Action Potential

Absolute vs. Relative Refractory Periods

The refractory periods are phases during which a neuron is less or not able to fire another action potential. These periods are crucial for the proper timing and directionality of neuronal signaling.

• Absolute Refractory Period: The time during which a second action potential cannot be initiated, regardless of stimulus strength. This occurs when voltage-gated Na+ channels are inactivated.

• Relative Refractory Period: The period following the absolute refractory period when a stronger-than-normal stimulus is required to initiate another action potential. Here, K+ channels are still open, and some Na+ channels have reset.

• Conformations: During the absolute refractory period, Na+ channels are inactivated; during the relative refractory period, K+ channels are open and Na+ channels are transitioning back to a resting state.

Excitability

Excitability refers to the ability of a neuron to respond to a stimulus and generate an action potential.

• During the relative refractory period, excitability is reduced because the membrane is hyperpolarized.

• Excitability depends on the state of ion channels and the membrane potential.

Graded vs. Action Potentials

Neurons use both graded and action potentials to transmit signals.

• Graded Potentials: Local changes in membrane potential that vary in size and decay over distance. They do not follow the all-or-none principle.

• Action Potentials: Large, rapid depolarizations that propagate along the axon without decrement. They follow the all-or-none principle.

• Threshold: If graded potentials reach a certain threshold, an action potential is triggered.

AP Propagation

Importance and Mechanisms

Action potential propagation is essential for rapid communication in the nervous system.

• Myelination: Myelinated axons conduct action potentials faster via saltatory conduction, where the impulse jumps between nodes of Ranvier.

• Unmyelinated Axons: Conduct impulses more slowly via continuous conduction.

• Prediction: If the axon is myelinated, the action potential travels faster and with less energy expenditure.

Types of Propagation

• Saltatory Propagation: Occurs in myelinated axons; action potentials jump from node to node.

• Continuous Propagation: Occurs in unmyelinated axons; action potentials move along every part of the membrane.

• Comparison: Saltatory conduction is faster and more efficient than continuous conduction.

Chemical Synapse

Structure and Function

Chemical synapses are specialized junctions where neurons communicate via neurotransmitters.

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives the signal via receptors.

• Steps: Action potential arrives at axon terminal → voltage-gated Ca2+ channels open → neurotransmitter released → binds to postsynaptic receptors → postsynaptic response.

Receptor Types

• Ionotropic Receptors: Ligand-gated ion channels that produce fast, direct changes in membrane potential.

• Metabotropic Receptors: G-protein-coupled receptors that initiate slower, indirect signaling cascades.

• Cellular Response: Ionotropic receptors lead to faster responses; metabotropic receptors lead to slower, modulatory effects.

EPSPs and IPSPs

Postsynaptic potentials determine whether a neuron will fire an action potential.

• EPSP (Excitatory Postsynaptic Potential): Depolarizes the postsynaptic membrane, increasing the likelihood of firing an action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of firing an action potential.

• Mechanism: EPSPs are typically caused by influx of Na+; IPSPs by influx of Cl- or efflux of K+.

Nervous System Overview

Main Divisions

The nervous system is divided into several major branches, each with distinct functions.

• Central Nervous System (CNS): Brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; transmits signals to and from the CNS.

• Afferent Division: Carries sensory information to the CNS.

• Efferent Division: Carries motor commands from the CNS to effectors (muscles, glands).

Communication Targets

• Somatic Nervous System: Controls voluntary movements (skeletal muscles).

• Autonomic Nervous System: Regulates involuntary functions (internal organs, glands).

Reflex Pathways

Definition and Components

A reflex is an automatic, rapid response to a stimulus, involving a specific neural pathway.

• Seven Generic Components:

  1. Stimulus

  2. Receptor

  3. Affector (sensory neuron)

  4. Integration center (interneuron)

  5. Effector (motor neuron)

  6. Effector organ

  7. Response

• Example: Knee-jerk (patellar tendon) reflex involves all seven components.

Spinal Cord Anatomy

• Cross-section: Shows gray matter (cell bodies) and white matter (myelinated axons).

• Dorsal Root Ganglion: Contains sensory neuron cell bodies.

• Integration: Sensory information enters via dorsal root; motor output exits via ventral root.

Membrane and Equilibrium Potentials

Key Concepts

Membrane and equilibrium potentials are fundamental to understanding neuronal signaling.

• Membrane Potential: The voltage difference across a cell's plasma membrane.

• Equilibrium Potential: The membrane potential at which the net flow of a particular ion is zero.

• Nernst Equation: Used to calculate equilibrium potential for a specific ion:

• Goldman-Hodgkin-Katz Equation: Used to calculate the resting membrane potential considering multiple ions:

• Application: Understanding these equations helps explain how neurons generate and propagate electrical signals.

Table: Comparison of Ionotropic vs. Metabotropic Receptors