BIO 1140 - Cell Transport Across Plasma Membrane (Pt. 2)

Outside the cell, the charges of Na+ ions are balanced by Cl- ions. Inside the cell, the charges of K+ ions are balanced by negative charges of organic molecules.

Small excesses of positive or negative charge on the two sides of the plasma membrane create a voltage difference.

This voltage difference across the membrane is called the membrane potential.

For a cell at rest, the exchange of anions and cations across the membrane is steady - Resting membrane potential in animal cells varies between –20 to –200 mV.

Ion channels create membrane potential - Ion channels create openings for the passive movement of inorganic ions

K+ leak channels are responsible for the resting membrane potential - At rest, plasma membrane is mostly permeable to K+ (moving out of the cell) - K+ is the main positive ion inside the cell

The Na+-K+ pumps also contribute to the resting potential - establish the K+ gradient - pumps 3 Na+ ions out of the cell for every 2 K+ ions in

Ion channels facilitate the passage of select inorganic ions. - Most channels are narrow and highly selective - Most channels in the cell are ion channels

Two characteristics distinguish Ion channels from pores : - Ion selectivity - Fluctuates between open/closure conformations (not continuously open)

A specific stimulus triggers gated-ion channels to switch between a closed and an open state by a change in their conformation. • Ligand Gated • Voltage Gated • Mechanically Gated • Always open (leak channels)

Voltage-gated ion channels are controlled by changes in the voltage across the membrane

Ligand-gated ion channels are controlled by the binding of a molecule - Stimulus molecule binds to receptor site - polar substance can diffuse across membrane

Mechanically-gated ion channels are controlled by a physical stimuli - physical stimuli include light, sound waves, pressure, stretch, touch and vibration

Ion channels are responsible for electric signal generation and transmission in neurons

A neuron (a nerve cell) consists of a cell body, axon and denrites. - An electric signal consists of changes in the membrane potential across the neuron’s plasma membrane.

• Cell body: contains nucleus and cell’s organelles • Dendrites: receive information from other neurons or sensory cells • Axon: carry information from cell body to target cells via axon terminals • Axon Terminals: branching of axon into fine nerve endings

Activities of neurons are recorded as changes in membrane potential - Electrodes are used to record electrical events in a cell - The resting potential of a neuron is usually between –60to –70 millivolts (mV) - The minus sign indicates that the inside of the cell is electrically negative compared with the outside

A rapid change in membrane potential is an action potential - Neuron stimulation shifts the membrane potential to a less negative value (toward zero, depolarization) - Action potentials are sudden, transient, large changes in membrane potential

Depolarization causes voltage-gated Na+ channels in the membrane to open transiently.

Action potentials result from rapid changes in voltage gated Na+ and K+ channels

The depolarized axonal membrane returns to its resting potential by the opening of voltage-gated K+ channels (repolarization).

• Action potentials travel along axons • Action potentials jump along myelinated axons • Action potential propagates in one direction

Neurons connect to their target cells at synapses - The signal is transmitted to the target cells at specialized junctions known as synapses - Presynaptic and the postsynaptic cells are separated from each other by synaptic cleft

Thousands of synapses form on the cell body and dendrites of a motor neuron in the spinal cord