Ion channels, APs, and neuronal functioning

Types of ion channels

Ungated (leak), voltage gated, ligand gates, and mechanically gated

Ungated(leak) channels

Leak at rest. More K+ than Na+ ungated channels. Other ions don’t usually pass through other ion channels

Voltage gated ion channels

Integral membrane proteins. 3 complimentary aspects: ion conductance, pore gating, and regulation. Na+ voltage gated and K+ voltage gated channels.

Na+ voltage gated channel

Key regions include voltage sensing, pore, and inactivation

K+ voltage gated channel

Some inactivate fast, slow, or not at all. Variability guarantees that there will always be some source of K+ for repolarization.

Property of ion channels

Rate of ionic flow determined by max channel conductance and ion electrochemical gradient

Action potentials

An abrupt, transient change in membrane potential. All-or-none. Fires at max strength regardless of stimulus strength

Process of AP

Stimulus causes cation influx. Membrane potential increases (Depolarizes). Occurs slowly until threshold. Sudden and rapid increase in membrane potential (Firing) from cation influx. Membrane potential then falls below resting potential (Hyperpolarization). Membrane potential returns to resting.

Refractory period

Period after an AP when a neuron either cannot fire another AP (absolute) or requires stronger stimulus for a weakened response (relative).

Absolute refractory period

Occurs when Na+ channels are already open or are inactivated. Inactivation blocks need repolarization in order to be removed to close the pore.

Relative refractory period

Some Na+ channels still inactive and some are closed and in resting state. K+ channels still open so membrane is hyperpolarized and need stronger stimulus.

AP in detail

Na+ channels begin to open (slow influx of Na+). Once threshold is reached, all Na+ channels open and rapid depolarization (K+ channels still closed). Once max strength is reached then K+ channels open and efflux of K+ (hyperpolarizes the cell below resting). Na+ channels are blocked (inactivated) with K+ still open. Then they both close and the cell is back to resting..

Conduction velocity

Speed of AP. Larger axon diameters means higher velocity. Larger diameters means less membrane resistance. Myelin increases conduction velocity. Nodes of Ranvier also increase velocity.

Cable theory

Neuron is treated as a perfectly cylindrical and electrically passive cable. V=IR (Ohm’s Law). Capacitance and resistance

Capacitance

Refers to the electrostatic forces acting through the lipid bilayer. stores electrical energy

Resistance

Due to cytosolic resistance to movement of charges

Length constant (Lambda)

Characteristic length at which voltage decays across a membrane. At distance lambda, the applied voltage would be at 1/3 of original strength. Larger lambda means faster conduction velocity and increased membrane resistance means larger lambda (Myelin increases membrane resistance).

Neurotransmission

Chemical synapses allow for simultaneous receiving of inputs from multiple terminals which allow for modulation of the signal. Uses small molecules called neurotransmitters from presynaptic cell.

Types of neurotransmission

Direct which has the neurotransmitter directly bind to ligand-gated ion channel (ionotropic). Indirect which has the NT bind to a GPCR and 2nd messengers open or close an ion channel (metabotropic).

Neurotransmitters

Acetylcholine, GABA, Glycine, Glutamate, Norepi/Epi, Dpamine, Serotonin, Endorphins, Enkephalins, Substance P, CO, NO

Acetylcholine

Between muscles and nerves, in brain, and heart. For memory, attention, learning, and the parasympathetic. Degeneration linked with Alzheimer’s

GABA

Inhibitory neurotransmitter. Inhibits neurotransmission. Opens Cl- on postsynaptic cell

Glycine

Similar to GABA

Glutamate

Excitatory. For learning and memory

Norepi & Epi

Dual roles as hormones and neurotransmitters. For attention & focus. Can be excitatory or inhibitory based on receptor. Pleasure/reward pathway, memory, and motor control

Dopamine

Behavior and cognition. Voluntary movement. Motivation and reward and inhibits prolactin. Sleep, mood attention, and learning. Degeneration linked with Parkinson’s

Serotonin

Regulates intestinal movement, mood, appetite, and sleep

Endorphins

Released during pleasurable experiences. Reduces pain. PNS

Enkephalins

Endorphins for CNS

Substance P

Increases perception of pain. From spine

CO

Regulates hormone release from hypothalamus

NO

Learning and muscle movement. Relaxes smooth muscles in blood vessels. Vasodilation

Removal of NTs from Synaptic Cleft

Acetylcholinesterase inhibitors, SSRIs, SSNRIs

Graded potential

Increase or decrease in membrane potential below the threshold and do not trigger APs. Precursor. EPSP and IPSPs

EPSPs

Change in mem. pot. closer to threshold

IPSPs

Change in mem. pot. further from threshold