Neuronal Physiology

A graded potential is a local electrical change in the membrane.

• It occurs in varying grades.• This change is progressively greater as the

triggering event becomes progressively stronger.

• Usually a flow of sodium ions into the ICF produces this kind of potential.

• Its duration of a graded potential is directly proportional to the duration of the triggering event.

Upward deflection = Decrease in potentialDownward deflection = Increase in potential

Repolarization

Hyperpolarization

Depolarization

Resting potential

Graded potentials spread by passive current flow.

• Resistance hinders electrical charge movement.

Graded potentials die out over short distances.

• Their spread is decremental.

• Examples of graded potentials are:– postsynaptic ,

receptor, end-plate, pacemaker, and slow-wave

An action potential occurs if a membrane reaches threshold potential.

• Voltage-gated channels in the membrane for sodium and potassium ions undergo conformational changes.

• The flow of sodium ions into the ICF reverses the membrane potential from -70 mV to +30 mV.

• The flow of potassium ions into the ECF restores the membrane potential to the resting state.

Gradedpotential(change inmembranepotentialrelative torestingpotential)

Magnitudeof stimulus

Resting potential

Time

Stimuli applied

Action potentials travel from the axon hillock to axon terminals.

• The three parts of a neuron are its dendrites, cell body, and axon.

• Dendrites signal toward the cell body.

• An axon signals away from the cell body. The axon is also called the nerve fiber.

• The axon hillock is the first part of the axon plus the part of the cell body where the axon exits. It has the lowest threshold for the action potential.

Once initiated, an action potential is conducted throughout the axon.

• A refractory period ensures one-way travel of an action potential. It also limits the frequency of action potentials.

• The conduction is contiguous.• An action potential occurs by all or none.• Stimulus strength is coded by action

potential frequency.

Change inmembranepotential inmV relativeto restingpotential—i.e., magnitudeof electricalsignal

Few mm

Distance

Initialactivearea Decremental spread

of graded potentialDecremental spread

of graded potentia

l

Restingpotential

Few mm

Myelination with nodes increases the speed of conduction of the action potential.

• This is saltatory conduction. The impulse jumps from node to node.

• The oligodendrocyte forms myelin in the CNS.

• The Schwann cell forms myelin in the PNS.

Nerve fibers can regenerate.

• Schwann cells in the peripheral nervous system guide the regeneration of cut axons.

• Regeneration of cut axons in the central nervous system is inhibited by oligodendrocytes.

Two neurons can interact at the synapse.

• A synapse is a junction between two neurons.

• The axon terminal of a presynaptic neuron can discharge a neurotransmitter.

• The neurotransmitter can diffuse across the synapse and excite a postsynaptic neuron.

A neurotransmitter can carry a signal across the synapse by a series of steps.• An action potential at the axon terminal of a presynaptic neuron

opens calcium ion channels.• Calcium flows from the ECF through the channels into the

synaptic knob.• This calcium influx induces the rupture of synaptic vesicles in the

axon of the presynaptic neuron. • These vesicles release neurotransmitter molecules which enter

the synaptic cleft by exocytosis.• These molecules diffuse and bind to receptors on the subsynaptic

membrane of the postsynaptic neuron. • This binding opens chemically-gated channels in this subsynaptic

membrane, leading to events that excite the postsynaptic neuron.

Presynaptic axonterminal

Voltage-gatedCa2+ channel

Ca2+

Neurotransmitter molecule

Synaptic cleft

Chemically gated ion channel forNa+, K+, or Cl-

Receptor for neurotransmitter

Postsynaptic neuron

Subsynaptic membrane

Synaptic vesicle

Synaptic knob

The signal at the synapse either excites or inhibits the postsynaptic neuron. • Excitatory synapse:

– An excitatory synapse mainly signals the influx of sodium ions into the postsynaptic neuron.

– This produces a EPSP and tends to depolarize the neuron.– This type of synapse is always excitatory.

• Inhibitory synapse:– An inhibitory synapse signals the outflow of potassium ions

from the postsynaptic neuron. It can also signal the influx of chloride ions.

– It produces an IPSP and tends to hyperpolarize the neuron. This type of synapse is always inhibitory.

Excitatory synapse

Thresholdpotential

Activation of synapse

EPSP

Inhibitory synapse

IPSP

Thresholdpotential

Activation of synapse

Excitatorypresynapticinputs

Inhibitorypresynapticinput

Recordingpotential ofpostsynapticcell

Temporal summation

Spatialsummation

EPSP–IPSPcancellation

Thresholdpotential

Resting potential

Recording potential of postsynaptic cell

Recording potential ofpostsynaptic cell C

Recording potential of postsynaptic cell C

Threshold potential

Resting potential

Other characteristics of neurotransmitters are:

• They are quickly removed from the synaptic cleft.• Some function through intracellular second-messenger

systems.• Presynaptic inputs determine a postsynaptic potential by

temporal summation and spatial summation.• By temporal summation, EPSPs occur close together in

time from a single presynaptic neuron.• By spatial summation, EPSPs originate from several

presynaptic inputs. • IPSPs can also undergo temporal and spatial

summation.

Neuropeptides act mainly as neuromodulators.

• They are molecules consisting of 2 to 40 amino acids.

• They do not produce EPSPs or IPSPs.• They modulate at the synapse,bringing about

long-term changes here.

The effectiveness of presynaptic input can be altered by presynaptic facilitation or inhibition.• presynaptic facilitation - There is enhanced

release of the neurotransmitter from the presynaptic axon terminal.

• presynaptic inhibition - There is reduced release of the neurotransmitter from the presynaptic axon terminal.

• Each is influenced by another nearby axon.

Drugs and diseases can alter synaptic transmission.

• Examples include: – Cocaine blocks the reuptake of dopamine.– Parkinson’s disease is due to a deficiency

of dopamine in a brain region controlling complex movements.

– Strychnine competes with glycine, an inhibitory neurotransmitter.

– Tetanus toxin prevents the release of GABA, affecting skeletal muscles.

Neurons are linked through complex converging and diverging pathways.

• By converging input a single cell is influenced by thousands of presynaptic cells.

• By divergence, branching axon terminals affects thousands of postsynaptic cells.

Presynapticinputs

Postsynapticneuron

Convergence of input(one cell is influencedby many others)

Presynapticinputs

Divergence of output(one cell influencesmany others)

Postsynapticneurons

Arrows indicate direction in which information is being conveyed.