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PHYSIOLOGY
Nervous System
Types of Neurons
AfferentSensory
EfferentMotor
Interneurons also known as association neuronsBetween neuron
Classes of Sensory Receptors also known as Neurons Mechano-receptors: mechanical forces- stretching alters membrane permeability (1) hair cells* (deflection = depolarization = AP's) ie. lateral line of fish (mechanoreceptor=
neuromasts detect water movement, etc) (2) stretch receptors of muscles (3) equilibrium receptor of inner ear (4) receptors of skin (touch, pain, cold, heat). Chemo-receptors: chemicals sense solutes in solvents, taste, smell Osmo-receptors: of hypothalamus which monitors blood osmotic
pressure Photo-receptors: light - eye, eyespots, infrared receptors of snakes, etc. Thermo-receptors: radiant (heat) energy Phono-receptors: sound waves Electro-receptors: detect electric currents... electric eels, etc.. Nociceptors: pain receptors... naked dendrites of skin (epidermis)
Neuroglial Cells of the CNS
Astrocytes
In the CNS only Most abundant Neuroglial Cell Formation of Synapses Plays a role in making exchanges between
capillaries and neurons Helps to form the Blood Brain Barrier
The BBB protects the brain from intruders
Astrocytes
Microglial Cells
MacrophageScavenges apoptotic cells
May go bad causing Alzheimer’s Disease Excessive secretion of Interleukin-1
Helps to maintain homeostasis in the brain
Ependymal Cells of the CNS
Ependymal Cells
Lines ventricles in the brain and the central cavity of the spinal cord
Cells have ciliaUsed to circulate the cerebrospinal fluid
Oligodendrocyte Cells of the CNS
Oligodendrocyte
Oligodendrocytes Production of myelin in
the CNS Can cover as many as
60 neurons with myelin
Schwann/Satellite Cells
Schwann CellsProduction of myelin in the PNSNot able to cover one neuron, must use multiple
Schwann CellsFormation of the Nodes of RanvierProduces Neuronal Growth Factor
Satellite CellsFunction unknown
Myelin Sheath
Myelin Insulates the axon for rapid conduction of
action potentials Nodes of Ranvier
Gray v. White matter in the brainMultiple Sclerosis is an autoimmune disease
http://www.youtube.com/watch?v=Naecv3h868c
Neuron
Receptive ZoneWhere the Graded Response occurs
Cell Body Same information as a regular cell but no centrioles
Amitotic Contains ligand regulated gates
Dendrites Projections to help form synapses Contains ligand regulated gates
Neuron
Conducting ZoneAxon Hillock
Begins action potentials Accumulation of K+ ions Contains voltage regulated gates for Na+/K+
Axon Propagation of action potentials Contains voltage regulated gates for Na+/K+ Anterograde vs. Retrograde and Polio
Secretory Zone
Terminal Boutons Contains voltage regulated gates for Ca+2
Contains vesicles filled with Neurotransmitter
Resting Membrane Potential
-70 mV Membrane is said to be polarized
Voltage generated by ionic movement through the membrane
Creates a current Current = Voltage/ Resistance Current generates a Kinetic Energy
More Na+ on the outside of the cell More K+ on the inside of the cell
Diffusion down their electrochemical gradient
Resting Membrane Potential
Maintained by the Na+/K+ATPase pumpsWill not allow the neuron to reach equilibrium
across the membraneActively transports 3Na+ out of the cell and
2K+ into the cell
Graded Response
Short lived Localized changes in membrane potential
Can depolarize or hyperpolarize the membrane Dependent on IPSP or EPSP
The magnitude of the graded potential varies directly with the stimulus strength The stronger stimulus causes greater voltage change
and the current flows farther The current dies out within a few millimeters of its
origin Graded response only signals over a very short distance
Graded Response
Ligand sensitive Na+ gates will open with a stimulusNa+ diffuses into the cell down its
