Excitable Tissues and Action Potentials

• Excitable tissues can change the electrical properties of their cell membranes when stimulated.
• Nerve and muscle tissues are excitable.
• All cells possess a resting potential, but only excitable cells can generate action potentials.

Resting Potential at Cell Membrane

• The cell membrane is selectively permeable.
• Intracellular and extracellular fluids have different substance concentrations.
• The concentrations of Na+, Cl-, and K+ ions are especially relevant.
• Anions are important for cell function.

Ion Channels

• Ion channels transport ions across the cell membrane's lipid part.
• They regulate ion passage and mediate changes in membrane potential.
• Ion channels facilitate action potential formation.
• Channels are selective, showing different opening and closing properties.

Ligand-Gated Channels

• These channels open when a neurotransmitter binds to a receptor.

Voltage-Gated Channels

• These channels open due to electrical charge differences across the cell membrane.

Mechanic Gates

• These gates are sensitive to tension and pressure.
• They open when the cytoskeleton stretches.

Ion Concentrations

• Na+ concentration is high in extracellular fluid.
• K+ concentration is high inside the cell.

Factors Contributing to Resting Potential

• Continuous diffusion of K+ ions happens through leak channels.
• The Na+/K+ ATPase pump maintains ion gradients.
• Anions contribute to the cell's negative charge.
• In almost all cells, the inside has a more negative potential than the outside.

Na-K ATPase Pump

• The Na-K ATPase pump transports 3 Na ions out of the cell and 2 K ions into the cell, consuming ATP.
• This active transport maintains balance, pumping sodium out and bringing potassium in.
• Although Na+ permeability is low at rest, some Na+ penetrates the cell, while K+ leaks out.
• When an action potential occurs, voltage-gated channels open, and the Na-K ATPase pump restores balance by pumping the ions back.

Resting Potential

• Resting potential measures the potential difference when the cells show no activity.
• Resting potential varies by tissue type, from -9 to -100 mV.
• Action potential involves potential changes from ion movement.

Action Potential

• When a cell at resting state receives a stimulus, the membrane resting potential changes quickly within milliseconds and reaches a positive value.
• It involves depolarization and repolarization.
• Na+ and K+ ions are responsible for the action potential.
• Depolarization occurs when the inside gains a more positive value than the outside.

Repolarization

• Repolarization is the return of the membrane potential from depolarization to resting potential.
• Communication across nerve cells propagates rapid changes in a membrane potential.
• Membrane potential rises rapidly from -70 mV to +30 or +40mV.

Stages of Action Potential

• Resting State
• Stimulation
• Depolarization
• Repolarization
• Hyperpolarization
• Return to Resting State

Action Potential Events

• Stimulation: Chemical, mechanical, or electrical stimulation with a polarized current (-70 mV) that raises it to -55 mV triggers an action potential. -If the stimulus does not reach the threshold value, action potential does not occur. -Threshold value: Minimum arousal intensity required to create an action potential.

• Stimulus above the threshold: If the stimulus reaches/exceeds the intensity, action potential occurs. -Voltage-gated ion channels: Voltage-gated sodium (Na+) and potassium (K+) channels play a major role in action potential formation.

• Depolarization: Voltage-gated sodium channels open, and sodium enters rapidly, increasing the membrane potential (e.g., to +40 mV).

• Repolarization: Voltage-gated Na+ channels begin to close, Na+ input slows, voltage-gated K+ channels open, and K+ exits reducing the membrane voltage down to -70mV.

• Hyperpolarization: K+ continues to exit. -The membrane voltage goes below -70mV.

• Return to rest state: Voltage-gated K+ channels close and membrane voltage rests at -70mV.

All-or-Nothing Law

• Stimulus below the threshold cannot create an action potential.
• Threshold and above-threshold stimuli create action potentials of the same amplitude.
• If a stimulus fails to surpass the threshold, the result is nothing.
• Exceeding the threshold may lead to full completion of action potential.

Action Potential Propagation

• Stimulation is received by dendrites and accumulates in the soma.
• If the excitation reaches the threshold value, action potential happens at the axon.
• Action potential spreads along the axon.

Conduction Velocity

• Dependent on the axon.
• Thick fiber transmits faster than thin fiber.
• Myelinated nerve fibers conduct faster compared to unmyelinated fibers.

Refractory Period

• Period in which a second action potential cannot be triggered.
• This is because Na+ channels are inactivated after the start of the action potential.
• Na+ transmission does not occur until these channels activate again.
• Divided into two phases.

Absolute Refractory Period

• No stimulus can trigger a new action potential

Relative Refractory Period

• Strong enough stimulus above the threshold may trigger next action potential.

Blocked Neural Conduction Examples

• Local anesthetics block voltage-gated Na+ channels, preventing action potentials.
• Cold temperatures block voltage gated channels.
• Compression of blood vessels feeding the neurons causes blockage
• Multiple sclerosis destroys myelin sheath which slows down conduction.

Neuron Structure

• A nerve cell with three parts: the cell body, dendrites, and axon.
• The cell body contains the membrane, cytoplasm, and nucleus.

Dendrites

• Numerous short extensions that extend from the cell body like tree branches

Axon

• Singular cell body extension which transmits messages of nerve impulses to other neurons.
• Messages pass down the axon to muscle fibers or endocrine glands.

Signals

• Dendrite extensions receive nerve impulses.
• These nerve impulses comes from neighboring neurons, where it becomes transmitted into the cell body.

Axons Terminals

• Connects with cell bodies/dendrites to transfer messages to cells.
• Point where a neuron transmits a nerve impulse to another neuron is called a synapse.
• Neuron that gives the signal = presynaptic neuron while neuron that receives the signal = postsynaptic neuron.
• End of the presynaptic neuron is the presynaptic terminal.
• Postsynaptic neuron axon may synapse with many cells or terminate in a single neuron.

Synaptic Region

• Tubers of the presynaptic neuron end open in tiny gaps between the postsynaptic cell membrane.
• Passage of a stimuli uses chemical substance, which classifies chemical synapse as an electrical synapse where there is electrical activity.
• Nerve axon often approaches to contact the cell. Synapse may stimulate or inhibit signal.
• Nerve impulses are called 'synapse'.

Neurotransmitters

• Molecules found inside membrane-lined synaptic vesicles at presynaptic neuron termination.
• The vesicles will join the axon to then be released into synaptic cleft within the exocytosis membrane.
• Exocytosis achieved by voltage-gated channels via action potentials, with exocytosis triggering calcium input to the axon terminal.
• Released neurotransmitters will rapidly make its way across membrane to reach the postsynaptic neuron.
• Neurotransmitters will bind with specific receptor proteins within the postsynaptic membrane.
• Neurotransmitter (acetylcholine) interaction causes the ion channels to open the postsynaptic membrane.

The Synaptic Potential

• There are voltage-gated channels (with depolarization) in the postsynaptic neuron's synapse.
• Neurotransmitters are (chemical gated)
• Depending on what neurotransmitter is used at postsynaptic, result is diverse.
• Change in chemical-gated channels create the local potential called synaptic potentials.
• Membrane/channel binding may cause either depolarization or hyperpolarization in membrane.

Excitatory Postsynaptic Potentials (EPSP)

• Membrane potential changes called, occurs with neurotransmitter binding.
• When a neurotransmitter binds to postsynaptic receptor and sodium ion channels open, ions enter therefore depolarizing the cell. As result, action potential is possible to be fired.

Inhibitory Postsynaptic Potential (IPSP)

• Action difficult due to neurotransmitters receptors which increases permeabiltiy of pottasium ions, ions diffuse out and causing hyperpolarizations.