Impulses, & Action Potential

salutatory conduction (myelinated) faster

depolarization only at Nodes of Ranvier where there is a high density of voltage gated ion channels

what prevents opening of voltage-gated Na+ channels

local anesthetics certain neurotoxins eg. Novocain & Iidocaine

Resting Membrane Potential (RMP)

negative ions along inside of cell membrane positive ions outside -potential energy difference at rest is -70mV -cell is polarized resting potential exists because -concentrations of ions different on outside & inside extracellular fluid rich in Na+ & Cl cytosol full of K+, organic phosphate & amino acids -membrane permeability differs for Na+ & K+ 50-100 greater permeability for K+ inflow of Na+ cant keep up with outflow of K+ Na+/K+ pump removes na+ as fast as it leaks in

action potentials

series of rapidly occurring events that change & then restore the membrane potential of a cell to its resting state 1. chemical or mechanical stimulus causes graded potential to reach -55mV or threshold 2. at -55mV, both Na+ & K+ gates open (K+ much slower) 3. Na+ rushes in to cell(depolarization) reaching+30mV 4. K+ gate opens, allows K+ in, pushing Na+ out 5. Cell becomes more negative (repolarization) 6. at -70mV, K+ gate closes, cell returns to resting state RMP Membrane potential of a neuron that is conducting an impulse. = nerve impulse. are all or none

continuous conduction (unmyelinated fibers) slow

step by step depolarization of each portion of the length of the axolemma

Sodium-potassium Pump

- Active transport mechanism in plasma membrane that transports Na+ and K+ in opposite directions & at different rates - Maintains an imbalance in distribution of position ions, resulting in inside surface becoming slightly negative with respect to its outer surface

Conduction of the Action Potential

1. At peak of action potential, plasma membrane's polarity is now reverse of the RMP 2.Reversal in polarity causes electrical current to flow between site of action potential & adjacent regions of membrane. Triggers voltage-gated Na+ channels to open. Exhibits an action potential. 3. Cycle continues to repeat, producing continuous conduction. 4. Action potential never moves backward because of refractory period 5. In myelinated fibers, action potentials in membrane only occur at nodes of Ranvier (impulse aka saltatory conduction) 6. Speed of nerve conduction depends on diameter & on presence or absence of a myelin sheath.

Fiber types

A fibers - largest (5-20 microns & 130 m/sec) -myelinated somatic sensory 7 motor to skeletal muscle B fibers - medium (2-3 microns & 15 m/sec) -myelinated visceral sensory & autonomic & preganglionic C fibers - smallest (0.5-1.5 microns & 2 m/sec) -unmyelinated sensory 7 autonomic motor

Polarized Membrane

A membrane that exhibits a membrane potential - Measured in volts (V) or millivolts (mV) - Indicates charge on inside surface of a polarized membrane

Local Potentials

Aka graded potentials. Slight shift away from resting membrane in a specific region of plasma membrane. 1. Excitation 2. Inhibition

Membrane potentials

All living cells maintain a difference in concentration of ions across their membranes. - Slight excess of positively charged ions on outside of membrane - Slight deficiency of positively charged ions on inside of membrane

Chemical synapse

Arrival of nerve impulse opens voltage-gated calcium channel release of neurotransmitter neurotransmitter crosses synaptic cleft binds to ligand-gated receptors one way information transfer

Absolute Refactory Period

Brief period (approx. 0.5ms) - Local area of a neuron's membrane resists restimulation - Will not respond to a stimulus, no matter how long

Explain why and how the neurotransmitter is removed from the synaptic cleft

If it is not removed it will continue to influence the postsynaptic neuron,muscle fiber or gland cell indefinitely 1. diffusion down the concentration gradient 2. enzymatic degradation eg. acetyl-cholinesterase 3. re-uptake into releasing neuron

2 types of ion channels

Leakage (nongated) channels are always open. -Nerve cells have more K+ than Na+ leakage channels, -as a a result, membrane permeability to K+ is higher. -Explains resting membrane potential of -70mV in nerve tissue. Gated channels open & close in response to stimulus -results in neuron excitability

Inhibition

Local potential. - When a stimulus triggers opening of additional K+ channels - Increases membrane potential (hyperpolarization) more negative

Excitation

Local potential. - When a stimulus triggers opening of additional Na+ channels - Allows membrane potential to move toward zero (depolarization)

Relative Refactory Period

Membrane is repolarized & restoring the RMP. - Few milliseconds after absolute refactory period - Will respond only to a very strong stimulus

graded potentials

Small deviations from resting potential of -70mV depolarization (membrane more positive) hyperpolarization (membrane more negative) -most often in the dendrites and cell body -communicate over short distances- localized -mainly ligand-gated channels -initiated by a stimulus -signals are graded, vary in amplitude.

how do graded potentials arise

Source of Stimuli - mechanical stimulation of membranes with mechanical gated ion channels (pressure), Chemical stimulation of membranes with ligand gated ion channels (neurotransmitter). graded/postsynaptic/receptor or generator potential -ions through ion channels -change membrane potential locally -charge varies with strength of stimuli flow of current is local charge only

explain the process of propagation of action potentential

as Na+ flows into the cell during depolarization, the voltage of adjacent areas is affected and their voltage-gated channels open. this is self propagating along the membrane

how do we differentiate a light touch from a firmer touch?

frequency of impulses -firm pressure generates impulses at a higher frequency number of sensory neurons activated -firm pressure stimulate more neurons than does a light touch

Synapses

functional junction between neurons or between a neuron and an effector such as muscle or gland 2 types electrical & chemical

Speed of impulse propagation

is not related to stimulus strength - larger myelinated fibers conduct impulses faster due to size and salutatory conduction

Electrical synapse

through gap junctions common in -visceral smooth muscle -cardiac muscle -developing embryo -brain faster coordinating

gated ion chanels

voltage gated -respond to direct change in membrane potential ligand gated -respond to specific chemical stimulus mechanically gated -respond to mechanical vibration or pressure