Nervous Systems - Function at The Cellular Level

Nervous Systems - Function at The Cellular Level

mean = 75.1 sd = 12.4 range = 52-99 50-59 = F 2 60-67: D 1 68-79: C 10 80-89: B 3 9099: A - 2 NERVOUS SYSTEMS NEURON FUNCTION Overview: neuron function 1)

2) 3) 4) Electrical properties excitability and resting potentials Stimulation the graded potential impulse transmission the action potential Transmission of action potentials between neurons the synapse The nervous system is responsible for processing

internal and external information (sensory input) and making decisions about how to respond INTEGRATION The nervous system coordinates the activity of cells, tissues and organs involved in responses At the level of individual cells, integration and decisionmaking is done by neurons

Anatomy of a neuron: a) dendrites - input b) cell body decision making; graded potential c) axon hillock - initiation of action potential d) axon - transmits action potential e) axon terminals multiple endings/neuron e) synapses communication with target cells dendri te cell

body axon hillock axon termin als All activity within the neuron is based on changes in electric charge of the cytoplasm

- due to movement of positive or negative ions across the cell membrane changes in charge changes in charge of the dendrite/cell of the axon are body region are always identical variable in amount action potential graded potential

ELECTRICAL PROPERTIES OF NEURONS Resting potential - electrical charge difference (membrane potential) between inside and outside of cell when neuron is inactive - inside of cell negative - outside of cell positive more K + inside more Na +

outside more Cl outside many anionic (negative) proteins inside Ions can move in or out through channels;

proteins can't Charge difference in inactive neuron is ~70 mV (millivolts) resting potential Membrane potential can change due to ion movement: a) diffusion ions move down concentration gradient, or towards opposite electrical charge b) active transport against concentration gradient; uses ATP resting potential stays constant

unless neuron is stimulated Ions diffuse based on two forces: - concentration gradient: high low concentration - electrical charge: region of opposite charge These may to oppose or reinforce each other Na+forces

- wants move in; concentration gradient and charge difference both inwards K+ - opposing forces: concentration out charge - in Unequal distribution of Na+ and K+ is maintained

by active transport (ATP): the Na/K pump Membrane is more permeable to K+ than to Na+ or other ions - Na/K pump moves Na+ out, K+ indiffusion through "leaky" membrane the Na/K pump: active transport

Most diffusion of ions in or out occurs through transmembrane proteins ion channels - facilitated diffusion no ATP needed - ion channels are selective and dynamic only one type of ion can pass; can be open or closed (gated) At rest, some K+ channels are always open free movement of K+ in or out Most gated channels open intermittently and briefly 2 types of channels in a typical neuron: 1) electrically gated (=voltage-gated) - membrane potential changes channel opens

2) chemically gated (=ligand-gated) - signaling molecule (neurotransmitter ) binds to receptor channel opens A graded potential is any electrical change from resting potential - opening of gated channels due to binding of neurotransmitters: ion movement in or out of dendrites or cell body change in membrane potential of cell body - amount of change varies (graded) depending on how many channels open and number of ions moving in or out If graded potential exceeds threshold (~10mV above resting potential, or -60 mV) , an action potential is initiated in the axon

Questions about resting or graded potentials? Gated channels and their role in ion movement? Where do graded potentials occur in the neuron? What causes graded potentials to occur? Which type of gated channels are most often involved?

Graded potentials can be excitatory or inhibitory Chemically-gated channels: opened by neurotransmitters - positive ions in (Na+)= depolarization less negative inside - negative ions in (Cl-) = hyperpolarization more negative A depolarizing graded potential is excitatory increases chance of an action potential - membrane potential moves closer to threshold (more positive) A hyperpolarizing graded potential is inhibitory - membrane potential moves farther from threshold (more negative) thresho

ld A single receiving (post-synaptic) neuron can have 1000s of synapses with different incoming (presynaptic) axon terminals - each synapse is either excitatory or inhibitory simultaneous stimulation at many synapses is "evaluated" by cell body = integration

Integration - cumulative effects of more than one graded potential Summation 2 mechanisms - temporal summation - multiple stimulation at one synapse in a short period of time - additive A graded potential that does not reach threshold rapidly fades cell returns to resting potential The same synapse releases neurotransmitters twice in a

short period of time larger graded potential action potential Spatial summation simultaneous stimulation from two or more nearby synapses; each synapse is excitatory or inhibitory - reinforcing or opposing effects - equal excitatory and inhibitory graded potentials cancel each other out Two excitatory synapses fire

simultaneousl y Excitatory and inhibitory synapse fire simultaneously The results of summation and integration in the dendrite/cell body region are "evaluated" at the axon hillock - if excitatory graded potential > threshold, then an action potential is initiated due to the action

of electrically-gated ion channels in the axon Questions about summation or integration? What is the effect on the overall function of the neuron if the graded potential doesn't reach threshold? The action potential: rapid transmission of information - all or none: does not vary in strength - electrically-gated ion channels open in response to depolarization self-propagating - the action potential passes undiminished as a wave down

the axon; a local, transient event SELF-PROPAGATION movement of Na+ ions in here causes voltagegated Na+ channels to open here repolarization due

to K+ movement quickly follows Na+ channels inactivated: refractory period axon is temporarily "off" - prevents movement of action potential backwards one-way propagation A single axon can transmit many action potentials at

the same time - each is a brief local event The action potential has 4 phases, each based on opening of voltage-gated channels and ion movement depolarizatio n Na+ in repolarization K+ out refractory period hyperpolarized return to resting

potential Na+ channels K+ channels Na+ channels open open inactive Na+ channels active, but closed Questions about action potentials?

At one point on axon Entire axon What is the functional effect of an action potential on other cells or tissues? SPEED OF ACTION POTENTIAL CONDUCTION Ranges from 1 to >100 m/s increased by:

a) larger size - giant axons - invertebrates b) increased body temperature faster diffusion c) fatty insulation - myelin sheaths - vertebrates Myelin fat produced by oligodendrocytes (Schwann cells) - myelinated axons are white and shiny white matter of brain and spinal cord Electrically gated Na+ channels are concentrated at gaps in the myelin sheath - nodes of Ranvier Saltatory conduction - AP jumps from one node to the next, travels faster than in an uncovered axon

-maximum speed of conduction in myelinated vertebrate axons - 120 m/s (= 270 mph) - small, unmyelinated axon of human autonomic nervous system 1m/s (= 2.25 mph) diffusion of Na+ ions inside the axon causes opening of Na+ channels at next node AP jumps from node

to node Information is transferred between neurons at the synapse Synapse - junction between axon terminals and target tissue (nerve, muscle or gland) Synaptic cleft - microscopic separation between axon and target - 10-20 nanometers

Rapid (instantaneous) diffusion from pre-synaptic neuron to post-synaptic receptors Neurotransmitter release at the axon terminal is caused by the action potential - neurotransmitters stored in synaptic vesicles - AP opens voltage-gated Ca++ channels; Ca++ ions cause vesicles to release contents at synapse (exocytosis) Neurotransmitters diffuse across synaptic cleft to postsynaptic neuron - neurotransmitters bind

to receptors on chemically-gated ion channels on postsynaptic membrane - channels open ion movement graded potential - neurotransmitter removed from receptor, inactivated

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