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Nervous System Physiology – outline notes

By at September 22, 2011 | 6:59 am | Print

Nervous System and Endocrine System = maintain homeostasis, control

Neural Tissue
Cells – Neuroglia and Neurons

Neuroglia (glial cells) – supporting cells

-CNS – 4 types
1. Ependymal cells
-line central canal of spinal cord and ventricles of brain
-ciliated – help circulate Cerebral Spinal Fluid in ventricles
-CSF – surrounds, cushions, supports brain
-CSF exchanges nutrients and wastes with interstitial fluid of brain across ependymal cells
-Choroid Plexus – ependymal cells and capillaries
-site of CSF production (third ventricle)
-CSF is continually replaced

2. Astrocytes
-largest, most abundant
-Blood-Brain Barrier
-separates interstitial fluid of brain and circulating blood
-Tight junctions – endothelial cells of capillaries
-astrocytes wrap capillaries and regulate permeability
-regulates interstitial fluid
–glucose and ions are pumped in
-permeable to:  lipids, CO2, O2
-most chemicals cannot pass

Blood supply to brain
↓         ↓
Blood-Brain Barrier  and Blood-CFS Barrier
(tight junctions and  (choroid plexus)
astrocytes)   ↓
↓   ↓
Neurons/Neuroglia   <——– > interstitial fluid <——– > CSF
(intracellular)

3. Oligodendrocytes
-membranes wrap axons in CNS
-myelin – white membranous insulation, speeds up nerve transmission
-axons wrapped in myelin sheath
-white matter
-nodes of Ranvier (ran-vE-A)
-gaps in myelin sheath along axon

4. Microglia
-phagocytes

PNS – 2 types

1. Satellite cells – surround neuron cell body
– regulates interstitial fluid (environment around cells)

2. Schwann cells – myelinate axons in PNS

Neurons (nerve cells)
-conducts nerve impulses
-amitotic
-can live over 100 years
-high metabolic rate – constant need for O2 and nutrients

Structure of a Neuron
-cell body (perikaryon)
-nucleus with nucleolus
-normal organelles
-Nissl bodies – cluster of rER; gives cell body gray appearance (gray matter)
-no centrioles – amitotic
-Nuclei – cluster of cell bodies in CNS
-Ganglia – cluster of cell bodies in PNS; surrounded by satellite cells
-processes – extensions from cell body; 2 types
Dendrites – receive information from other neurons
-numerous
-conduct Graded Potentials toward cell body
Axon – the initial segment arises from axon hillock of cell body
-axons branch and terminate in synaptic terminals
-synaptic terminals contain synaptic vesicles with neurotransmitters
-conduct Action Potentials away from cell body
-Tracts – bundles of axons in CNS
-Nerve – bundles of axons in PNS

Neurophysiology

Membranes

-Polarized / Transmembrane potential – charge difference across membrane
-Resting membrane potential – transmembrane potential at rest
-Membranes selectively permeable and  negative on inside
-intracellular fluid (cytosol) – high in K+ and proteins-
-extracellular fluid (interstitial fluid) – high in Na+ and Cl-
-proteins do not pass
-membrane more leaky to K+ than Na+ = negativity on inside
-creates a potential difference
-Na+/K+ pump = pumps out 3 Na+ / pumps in 2 K+
-ATP dependent, against gradient
-maintains resting membrane potential

-Ohm’s Law
Current = Voltage / Resistance

Volts = measure of potential difference across membrane (-70 mV)
Current = movement of charged particles
Resistance = restricts ion movement (membrane)

Membrane channels
-proteins in membranes – control movement of ions across membrane
-2 classes of channels

1. Leak channels – always open
2. Gated channels – closed for resting cell
1) Chemically gated – chemical stimuli (neurotransmitter)
-dendrites and cell body
-Graded Potentials
2) Voltage gated – open due to changes in membrane potential
-axons and sarcolemma
-Action Potentials

Action Potentials
-occur on axons
-voltage gated channels

STEPS
Resting membrane potential (-70mV)
-voltage gated channels are closed

1. Depolarization to Threshold
-local current changes (graded potentials) bring membrane to threshold (-60 mV)
-depolarization – inside of membrane becomes less negative (more positive)

2. Rapid Depolarization  
-at threshold, -60mV, voltage gated Na+ channels open
-Na+ rushes in
-membrane depolarizes from -60mV to +30 mV
–All-or-None principle – action potential will occur when membrane
potential reaches  threshold, -60 mV
3. Repolarization
-at +30mV, Na+ channels close and voltage gated K+ channels open
-K+ leave cell
-inside becomes more negative (less positive)

