Neuron Physiology

What is neuron physiology?

Electrophysiology – cellular mechanisms for producing electrical potentials and currents; basis for neural communication and muscle contraction.

What is charge in neuron physiology?

The property of particles that produces electrical effects.

What is voltage in neuron physiology?

Difference in electrical charge across a membrane.

What is electrical current in neurons?

Movement of charged particles, usually ions, across membranes.

What are pumps in neurons?

Membrane proteins that maintain gradients by moving substances against their concentration gradient; require energy.

Which pumps are present in neurons?

Sodium-potassium (Na+/K+) pumps and calcium (Ca2+) pumps.

What are channels in neurons?

Protein pores in the membrane allowing ions to move down their concentration gradient; specific for certain ions.

What are leak channels?

Always open channels that allow continuous ion diffusion.

What are chemically gated channels?

Normally closed channels that open when a neurotransmitter binds.

What are voltage-gated channels?

Normally closed channels that open in response to changes in membrane voltage.

What are Na+ channel states?

Resting (gate closed), activation (gate opens with voltage change, Na+ enters), inactivation (activation gate open, inactivation gate closed, prevents Na+ entry, channel reset).

Why can Na+ channels be in three states?

Because they have two gates: activation and inactivation gates.

What are modality-gated channels?

Normally closed channels that open in response to a stimulus other than chemical or voltage changes; found in sensory neurons.

Where are pumps and channels distributed in neurons?

Entire plasma membrane: leak channels and Na+/K+ pumps maintain RMP; receptive segment: chemically gated channels; initial and conductive segments: voltage-gated Na+ and K+ channels; transmissive segment: voltage-gated channels and Ca2+ pumps.

How do neurons follow Ohm’s Law?

Neuron current depends on voltage (potential energy) and resistance; C = V/R; ions generate current through diffusion; membrane resistance influenced by gated channels.

What are graded potentials?

Small, short-lived changes in RMP in receptive segment; can depolarize or hyperpolarize; vary in magnitude.

Caused by opening of chemically gated ion channels

What are postsynaptic potentials?

Graded potentials in a postsynaptic neuron: EPSP (depolarization) or IPSP (hyperpolarization).

How is an EPSP generated?

Depolarization caused by Na+ entry.

How is an IPSP generated?

Hyperpolarization caused by K+ exit or Cl- entry.

Where does summation of EPSPs and IPSPs occur?

At the axon hillock (initial segment).

What is threshold potential?

Minimum voltage change (~-55 mV) required to trigger an action potential.

Needs multiple EPSPs in initial segment

What is spatial summation?

EPSPs from multiple locations on the receptive region combine to potentially reach threshold.

What is temporal summation?

EPSPs from repeated release of neurotransmitter by a single presynaptic neuron combine to potentially reach threshold.

What is the all-or-none law in neurons?

If threshold is reached, an action potential occurs; if not, no response.

What occurs in the conductive segment of a neuron?

Propagation of action potential along the axon; depolarization (Na+ influx) and repolarization (K+ efflux).

AP goes to synaptic knob

How is an action potential generated: depolarization?

Na+ channels open, Na+ enters, membrane potential becomes positive; Na+ channels close and inactivate; AP propagates.

How is an action potential generated: repolarization?

K+ channels open slowly, K+ exits, restoring negative potential; K+ channels close, RMP reestablished; AP propagates.

What is the refractory period?

Period after an AP when firing another AP is impossible or difficult.

What is the absolute refractory period?

No stimulus can initiate another AP; Na+ channels are inactive; ensures AP moves toward synaptic knob.

What is the relative refractory period?

Another AP possible, but requires stronger stimulus; some K+ channels still open; cell slightly hyperpolarized.

What is continuous conduction?

AP propagation in unmyelinated axons; each region depolarizes sequentially. (Opening channels)

What is saltatory conduction?

AP propagation in myelinated axons; AP occurs only at nodes of Ranvier; faster and more energy-efficient.

What occurs at the transmissive segment?

AP arrival opens Ca2+ channels; Ca2+ enters, triggers vesicle exocytosis; neurotransmitter released into synaptic cleft; binds postsynaptic receptors.

Starts at synaptic knob

Neurons can synthesize more than one neurotransmitter

What are the differences between graded potentials and action potentials?

Graded potentials: occur in receptive region, local, can be positive/negative, graded; Action potentials: occur on axon, all-or-none, propagate entire length, depolarization and repolarization.

What factors affect velocity of AP propagation?

Axon diameter (larger = faster), myelination (myelinated = faster); less resistance increases conduction speed.

What are nerve fiber groups?

Group A: fast, large diameter, myelinated (150 m/s); Group B: 15 m/s, smaller diameter/unmyelinated; Group C: 1 m/s, small/unmyelinated.

A - All somatic motor and some sensory

B/C - Some somatic and some visceral

How does AP firing frequency relate to stimulus strength?

Stronger stimulus → higher firing frequency → more tension in motor nerves; frequency can influence type of neurotransmitter released.