Basic principles of electricity Voltage is a measure of the potential difference between separated electrical charges of opposite sign.
Current refers to the flow of electrons fromone point to another.Resistance is hindrance to charge flow.Ohm’s law: current (I) = voltage (V) resistance (R)
Chemically gated channels are ion channelsthat open when the appropriateneurotransmitter binds.Voltage gated channels are ion channelsthat open and close in response to changesin the voltage.
The resting potential Resting membrane potential is the voltage that exists across the plasma membrane during the resting state of an excitable cell. When a membrane is in its resting potential, it is said to be polarized.
Membrane potentials Depolarization refers to the loss of a state of polarity. Hyperpolarization occurs when the membrane potential or voltage increases, becoming more negative than the resting potential.
Graded potentials A graded potential is local change in membrane potential that varies directly with the strength of the stimulus.
Action potentials An action potential is a self-propagating wave of depolarization. A nerve impulse is another name for an action potential.
Generation of an action potential Resting state:active channels closed. Depolarization phase: increase in sodium permeability and reversal of the membrane potential; threshold. Depolarizing phase: decrease in sodium permeability.
Repolarizing phase: Increase in potassiumpermeability; repolarization.Undershoot: Potassium permeabilitycontinues; undershoot.
An action potential must be propagated, ortransmitted, along the axon’s entire length ifit is to serve as the neuron’s signalingdevice.
The all-or-none phenomenon refers to thefact that an action potential either happenscompletely or it doesn’t happen at all.
The absolute refractory period is the periodfollowing stimulation during which noadditional action potential can be evoked.The relative refractory period, whichfollows the ARP is the interval when athreshold stimulus is unable to trigger anaction potential.
The larger the axon’s diameter, the faster itconducts impulses.The myelin sheath greatly increases the rateof impulse propagation.Saltatory conduction refers to the triggeringof action potentials at nodes, such that itjumps from node to node along the axon.
The synapse A synapse is a unique junction that mediates the transfer of information from one neuron to another neuron, or to an effector cell. Synapses that occur between the axonal endings of one neuron and the dendrites or cell bodies of other neurons, are called axodendritic or axosomatic, respectively.
Neurons conducting impulses toward thesynapse are called presynaptic neurons, andneurons that transmit the electrical signalaway from the synapse are calledpostsynaptic neurons.Neurotransmitters are chemicals that act toopen or close ion channels that influencemembrane permeability.
Electrical synapses are bridged junctionsthat correspond to the gap junctions foundbetween certain other body cells.Chemical synapses are specialized forrelease and reception of neurotransmitters.
A synaptic vesicle is a small membranoussac containing the neurotransmitteracetylcholine.A synaptic cleft is the fluid-filled space at asynapse.
Information transfer acrosschemical synapses Calcium gates open in the presynaptic axonal terminal. Neurotransmitter is released by exocytosis. Neurotransmitter binds to postsynaptic receptors. Ion channels open in the postsynaptic membrane.
Synaptic delay is the time required for animpulse to cross a synapse between twoneurons.
Postsynaptic potentials andsynaptic integration An excitatory postsynaptic potential is a local depolarization of the postsynaptic membrane that brings the neurons closer to threshold for actin potential generation. An inhibitory postsynaptic potential results in hyperpolarization of the postsynaptic neuron and drives the neuron away from the threshold for firing.
EPSPs can add to together to influence theactivity of a postsynaptic neuron.Temporal summation occurs when one ormore presynaptic neurons transmit impulsesin rapid-fire order, which results in wavesof neurotransmitter released in quicksuccession.
Spatial summation occurs when thepostsynaptic neuron is stimulated by a largenumber of terminals from other neurons atthe same time.When partially depolarized neurons aremore easily excited by successivedepolarization events, the neurons are saidto be facilitated.
Synaptic potentiation refers to theenhancement of the neuron’s ability toexcite the postsynaptic neuron, due torepeated or continuous use of a synapse.
Presynaptic inhibition occurs when therelease of an excitatory neurotransmitter byone neuron is inhibited by the activity ofanother neuron.Neuromodulation occurs when chemicalsother than neurotransmitters modifyneuronal activity.
Neurotransmitters and theirreceptors Acetylcholine is a neurotransmitter released by some nerve endings, and at neuromuscular junctions. Acetylcholinesterase (AChE) is an enzyme that degrades acetylcholine, so that it transmit an action potential across a synapse.
Biogenic amines Catecholamines include dopamine, norepinephrine, and epinephrine. Indolamines include serotonin and histamine.
Amino acids Amino acid neurotransmitters include gamma-aminobutyric acid (GABA), glycine, aspartate, and glutamate.
Peptides The neuropeptides include substance P, endorphins, and enkephalins.
Novel messengers Adenosine triphosphate (ATP), nitric oxide (NO), and carbon monoxide (CO), have recently been shown to be neurotransmitters.
Channel-linked receptors mediate directtransmitter action, and are activated inresponse to ligand binding.The effects of G protein-linked receptorsare mediated by second messengers, such ascyclic AMP.
Organization of neurons Neuronal pools are functional groups of neurons that process and integrate information.
Types of circuits The patterns of synaptic connections in neuronal pools are called circuits. When one incoming fiber triggers responses in ever-increasing numbers of neurons along the circuit, it is said to be a diverging circuit.
Converging circuits occur when the poolreceives inputs from several presynapticneurons, and the circuit has a concentratingeffect.Reverberating circuits occur when theincoming signal travels through a chain ofneurons with collateral synapses.
Parallel after-discharge circuits occur whenthe incoming fiber stimulates severalneurons arranged in parallel arrays thateventually stimulate a common output cell.
Patterns of neural processing When one neuron stimulates the next in sequence, eventually causing a specific response, it is known as serial processing. A reflex is an automatic reaction to stimuli. A reflex arc is a neural pathway over which reflexes occur.
Inputs may be segregated into manydifferent pathways, and simultaneouslyanalyzed by neural circuitry in parallelprocessing.