BIOS 3450: Ch. 4 Principles of Neural and Hormonal Communication

Excitable cells

Cells such as neurons and muscles that use changes in membrane potential to rapidly propagate electrical signals.

Why excitable cells have ion gradients

Cells use primary or secondary active transport to move ions against their concentration gradients.

Chemical gradient

A gradient caused by unequal concentrations of a substance across the plasma membrane.

Electrical gradient

A gradient caused by unequal distribution of electrical charges across the plasma membrane.

Why polar ions need assistance

Polar ions cannot cross the nonpolar lipid membrane without assistance.

Gated channels

Channels that open or close in response to a specific stimulus.

Voltage-gated channels

Channels that open or close based on changes in membrane potential.

Chemically gated channels

Channels that open or close when a chemical messenger binds to them.

Mechanically gated channels

Channels that open or close in response to mechanical deformation.

Thermally gated channels

Channels that open or close in response to temperature.

Resting membrane potential

The membrane potential of a neuron when it is at rest; approximately -70 mV.

Na+ equilibrium potential

Approximately +60 mV.

K+ equilibrium potential

Approximately -90 mV.

Effect of increased Na+ permeability

Drives the membrane potential toward the Na+ equilibrium potential of +60 mV.

Effect of increased K+ permeability

Drives the membrane potential toward the K+ equilibrium potential of -90 mV.

Membrane potential reference

Membrane potential is always referenced to the inside of the cell.

Regional membrane potential

A local change or fluctuation in membrane potential.

Depolarization

A decreased membrane potential; the inside of the cell becomes less negative.

Repolarization

The return of the membrane potential toward its resting value after depolarization.

Hyperpolarization

An increased membrane potential; the inside of the cell becomes more negative.

Regional graded potential

A local membrane potential change whose signal strength decreases with distance.

Graded potential propagation

A trigger opens ion channels, causing regional depolarization; current then flows to adjacent areas.

Graded potential strength

Signal strength decreases with distance.

Voltage-gated Na+ channel

A voltage-gated channel with two gates: a fast activation gate and a slow inactivation gate.

Voltage-gated K+ channel

A voltage-gated channel with one activation gate that responds more slowly.

Voltage-gated Na+ channel states

The channel has three functional conformational states related to activation and inactivation.

Voltage-gated K+ channel states

The channel has two conformational states: open or closed.

Activation threshold for voltage-gated Na+ and K+ channels

Approximately -50 mV.

Typical neuron

Consists of a cell body, dendrites, axon hillock, axon, and axon terminals.

Cell body

Integrates incoming signals and produces graded potentials.

Dendrites

Receive signals from other cells and transmit them toward the cell body.

Axon hillock

The region where graded potentials are funneled together and compared to the threshold for firing an action potential.

Axon

Conducts the action potential.

Axon terminal

Transfers the signal to the target cell.

Voltage-gated channel density in the axon hillock

Voltage-gated channels are more densely concentrated at the axon hillock.

Why the axon hillock is important

Graded potentials converge there and can reach the threshold needed to activate voltage-gated channels.

Sub-threshold graded potential

A graded potential that does not reach threshold and fails to activate voltage-gated channels.

Action potential

A large, rapid, self-propagating change in membrane potential.

Action potential initiation

A cumulative graded potential reaches threshold and activates densely populated voltage-gated Na+ channels.

Positive feedback during an action potential

Opening of voltage-gated Na+ channels causes further depolarization, which opens more Na+ channels.

Na+ activation gates during an action potential

Open rapidly.

Na+ inactivation gates during an action potential

Close after activation.

K+ activation gates during an action potential

Open slowly.

Refractory period

The period following an action potential during which the neuron is less able or unable to generate another action potential.

Absolute refractory period

The period when another action potential cannot be generated.

Relative refractory period

The period when another action potential can occur but requires a stronger stimulus.

Purpose of the refractory period

Ensures that action potentials propagate in one direction.

Return to baseline

After the action potential, Na+ gates reset and K+ gates close, allowing the membrane potential to return toward baseline.

Axon diameter

An increase in axon diameter increases conduction speed.

Myelination

The formation of an insulating layer around an axon that increases conduction speed.

Saltatory conduction

The process in which electrical signals rapidly jump between gaps in the myelin sheath.

Schwann cells

Cells that form myelin in the peripheral nervous system (PNS).

Oligodendrocytes

Cells that form myelin in the central nervous system (CNS).