electrochemical gradient Depolarization of the membrane
K+ is repelled down the membrane towards the axon hillock
K+ can diffuse out of the cell because the plasma membrane is very “leaky”
Action Potentials
Begins at the axon hillock Voltage regulated Na+ and K+ gates
Along with Na+/K+ATPase pumps along the entire membrane
All or nothing response
Action Potentials
Depolarization -50mV due to the accumulation of K+ at the axon hillock
triggers an action potential At -50mV Na+ voltage regulated gates open
Na+ diffuses into the cell down its electrochemical gradient Na+ repels K+ down the membrane
Positive Feedback “on” The more positive the voltage, due to Na+ diffusing into the cell,
the more Na+ gates open. This creates a more positive voltage and more Na+ gates open
Positive Feedback “off” +30mV
Action Potential
RepolarizationAt +30mV
All Na+ gates close quickly All K+ gates open
K+ diffuses out of the cell down its electrochemical gradient
K+ gates close slowly at -70mV K+ continues to diffuse out of the cell until it reaches -
90mV All K+ gates are closed
Action Potential
HyperpolarizationAt -90mV the Na+/K+ATPase pump turns on
Pumps 3Na+ out and 2K+ into the cell Re-establishes resting membrane potential
Propagation of an Action Potential
As the influx of Na+ repels the K+ down the membrane there is an accumulation of K+The K+ accumulation with change the
membrane voltage to -50mVThe occurs when the previous action potential
reaches +30mV Repolarization is chasing Depolarization
down the membrane
Refractory Period
Absolute refractory From the opening of the Na+ channels until the Na+
channels begin to reset to their original resting state Cannot re-stimulate the neuron during this time
Relative refractory The interval following the absolute refractory period
Na+ channels have returned to their resting state K+ channels are still open and repolarizing the membrane
Can re-stimulate the neuron during this time with a great stimulus
Synapse
Presynaptic neuron Postsynaptic neuron Synaptic Cleft
About 10 angstroms between neurons Synaptic Vesicles
Filled with neurotransmitter
Synapse
Voltage regulated Calcium channels Membrane reaches -50mV due the
accumulation of K+ Calcium channels open
Calcium diffuses in down its electrochemical gradient
2 Calcium ions bind to the vesicleThe vesicle fuses with the membrane for
exocytosis of the NT
Synapse
The Neurotransmitter crosses the synaptic cleft NT binds to the receptors on the postsynaptic
neuron Neurotransmitter are removed from the
synaptic cleft by:ReuptakePhagocytosisEnzymatic Degradation
Events at the SynapseEvents at the Synapse
AP reaches axon terminal
Voltage-gated Ca2+ channels open
Ca2+ entry
Exocytosis of neurotransmitter containing vesicles
CaCa2+ 2+ = Signal for = Signal for Neurotransmitter Neurotransmitter
ReleaseRelease
1. Axon Diameter1. Axon Diameter
Fig. 8-18
Demyelination Demyelination diseases diseases (E.g. ?)(E.g. ?)
2. Signal Transduction in Myelinated Axon:2. Signal Transduction in Myelinated Axon:
Animation
3 Classes of3 Classes of Neurotransmitters (of 7)Neurotransmitters (of 7)
1. Acetyl Choline (ACh)– Made from Acetyl CoA and choline– Synthesized in axon terminal– Quickly degraded by ACh-esterase– Cholinergic neurons and receptors – Nicotinic (agonistic)
and muscarinic (antagonist)
2. Amines– Serotonin (tryptophane) and Histamine (histidine)
SSRI = antidepressants– Dopamine and Norepinephrine (tyrosine)– Widely used in brain, role in emotional behavior (NE used in ANS) – Adrenergic neurons and receptors - and
• Gases– NO (nitric oxide) and CO
1. Others: AA, (e.g., GABA), lipids, peptides, purines
Neurotransmitters
Cholinergic ReceptorsNicotinicMuscarinic
CatecholamineAlphaBeta
Nicotinic Receptors
Stimulated by ACh and nicotine, not stimulated by muscarine.
