Structures and Processes of the Nervous System – Part 2 Section 8.2

The Electrical Nature of Nerves UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 The Electrical Nature of Nerves Neurons use electrical signals to communicate with other neurons, muscles, and glands. The signals, called nerve impulses, involve changes in the amount of electric charge across a cell’s plasma membrane.  

Resting Membrane Potential UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Resting Membrane Potential In a resting neuron, the cytoplasmic side of the membrane (inside the cell) is negative, relative to the extracellular side (outside the cell) This charge separation across the membrane is a form of potential energy called membrane potential. When microelectrodes are inserted in a resting neuron, a voltmeter can indicate an electrical potential difference (voltage) across a neural membrane.

Resting Membrane Potential UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Resting Membrane Potential The resting membrane potential is the potential difference across a membrane in a resting neuron (about -70 mV). It provides energy for generating a nerve impulse. The process of generating a resting membrane potential of -70 mV is called polarization.

Sodium-Potassium Pump UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Sodium-Potassium Pump The sodium-potassium pump is the most important factor that contributes to the resting membrane potential. This system uses ATP to transport three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell. The overall result of this process is a constant membrane potential of -70 mV. Na/K Pump

UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential Recall that a neuron is polarized due to the charge difference across the membrane. Depolarization occurs when the cell becomes less polarized During depolarization, the inside of the cell becomes less negative relative to the outside of the cell. An action potential causes depolarization to occur. Continued…  

UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential An action potential is the movement of an electrical impulse along the plasma membrane of an axon. It is an all-or-none phenomenon: if a stimulus causes the axon to depolarize to a certain level (the threshold potential), an action potential occurs. Threshold potentials are usually close to -50 mV. The strength of an action potential does not change based on the strength of the stimulus.

Action Potential UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential The graph shows the changes that occur to membrane potential as an action potential travels down an axon.

Action Potential: Step 1 UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential: Step 1 An action potential is triggered when the threshold potential is reached.

Action Potential: Step 2 – depolarization UNIT 4 Chapter 8: The Nervous System and Homeostasis Action Potential: Step 2 – depolarization Voltage-gated sodium (Na+) channels open when the threshold potential is reached. Sodium ions move down their concentration gradient and rush into the axon, causing depolarization of the membrane. The membrane potential difference is now +40 mV.

Action Potential: Step 3 - hyperpolarization UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential: Step 3 - hyperpolarization Voltage-gated sodium channels close due to change in membrane potential. Voltage-gated potassium (K+) channels open. Potassium ions move down their concentration gradient and exit the axon, causing the membrane to be hyperpolarized to -90 mV.

Action Potential: Step 4 – resting membrane restored UNIT 4 Section 8.2 Action Potential: Step 4 – resting membrane restored Voltage-gated potassium channels close. The sodium-potassium pump and naturally occurring diffusion restore the resting membrane potential of -70 mV. The membrane is now repolarized.

UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Action Potential After an action potential occurs, the membrane cannot be stimulated to undergo another action potential. This brief period of time (usually a few milliseconds) is called the refractory period of the membrane. The events that occur in an action potential continue down the length of the axon until it reaches the end, where it initiates a response at the next cell.

Myelinated Nerve Impulse UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Myelinated Nerve Impulse A nerve impulse consists of a series of action potentials. Conduction of a nerve impulse along a myelinated neuron is called saltatory conduction because action potentials “jump” from one node of Ranvier to the next. Saltatory conduction is faster (120 m/s) than the conduction of nerve impulses in unmyelinated neurons (0.5 m/s).

Signal Transmission Across a Synapse UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Signal Transmission Across a Synapse synapse: the junction between two neurons, or between a neuron and an effector) neurons are not directly connected; the small gap between them is called the synaptic cleft A nerve impulse cannot jump from one neuron to another Q: How does the nerve impulse proceed from the presynaptic neuron (which sends out the information) to the postsynaptic neuron (which receives the information)? A: Chemical messengers called neurotransmitters carry the nerve impulse across the synapse from one neuron to another, or from a neuron to an effector.

Signal Transmission Across a Synapse: Step 1 UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Signal Transmission Across a Synapse: Step 1 The nerve impulse travels to the synaptic terminal.

Signal Transmission Across a Synapse: Step 2 UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Signal Transmission Across a Synapse: Step 2 Synaptic vesicles containing neurotransmitters move toward and fuse with the presynaptic membrane.

Signal Transmission Across a Synapse: Step 3 UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Signal Transmission Across a Synapse: Step 3 Synaptic vesicles release neurotransmitters into the synaptic cleft by exocytosis. Neurotransmitters diffuse across the synapse to reach the postsynaptic neuron or the cell membrane of an effector.

Signal Transmission Across a Synapse: Step 4 UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Signal Transmission Across a Synapse: Step 4 Neurotransmitters bind to specific receptor proteins on the postsynaptic membrane. The receptor proteins trigger ion channels to open. Depolarization of the postsynaptic membrane occurs, and an action potential is initiated if the threshold potential is reached.

Neurotransmitters UNIT 4 Chapter 8: The Nervous System and Homeostasis Section 8.2 Neurotransmitters Neurotransmitters have either excitatory or inhibitory effects on the postsynaptic membrane. Excitatory molecules, like acetylcholine, cause action potentials by opening sodium channels. Inhibitory molecules cause potassium channels to open, causing hyperpolarization. Neurotransmitters