CHAPTER 3A: Nerve Physiology

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Nervous system

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Nervous system

A network of billions of nerve cells linked together in a highly organized fashion to form the rapid control center of the body.

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3 basic functions

sensation, integration, reaction

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Sensation

Monitors changes/events occurring in and outside the body, has receptors.

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Integration

Monitors changes/events occurring in and outside the body. Such changes are known as stimuli and the cells that monitor them are receptors.

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Reaction

The activation of muscles or glands (typically via the release of neurotransmitters (NTs))

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Similarities of nervous system and endocrine system

They both monitor stimuli and react so as to maintain homeostasis.

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Nervous system

rapid, fast-acting system whose effects do not always persevere.

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Endocrine system

acts slower via blood-borne chemical signals called hormones and its actions are usually much longer lasting.

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2 types of nervous system

central nervous system and peripheral nervous system

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2 division of peripheral nervous system

sensory (afferent) and motor (efferent)

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2 types of motor division

autonomic motor and somatic motor

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2 types of somatic motor

sympathetic and parasympathetic

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Central nervous system

(The brain + the spinal cord) The center of integration and control

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Peripheral nervous system

The nervous system outside of the brain and spinal cord

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Peripheral nervous system consists of

31 Spinal nerves (to and from the spinal cord), 12 Cranial nerves (to and from the brain)

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Responsibility of peripheral nervous system

Communication between the CNS and the rest of the body

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Sensory division

afferent, from PNS to CNS, has receptors

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Motor division

efferent, from CNS to the body, sends information to effectors

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Autonomic motor

inovoluntary, innervates cardiac muscle, smooth muscle, glands

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Somatic motor

voluntary, innervates skeletal muscle

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Sympathetic

fight or flight, NTs is noradrenaline, adrenegic system

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Parasympathetic

rest and digest, NTs is acetylcholine, cholinergic system

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Myelin sheath

Insulates the axon to help protect the neuron cell & speed up transmission of electrical impulses

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Axon terminal

Transmits electrical & chemical signals to other neuron cells and effector cells

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Axon

Transfers signals to other cells/organs

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Dendrites

Receives signals from other cells and carry those signals to the cell body

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Cell body

Organizes and kepps the cell function

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Cell membrane

Protects the cell

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Axon Hillock

Generates impulse in the neuron

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Node of Ranvier

Gaps in the myelin sheath coating on the neural axons

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Schwann Cell

Produces the myelin sheath

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Input Zone

Receives incoming signals from other neurons.

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Trigger zone

(Axon Hillock), Region that triggers and initiates the propagation of the action potential.

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Conduction Zone

Conducts action potential in an undiminishing fashion

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Output Zone

Portion that releases neurotransmitter that influences other cell.

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Synapse

Specific location of interaction between a neuron and another neuron or an effector.

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Synapse

It is the site where one cell the presynaptic neuron) controls another cell's function (the postsynaptic neuron or effector).

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Chemical Synapse

The currents escape between neurons and do not enter the postsynaptic neuron.

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Chemical Synapse

Released molecules of neurotransmitter carry the signal across the cleft.

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Electrical Synapse

Low-resistance channels allow currents to pass directly between neurons in electrical synapses.

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Electrical Synapse

Neurons are electrically coupled by current flow through the low-resistance pathways of gap junctions.

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Action potential

(Nerve impulse), Is an electrical charge that moves along a neuron’s membrane

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Action potential

Exhibited only by excitable cells (neurons, all muscle type – cardiac, smooth, skeletal)

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Nature of Action potential

All-or-nothing in nature.

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Threshold

Membrane potential must reach a certain level of depolarization before an action potential can begin.

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Threshold potential

varies, but it is usually around 15 millivolts (mV) higher than the cell's resting membrane potential.

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Occurrence of A.P.

Will not occur if the membrane depolarization does not reach the threshold level

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Stereotypical size and shape

Depolarizes to same potential and repolarizes to the same RMP

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Propagating

non-decremental manner

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All-or-nothing in nature

if threshold is reached, a full size AP will be produced

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Stereotypical size and shape

graphing it will get the same thing again and again.

