Action Potentials & Nervous System

Bio 219

Napa Valley CollegeDr. Adam Ross

Review:

• Membrane potentials exist due to unequal distribution of charge across the membrane

• Concentration gradients drive ion flow across membrane• Movement also regulated by permeability

• Equilibrium potential is the electrical charge at which and ion will no longer flow down its concentration gradient

Resting membrane potential

-70mV

Na+/K+ ATPase

Neuron structure

Neuron structure

• Cell body• Contains nucleus and most organelles

• Dendrites• Branch from cell body

• Receive signals from other cells

• Axon• Extends from cell body, conducts action potentials

• Axon hillock – joins cell body

• Axon terminal – form synapses with other cells• Contain neurotransmitter

Nervous System

Action Potentials

• Neurons send electrical messages through the body via action potentials

• Regulated events that use membrane potential and ionic concentration gradients to send messages

• Results from an initial Na+ influx into the cell• This depolarizes the cell

• Followed by K+ efflux from cell• This repolarizes the cells

Graded Potentials

• One type of neuronal signal

• Small, localized changes in membrane potential

• Can be hyperpolarization or depolarization

• Size depends on stimulus strength

Action Potentials

• Large changes in membrane potential

• Formed along the axon

• Rapid depolarization followed by repolarization

• Conducted along the axon

• All or none event• Size of AP is not dependent on size of stimulus

Action Potential

Figure 7.4

Extracellular

fluid

Neuron plasma

membrane

Cytoplasm

Sodium-potassium pump

The sodium-potassium pump

uses cellular energy (ATP) to

pump sodium ions out of the

cell and potassium ions into

the cell

Continually open ion channels “Gated” ion channels Sodium-potassium pump

Ion channels

Ion channels can be open continuously or opened and

closed by a molecular gate

Cross section

Axon membrane

Steps of action potential• 1. The axon is depolarized

• Usually the result of sodium influx into the cell

Steps of an action potential

• 2. Cell is repolarized via K+ efflux

Steps of an action potential

• 3. Undershoot• Voltage gated K+ channels remain open

• High K+ permeability results in hyperpolarization

• Resting state is restored at the end of the undershoot phase

Three phases of AP

• Rising phase• Results from influx of Na+

• Cell is depolarizing

• Falling Phase• Voltage gated Na+ channels close and inactivate (refractory period)

• Voltage gated K+ channels open (K+ rushes out_

• Undershoot• Voltage gated K+ channels remain open, too much K+ leaves

• Cell is restored by Na/K ATPase

Properties of AP• 1. threshold - stimulus must be greater than a certain strength to evoke an

AP

• 2. "all or none" - once threshold is reached, size of the AP is constant regardless of stimulus

• 3. regenerative - AP is regenerated and does not decrease in strength along the axon

• 4. refractory period - short delay following an AP before another AP can be formed• absolute refractory period (ARP) - period in which another AP can not be formed

• relative refractory period (RRP) - period in which a larger stimulus is required to form another AP

Importance of refractory period

• Importance of refractory period:

• ARP sets an upper limit on frequency of action potentials

• During RRP, a stronger stimulus can result in increased frequencyof APs

Stimulus intensity is coded by the frequency of APs._I___I____I_I_I_I____IIIIIIIIIIII_

• Refractory period prevents AP from traveling backward along the axon

Myelin

• Schwann cells wrap around neurons, covering them with myelin

• This covers the ion channels under the myelin

• Uncovered parts of neuron are called “nodes of Ranvier”

• Allows for saltatory conduction from one node of ranvier to the next

Nodes of Ranvier

(a)

Cell

body

Dendrites

Myelin sheath

Node of

Ranvier

Nucleus

Schwann cell

In saltatory conduction, the

nerve impulses jump from one

node of Ranvier to the next.

Conduction of action potentials

• Myelinated axons• AP depolarization jumps between nodes of Ranvier (saltatory)

• Myelin insulates neuron against loss of ions

• Fast conduction speed of up to 120 m/s

• Unmyelinated axons• AP spreads short distance down axon (local current flow)

• Depolarization stimulates formation of AP farther down axon

• Axons are leaky and need to regenerate AP often along axon

• Increasing axon diameter increases speed

• Slow conduction speed compared to myelinated neurons

Organization of the Nervous System

• Central Nervous System• Spine and brain

• Periphereal Nervous System• Autonomic

• Contains: Sympathetic and Parasympathetic

• Automatic

• Regulates things like heart rate, breathing and digestion

• Somatosensory• Motor- you can control

• Senses- touch, sight, taste, sound, smell