Warm Up What is actually occurring inside the cell when a neuron has an “impulse” traveling through it?

Action Potential Chapter 7 (pg 237-242)

Objective Checklist Describe the events that lead to the generation of a nerve impulse and it conduction from one neuron to another. Explain how the sodium-potassium pump restores the resting membrane potential. Define reflex arc, and list its elements.

Functional Properties of Neurons Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse

Neurons at Rest The plasma membrane at rest is polarized (negatively charged) Fewer positive ions are inside the cell than outside Inside = potassium (K+) Outside = sodium (Na+)

Initiating a Nerve Impulse Many different types of stimuli can excite neurons to become active and generate an impulse Examples: Light excites eye receptors Sound excites ear receptors Pressure excites cutaneous receptors

Initiating a Nerve Impulse A stimulus causes sodium (Na+) channels to open. Sodium ions diffuse into the cell, making the inside of the cell more positive. This area of the membrane is said to be depolarized.

Conducting a Nerve Impulse Voltage-gated sodium channels respond to a change in charge Local depolarization causes adjacent sodium channels to open. The depolarization travels down the nerve axon and is known as an action potential.

Aside: All or Nothing The nerve impulse is an all-or-none response. It does not occur unless the local depolarization is strong enough. It is either propagated over the entire axon, or it doesn’t happen at all.

Restoring Resting Potential Potassium ions (K+) rush out of the neuron after sodium ions rush in, which repolarizes the membrane. Repolarization restores the negative charge inside the cell. Until repolarization occurs, a neuron cannot conduct another impulse.

Restoring Ion concentrations After repolarization occurs, the initial concentrations of the sodium and potassium ions are restored using sodium-potassium pumps. ATP is used to pump excess sodium ions out of the cell and to bring potassium ions back in to the cell. Three sodium ions are pumped out; two potassium ions are pumped in. Animation: Nerve Impulse Animation: Na/K Pump

Transmitting Impulses to other Neurons Neurotransmitters are released from the axon terminal They cross the synapse and stimulate dendrites on an neighboring nerve A nerve impulse is an electrical and a chemical event. Why?

Saltatory Conduction Impulses travel faster when fibers have a myelin sheath. That is because electrical current cannot flow across the myelin. The impulse jumps from node to node, a process known as saltatory conduction.

Introduction to Reflexes Reflex – rapid, predictable, and involuntary responses to stimuli Reflex arc – the neural pathway that a reflex travels; involves the PNS and CNS

Types of Reflex Arcs Autonomic reflexes Somatic reflexes Regulate activity of smooth muscle, the heart, and glands Examples: salivary reflex, pupillary reflex* Somatic reflexes Activation of skeletal muscles Examples: patellar reflex, withdrawal reflex The * denotes a reflex that requires brain involvement, which is uncommon

Examples of Somatic Reflex Arcs Which one of these do you think takes longer? Why? Most reflex arcs have five elements and require interneurons

Importance of Reflex Arcs Most reflex arcs do not require communication with the brain. Reflex transmission is important in evaluating the condition of the nervous system. Changes in reflex arcs often occur before other symptoms of disease have become obvious.

Objective Checklist Describe the events that lead to the generation of a nerve impulse and it conduction from one neuron to another. Explain how the sodium-potassium pump restores the resting membrane potential. Define reflex arc, and list its elements.