Neuro B100: Neural Conduction

Propagation

the movement of an action potential along an axon from the axon hillock to the axon terminals

Action Potential Propagation Step 1

Depolarization occurs in one region of the axon, creating local electrical currents

Action Potential Propagation Step 2

Currents spread into neighboring membrane regions

Action Potential Propagation Step 3

Threshold is reached in the neighboring region, generating a new action potential

Action Potential Propagation Step 4

The process repeats along the entire length of the axon, continuously regenerating the signal

Direction of a Propagation

Directionality depends on the state of the membrane surrounding the action potential; axon does not force AP in one direction

What creates the normal directionality of action-potential propagation?

refractory periods

What would happen if the axon did not regenerate the signal?

neural signals weaken,electrical charge naturally dissipates through membrane leakage and internal resistance as it travels down the axon

Conduction speed

the speed at which an electrical impulse or action potential travels down the length of an axon

What two factors determine conduction speed?

axon diameter, myelination

Conduction velocity range

less than 1 m/s to more than 100 m/s

Which generates a faster conduction speed: large, myelinated v small, unmyelinated?

larger, myelinated axons

Importance of Conduction speed

Less time between a stimulus and the neural response, important for time-critical functions, rapid reflexes, fast sensory responses, coordinated movements

Axon diameter and conduction speed

larger diameter means faster signals, less internal resistance, rapid spread

Myelin

lipid-rich insulating layer around an axon that blocks ion leakage, radically boosting conduction speed while saving cell energy

Myelin production in CNS

oligodendrocytes produce myelin, one oligodendrocyte can myelinate parts of multiple axons

Myelin production in PNS

Scwann cells produce myelin, one Schwann cell myelinates one

segment of one axon and do metabolic support

Nodes of Ranvier

Small gaps between adjacent segments of myelin with high concentration of voltage-gated ion channels where signals regenerate

Saltatory conduction

Action potentials "jump" from node to node and regenerate signals, which is faster than continuous conduction

What is the relationship between myelin and ions?

Myelin reduces leakage of ions through membrane

Demyelination

loss or destruction of the myelin sheath around axons, which causes electrical current to leak out, cripples signal efficiency, and slows down conduction speed

Multiple sclerosis (MS)

autoimmune disease in which the immune system attacks CNS and its myelin

Symptoms of MS

vision problems, numbness, muscle weakness, fatigue, balance problems

Guillain-Barré Syndrome

rare autoimmue neurological disorder in which immune system attacks PNS and its myelin, breaking down communication between affected areas and CNS

Symptoms of Guillain-Barré Syndrome

severe pain, muscle weakness, numbness, loss of reflexes

What is a third factor that affects conduction speed?

Temperature

How do higher temperatures affect conduction speed?

increases conduction speed by affecting kinetics of ion channels and other membrane properties