Found at all ganglionic synapses. Also found at neuromuscular junctions. A ligand sensitive gate
Muscarinic Receptors
Stimulated by ACh and muscarine, not stimulated by nicotine. Found at target organs when ACh is released by post-ganglionic neurons (all
of parasympathetic, and some sympathetic). Stimulated selectively by Muscarine, Bethanechol. Blocked by Atropine. Stimulation causes:
Increased sweating. Decreased heart rate. Decreased blood pressure due to decreased cardiac output. Bronchoconstriction and increased bronchosecretion. Contraction of the pupils, and contraction of ciliary body for near vision. Tearing and salivation. Increased motility and secretions of the GI system. Urination and defecation. Engorgement of genitalia.
Catecholamine Receptors
NE and epinephrine, each act on α- and β-adrenergic receptors
Two subclasses of α-adrenergic receptors Activation of α1-receptors usually results in a slow depolarization
linked to the inhibition of K+ channels activation of α2-receptors produces a slow hyperpolarization due
to the activation of a different type of K+ channel. There are three subtypes of β-adrenergic receptor Agonists and antagonists of adrenergic receptors
β-blocker propanolol (Inderol®). However, most of their actions are on smooth muscle receptors,
particularly the cardiovascular and respiratory systems
α1 adrenergic receptors
Mainly involved with contraction of smooth muscle
G protein, cAMP action
α2 adrenergic receptors
Three types of receptorsα2A, α2Β, and α2C
These receptors have a critical role in regulating neurotransmitter release from sympathetic nerves and from adrenergic neurons in the central nervous system
β1 adrenergic receptors
Specific actions of the β1 receptor include: Increases cardiac output
by raising heart rate and increasing the volume expelled with each beat (increased ejection fraction).
Renin release from juxtaglomerular cells.Lipolysis in adipose tissue.
β2 adrenergic receptors
Specific actions of the β2 receptor include: Smooth muscle relaxation, e.g. in bronchi. Relax non-pregnant uterus. Relax detrusor urinae muscle of bladder wall Dilate arteries to skeletal muscle Glycogenolysis and gluconeogenesis Contract sphincters of GI tract Thickened secretions from salivary glands. Inhibit histamine-release from mast cells Increase renin secretion from kidney
β3 adrenergic receptors
Specific actions of the β3 receptor include:Enhancement of lipolysis in adipose tissue. CNS effects
Neurological Communication
There’s no one-to-one communication between neurons
May be as many as 500 neurons communicating with a single neuronConvergenceDivergence
Postsynaptic ResponsesPostsynaptic Responses
Can lead to either EPSP or IPSP Any one synapse can only be either excitatory or inhibitory
Fast synaptic potentialsOpening of chemically gated ion channel
Rapid & of short duration
Slow synaptic potentialsInvolve G-proteins and 2nd messengers
Can open or close channels or change protein composition of neuron
Integration of Neural InformationIntegration of Neural InformationTransferTransfer
Multiple graded potentials are integrated at axon hillock to evaluate necessity of AP
1. Spatial Summation: stimuli from different locations are added up
2. Temporal Summation: sequential stimuli added up
1. Spatial Summation1. Spatial Summation
2. Temporal Summation2. Temporal Summation
General Adaptation Syndrome
General Adaptation Syndrome
Hans Selye Alarm Phase
A stressor disturbs homeostasis
Cerebral Cortex alerts Hypothalamus which alerts the Sympathetic Nervous System
General Adaptation Syndrome
Resistance Phase Body reacts to
stressor Attempts to return to
homeostasis Down and Up
Regulation
General Adaptation Syndrome
Exhaustion Phase Physical and Psychological
energy is sapped Atypical depression
Mood disorder Dysphoria -generally
characterized as an unpleasant or uncomfortable mood, such as sadness (depressed mood), anxiety, irritability, or restlessness
Serious illness(es) may occur Hits person at weakest genetic
point Autoimmune Disease(s) Endorphins Increase and inhibit
the immune system response
General Adaptation Syndrome
Final Phase is Death
Dermatomes
Dermatomes
Bipolar Neuron
Two processes An axon and a
dendrite They extend in opposite
directions
Used for sensory organs
Olfactory neurons Retina
Unipolar Neurons
Presence of only a single axon, branching at the terminal end.
True unipolar neurons not found in adult human; common in human embryos and invertebrates