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Propagating

spreads, if one cell is AP, the other cells will be AP too

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All-or-none

”on” and ”off” state. It will be on “ON” state once threshold is reached

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Location of Action potentials

Recreated at points with open sodium channels.

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Depolarization wave strength

sustained along the axon.

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Resting membrane potential

Electrical potential difference across the plasma membrane when it is not excited.

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Resting membrane potential

All cells have..

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Resting membrane potential

Determined by the uneven distributions of ions (charged particles) between the inside and outside of the cell and by the different permeability of the membrane to different types of ion.

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Resting membrane potential

It is expressed by its value inside the cell relative to the extracellular environment

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30-90 mV

potential difference across the membrane with the inside of the cell more negative

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Normal Nerve RMP

- 70 mv

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Normal Cardiac RMP

- 90 mv

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Polarized

there is membrane potential = positive than resting potential in the membrane

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Hyperpolarized

membrane potential = more negative than its resting potential

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K+ and organic anions

present at higher concentrations inside (intracellular) the cell than the outside.

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Na+ and Cl

are usually present at higher concentrations outside (extracellular) the cell

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Positively charged cations

Sodium (Na+) and Potassium (K+)

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Negatively charged anions

Chloride (Cl-) and organic anions

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Charged ions

cannot pass directly through the hydrophobic lipid regions of the membrane

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Specialized channel proteins

Provide hydrophilic tunnel across the membrane.

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Leak channels

open in resting neurons

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Potassium channels

allow K+ to pass through

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Sodium channels

allow Na+ to pass through

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Mecahnically Gated Ion Channels

Found in sensory neurons and open in response to physical force.

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Chemically Gated Ion Channels

Known also as ligand-gated channels

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Chemically Gated Ion Channels

Open and close by hormones, 2nd messengers

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Skeletal Muscle AChR

(Nm Receptor), open gates for Na+ and K+ when Ach binds.

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Chemically Gated Ion Channels

In most neurons respond to a variety of ligands, such as extracellular transmitters and neuromodulators or intracellular signal molecules

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Voltage Gated Ion Channels

Open or close by changes in membrane potential.

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Example of voltage gated ion channels

Activation vs Inactivation of nerve Na channel.

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Voltage Gated Ion Channels

Respond to changes in the cell's membrane potential.

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Equilibrium potential

Nernst Potential, electrical potential difference across the cell membrane that exactly balances the concentration gradient of an ion.

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Equilibrium potential

The steeper the concentration gradient is, the larger the electrical potential that balances it has to be.

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Electrochemical equilibrium

chemical and electrical driving forces that act on an ion are equal and opposite

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Electrochemical equilibrium

High Na+ permeability causes a positive resting membrane potential due to its concentration gradient driving Na+ into the cell, making the interior positive relative to the outside

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Opening and closing of ion channels

This alters the membrane potential

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Hyperpolarized

If more potassium channels where to open up making it easier for K+ to cross the cell membrane (closer to to potassium equilibrium potential)

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Depolarized

If additional sodium channels were to open up making it easier for Na+ to cross the membrane.

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Opening and closing of ion channels

Changing the number of open ion channels provides a way to control the cell’s membrane potential and a great way to produce electrical signals

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Sodium Potassium Pump (Na + K+ ATPase)

a protein that maintains the Na+ and K+ concentration gradient across the membrane of the cell.

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Sodium Potassium Pump (Na + K+ ATPase)

It actively transports Na+ and K+ against their electrochemical gradients

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ATP hydrolysis

powers this uphill movement

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1 molecule of ATP that is broke down

3 Na+ ions (inside to the outside) and 2 K+ ions (outside to the inside)

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Small contribution of pump

resting membrane potential is slightly more negative.

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Big contribution of the pump

Indirect: it maintains steady Na+ and K+ gradients, which give rise to the membrane potential as Na+ and K+ move

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Mnemonic TRI-Na-TO-K-en

3 NA, 2 K, negative

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Polarization

Charges are separated across the plasma membrane, so that the membrane has potential to do work.

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Membrane in staete of Polarization

Anytime the potential is not 0mV, either the positive or negative direction

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Depolarization

A change in the potential that makes the membrane less polarized that at resting potential.

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