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