Membrane Potentials: Graded Potentials

Definition and Production of Graded Potentials

Graded potentials are local changes in membrane voltage that occur in response to a stimulus. They are essential for initiating action potentials and are produced when ion channels open, allowing ions to flow across the membrane. - Key Point 1: Graded potentials occur primarily in the dendrites and cell body of a neuron. - Key Point 2: The magnitude of a graded potential depends on the strength of the stimulus; stronger stimuli produce larger voltage changes. - Key Point 3: Graded potentials can be depolarizing (making the inside less negative) or hyperpolarizing (making the inside more negative). - Example: Sodium ions (Na+) entering the cell cause depolarization, while potassium ions (K+) leaving cause hyperpolarization.

Locations and Stimuli Initiating Graded Potentials

Graded potentials are initiated at the dendrites or cell body by various stimuli, including neurotransmitter binding, mechanical deformation, or voltage changes. - Key Point 1: Chemically gated, voltage-gated, and mechanically gated ion channels are involved in generating graded potentials. - Key Point 2: Graded potentials decay with distance from the site of stimulation, making them local signals.

Membrane Potentials: Action Potentials

Definition and Triggering of Action Potentials

An action potential is a rapid, all-or-none electrical event that propagates along the axon, allowing long-distance communication within the nervous system. - Key Point 1: Action potentials are triggered when the membrane potential reaches a threshold value, typically around -55 mV. - Key Point 2: Action potentials do not fade with distance and are essential for transmitting signals from the axon hillock to the axon terminals.

Phases of the Action Potential

The action potential consists of several distinct phases, each involving specific ion channel activity and ion movement.

Comparison of Graded and Action Potentials

- Location: Graded potentials occur in dendrites and cell body; action potentials occur in axons. - Amplitude: Graded potentials vary in size; action potentials are all-or-none. - Duration: Graded potentials are brief and local; action potentials are longer and propagate. - Channel Types: Graded potentials use chemically, mechanically, and voltage-gated channels; action potentials use voltage-gated channels. - Function: Graded potentials initiate action potentials; action potentials transmit signals.

Resting Membrane Potential (RMP)

Establishment and Maintenance of RMP

The resting membrane potential is the voltage difference across the plasma membrane when the neuron is not transmitting signals. - Key Point 1: RMP is typically -70 mV, with the inside of the neuron being more negative than the outside. - Key Point 2: The Na+/K+ ATPase pump maintains ion gradients by moving 3 Na+ out and 2 K+ in, using ATP. - Key Point 3: Permeability to K+ is especially important for maintaining RMP.

Action Potential Propagation and Coding for Stimulus Intensity

Propagation of Action Potentials

Action potentials begin at the axon hillock and propagate along the axon toward the terminals. - Key Point 1: Propagation is unidirectional due to refractory periods. - Key Point 2: Myelinated axons conduct action potentials faster via saltatory conduction (jumping between nodes of Ranvier).

Coding for Stimulus Intensity

The central nervous system codes for stimulus intensity by the frequency of action potentials, not their amplitude. - Key Point 1: Stronger stimuli generate more frequent action potentials. - Key Point 2: Subthreshold stimuli do not generate action potentials.

Refractory Periods

Absolute and Relative Refractory Periods

Refractory periods are phases during which a neuron cannot or is less likely to fire another action potential. - Absolute Refractory Period: No second action potential can occur, regardless of stimulus strength; voltage-gated Na+ channels are open or inactivating. - Relative Refractory Period: A second action potential can occur, but only with a stronger stimulus; membrane is hyperpolarized and some Na+ channels are recovering.

Abnormalities of Nervous Tissue

Multiple Sclerosis and Local Anesthesia

Disorders affecting nervous tissue can alter normal physiological function. - Multiple Sclerosis (MS): Demyelinating disorder that slows or blocks action potential conduction in the CNS. - Local Anesthesia: Blocks voltage-gated Na+ channels, preventing action potential generation and transmission.

Key Ions in Membrane Potentials

Roles of Sodium, Potassium, Calcium, and Chloride

- Sodium (Na+): Major extracellular ion, influx causes depolarization. - Potassium (K+): Major intracellular ion, efflux causes repolarization and hyperpolarization. - Calcium (Ca2+): Important for neurotransmitter release. - Chloride (Cl-): Contributes to inhibitory postsynaptic potentials.

Summary Table: Ion Channel Types

Equations and Formulas

Nernst Equation for Equilibrium Potential

The equilibrium potential for an ion can be calculated using the Nernst equation:

Goldman-Hodgkin-Katz Equation for Membrane Potential

The membrane potential considering multiple ions is given by:

Central and Peripheral Nervous System Overview

Structural Organization

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). - CNS: Includes the brain and spinal cord. - PNS: Includes cranial nerves, spinal nerves, and ganglia.

Additional info:

- Academic context was added to clarify the phases of action potentials, the role of ion channels, and the clinical relevance of disorders such as multiple sclerosis and local anesthesia. - Tables and equations were inferred and expanded for completeness and exam preparation.