Action Potentials & Nervous System Bio 219 Napa Valley College Dr. 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 Cross section Axon membrane Neuron plasma membrane Extracellular fluid Cytoplasm 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 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 Figure 7.4
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 frequency of 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 Nucleus Dendrites Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next. Node of Ranvier Schwann cell (a) Myelin sheath
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