4. Hyperpolarization (-90mV)
-K+ channels close too slowly

**Resting membrane potential is restored (-70mV)
** Na+/K+ pump restores ion levels across membrane

Propagation of Action Potential
-actions potentials spread across entire membrane
-begins at initial segment
-positive ions in axoplasm move laterally from local area of polarity reversal to
adjacent area that is negative and in interstitial fluid the positive ions move to area
of negative charge
*this opens Na+ voltage gated channels and triggers another action
potential ——–over and over across entire membrane

 Propagation velocities of axons
1. Myelin
Saltatory propagation – “jumping”
-myelinated axons
-action potentials only occur at node of Ranvier
-nodes are 1-2mm apart

2. Axon diameter
-larger the diameter, faster propagation speed

Fiber types
Type A
– largest diameter, heavy myelin
-140 m/s (300 miles/hour)
-motor neurons to skeletal muscle
Type B –

Type C
– unmyelinated, smallest diameter
-1 m/s (2 miles/hour)
-neurons to smooth muscle, glands

Chemical Synapse
-neurotransmitters
-axon of presynaptic neuron ———->dendrites of postsynaptic neuron

Cholinergic synapses – ACh
1.  Action potential arrives and depolarizes synaptic knob
-voltage gated Ca2+ channels open and Ca2+ rushes into synaptic knob
-exocytosis of ACh
2. ACh diffuses across synaptic cleft (20nm) and binds to postsynaptic receptors on
dendrites
-postsynaptic receptors are chemically gated ion channels
-produce Graded Potentials
3.  ACh is removed by AChE

Graded Potentials
-transient changes in transmembrane potential
-chemically gated channels open

 Postsynaptic Potentials – graded potentials of postsynaptic membrane (dendrites)
-caused by neurotransmitter binding to postsynaptic receptors
1. Excitatory Postsynaptic Potential (EPSP)
-excitatory synapse, excite membrane
-neurotransmitter binding causes depolarization
-Na+ channels open
-promote Action Potentials

2. Inhibitory Postsynaptic Potential (IPSP)
-inhibitory synapse, inhibit membrane
-neurotransmitter binding causes hyperpolarization
-K+ channels open
-Cl- channels open
-inhibit Action Potentials

*receptors are linked to ion channels
*receptors determine if N.T. will excite or inhibit membrane

Summation
-EPSP and IPSP are added together at axon hillock (accountant) and spread to the    initial segment of the axon
-single EPSP depolarizes membrane about 0.5 mV
-combined EPSP must reach threshold(-60mV) at initial segment for action
potential to occur and be propagated

-Facilitation – neuron brought closer to threshold
-summation of EPSPs
-drugs
-nicotine – stimulates ACh receptors, produces prolonged EPSPs
-caffeine, theobromine (cocoa), theophylline(tea)
-lower threshold at initial segment, smaller depolarization will
trigger action potential
-also, increase ACh released at synaptic knob

Disrupting Neural Function
1. Changes in pH / [H+]
-normal pH = 7.35-7.45
-rise in pH facilitates neurons
-at 7.8, spontaneous action potentials = convulsions
-drop in pH inhibits neurons
-at 7.0, decreased action potentials = person unresponsive

2.  Ion (electrolytes) concentration changes; K+, Na+, Ca2+

3. Body temperature
-increase temperature facilitates neurons, hallucinations or convulsions
-decrease temperature inhibits neurons, lethargic and confused

4. Drugs
-Botulinus toxin – blocks ACh release; paralysis
-Botox – cosmetic purposes
-Venom of black widow spider – massive release of ACh; intense cramps, spasms
-Anticholinesterase drugs – blocks AChE
-military nerve gases
-extreme state of contraction
-Atropine – blocks ACh from binding; muscarinic receptors
-cardiac and smooth muscle
-can counteract anticholinesterase drugs
-Curare – blocks ACh receptors at neuromuscular junction
-nicotinic receptors
-plant extract used by S. American tribes to paralyze prey
-Nicotine – binds to and stimulates receptors on postsynaptic membrane

Neurotransmitters
-chemicals stored and released by synaptic knob
-over 50 identified

Structural Classifications

1. Acetylcholine
-1st discovered, best studied

2. Biogenic Amines
-Catecholamines
Norepinephrine, Epinephrine, Dopamine
-Indolamines
-Serotonin, Histamine

3. Amino Acids
-GABA (gamma aminobutyric acid), Glycine —————> inhibitory
-Aspartate, Glutamate ————> excitatory

4. Neuropeptides
-Substance P, Neuropeptide Y
-Opioids
-endorphins, enkephalins
-Hormones
-CCK, ADH, OT
5. Gases
-NO, CO

 

 

 

 

 

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