Electrical synapse

A synapse in which neurons are connected by gap junctions and ions flow directly between cells.

Electrical synapses

They are less common and allow direct, relatively unregulated propagation of electrical signals.

Chemical synapse

The most common type of synapse, where a neurotransmitter is released from one neuron and acts on another cell.

Presynaptic neuron

The neuron that sends the signal.

Postsynaptic neuron

The neuron that receives the signal.

Synaptic knob

The enlarged end of a presynaptic axon terminal containing synaptic vesicles filled with neurotransmitters.

Synaptic vesicles

Vesicles in the axon terminal that store neurotransmitters.

Neurotransmitter

A chemical messenger released by a neuron that communicates with another cell.

Chemical synapse sequence

Action potential arrives → voltage-gated Ca2+ channels open → Ca2+ enters → vesicles fuse with membrane → neurotransmitters are released → neurotransmitters bind receptors on the postsynaptic cell.

Role of Ca2+ in chemical synapses

Ca2+ enters through voltage-gated Ca2+ channels and triggers synaptic vesicles to release neurotransmitters.

Exocytosis at a chemical synapse

The release of neurotransmitters when synaptic vesicles fuse with the plasma membrane.

Postsynaptic receptors

Receptors that bind neurotransmitters and alter ion permeability and membrane potential.

Removal of neurotransmitters

Neurotransmitters are rapidly removed from the synapse.

Neural integration

The process by which neurons combine and interpret signals from multiple presynaptic neurons.

Signal strength coding in neurons

Presynaptic signal strength is coded by firing rate.

Excitatory signal

A signal that produces a depolarizing graded potential.

Excitatory graded potential

Produced by increased influx of positive ions or exit of negative ions; Na+ influx is dominant.

Inhibitory signal

A signal that produces a polarizing graded potential.

Inhibitory graded potential

Produced by increased K+ permeability causing K+ to leave or increased Cl- permeability causing Cl- to enter.

Primary negative ion in ECF

Cl-.

Temporal summation

An increase in graded potential caused by rapid stimulation over time.

Spatial summation

An increase in graded potential caused by signals arriving from multiple nearby/proximal locations.

Cancellation of neural signals

Excitatory and inhibitory signals can cancel each other.

Axon hillock decision

The axon hillock is deterministic; cumulative excitatory and inhibitory signals either reach threshold and produce an action potential or do not.

Neural convergence

Graded input from multiple neurons converges onto a single neuron for signal integration.

Neural divergence

A single neuron synapses on multiple other neurons to distribute a signal for parallel processing.

Convergence

Multiple neurons → one neuron.

Divergence

One neuron → multiple neurons.

Chemical messengers

Extracellular molecules that allow cells to communicate with one another.

Four categories of extracellular chemical messengers

Paracrines/autocrines, neurotransmitters, hormones, and neurohormones.

Paracrine messenger

A chemical messenger that produces local effects on nearby cells.

Autocrine messenger

A chemical messenger that acts on the cell that released it.

Neurotransmitter

A chemical messenger released by neurons to communicate with target cells.

Hormone

A chemical messenger secreted into the blood by endocrine glands.

Neurohormone

A chemical messenger released by neurons into the circulation.

Peptide/protein messengers

Chemical messengers such as insulin, growth hormone, and cytokines.

Amino acid and derivative messengers

Chemical messengers such as dopamine and serotonin.

Steroid hormones

Hydrophobic chemical messengers derived from cholesterol, including cortisol, estrogen, and testosterone.

Lipid messengers

Chemical messengers such as prostaglandins and leukotrienes.

Gas messengers

Chemical messengers such as nitric oxide (NO).

Examples of molecules with signaling roles

Glucose, lactate, O2, CO2, lipids, amino acids, ATP, ADP, AMP, and adenosine.

Water-soluble chemical messengers

Hydrophilic messengers that cannot pass through the plasma membrane without assistance.

Receptors for water-soluble messengers

Usually located on the cell surface.

Lipid-soluble chemical messengers

Hydrophobic messengers that can pass through the plasma membrane.

Receptors for lipid-soluble messengers

Located inside the cell.

Three major types of extracellular receptors

Chemically gated receptor channels, receptor-enzyme complexes, and G-protein coupled receptors (GPCRs).

Chemically gated receptor channel

A receptor that directly opens or closes a membrane channel when a chemical messenger binds.