons & Drugs Nerve gas, some pesticides: inactivates

AChE for up to several weeks

Prostigmine: temporarily blocks AChE,

helps in myasthenia gravis


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Poisons & Drugs Caffeine: reduces threshold

Botulism toxin: prevents release of ACh

Nicotine: mimics ACh, turns on a wide

variety of excitatory stimuli


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Central Nervous System

Structural classification

Cerebrum

Cerebellum Brainstem

Spinal cord

Also classify by developmental region(something-cephalon)


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CNS Development

Neural tube forms by the 4th week of

embryonic development, divided into:

Prosencephalon: forebrain

Mesencephalon: midbrain

Rhombencephalon: hindbrain

Spinal cord


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Forebrain

Prosencephalon (forebrain) develops

into telencephalon (left and right

hemispheres of cerebrum) anddiencephalon (thalamus, epithalamus,

hypothalamus)


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Meninges

Singular = meninx

Dura mater= tough mother

Arachnoid mater= spider mother

Pia mater= tender mother


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DuraM

ater In the cranial cavity, the dura mater is

divided into a periosteal layer and a

meningeal layer These two layers separated in some

places by a dural sinus, which empties

into the venous circulation

In the vertebral cavity, the dura mater is

separate from the bone (epidural space)


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DuraM

ater Meningeal layer folds inward to

produce:

Falx cerebri, in longitudinal fissure,between left and right hemispheres

Falx cerebelli, between left and right

halves of cerebellum

Tentorium cerebelli, in transverse fissure,

between cerebrum and cerebellum


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M

eningeal Spaces Epidural space: in vertebral cavity,

between dura mater and bone

Subdural space: between dura materand arachnoid mater

Subarachnoid space: between

arachnoid mater and pia mater(where the CSF is)


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Cerebrospinal Fluid

Cerebrospinal fluid(CSF): produced by

choroid plexus, a network of modified

capillaries within each ventricle, as wellas within subarachnoid space and by

the ependymal cells lining ventricles


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Cerebrospinal Fluid

Three main purposes:

Buoyancy (helps support weight of brain)

Protection (shock absorber)

Chemical stability (helps carry nutrients

and wastes around)


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Cerebrospinal Fluid

We each produce around 500 or600 mL of CSF per day

Capacity of ventricles and canalsonly ~150 mL

So we recycle all our CSF three or fourtimes per day

Arachnoid villiprotrude through duramater into sinuses


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Lumbar Puncture

Also called spinal tap, usually into

subarachnoid space between L3 and

L4 vertebrae


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Blood Brain Barrier

Capillaries have tight junctions between

cells, no leaking around endothelium

Astrocytes cover capillary all nutrientsand wastes go though astrocyte

between blood and neuron

CSF significantly different from blood,has more Na and Cl, less K


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Telencephalon

Frontal lobe

Voluntary control of skeletal muscle

Motivation, mood, intelligence

Parietal lobe

Evaluation of sensory information (pain,

touch, pressure, temperature, taste,but not vision, hearing or smell)


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Telencephalon

Occipital lobe

Vision

Temporal lobe

Smell and hearing

Memory

Insula Limbic system (emotion, memory)


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Frontal Lobe

Primary motor cortex: pyramidal cells,

precise control of voluntary movement

Premotor cortex: extrapyramidal tracts,control of learned, coordinated

movements


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Frontal Lobe

Frontal association area: sort of where

you are

Ever hear of a frontal lobotomy?

Brocas area: controls speech, left

hemisphere in 95% of people


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Diencephalon

Thalamus: almost all sensory input tocerebral cortex passes through the

thalamus Hypothalamus: homeostasis controls

autonomic nervous system, endocrinesystem

Epithalamus: Mostly pineal gland (moreabout this in 202)


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Metencephalon

Pons: sleep, hearing, equilibrium, taste,

eye movements, facial expressions,

Cerebellum: white matter calledarbor vitae, Purkinje cells, muscular

coordination


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Myelencephalon

Medulla oblongata: cardiac centers,

vasomotor center, respiratory centers


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Consciousness

Sleep and consciousness controlled by

the reticular formation, a loosely

organized region spread through thepons, midbrain and medulla


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Memory

Short term memory (minutes to weeks) in

temporal lobe

Also called primary memory Longer term memories processed through

amygdala and hippocampus (parts of limbic

system) on way to cerebral cortex

Secondary memory fades Tertiary memory stays


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Spinal Cord

No periosteal layer to dura mater only

meningeal layer

Gray matter and white matter arrangeddifferently from in brain


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Riding a Bike

Frontal association area

Decides if and where, makes the plan

Premotor cortex Since weve ridden a bike before

Primary motor cortex

Sends the action potentials to themotor units


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Riding a Bike

Brain stem

Smoothes the motion profile

Cerebellum Adjusts the motion profile as needed

to hit the target

Parietal lobe

Evaluates the sensory feedback (how wellis the plan working?)

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4 Nervous System I