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