Nervous System and Action Potential Notes
Nervous System Overview
Organization: Central Nervous System (CNS) and Peripheral Nervous System (PNS).
CNS: Comprises the brain and spinal cord.
PNS: Sensory neurons (afferent neurons) and motor neurons (efferent neurons).
Motor Neurons:
Autonomic Division: Sympathetic and parasympathetic.
Somatic Division: Controls voluntary muscles.
Emergent Properties: Characteristics developed based on individual interactions with the environment.
Neurons and Neural Communication
Neurons: Main cells of the nervous system, classified by structure.
Types of Neurons:
Multipolar: Most common, found in motor and interneurons.
Bipolar, Pseudounipolar, Anaxonic: Other classifications based on the number of processes.
Neuronal Structure:
Dendrites: Receiving ends of neurons, along with cell bodies.
Axon: Sending end, typically myelinated to increase conduction speed.
Synapses: Connections between neurons—can be monosynaptic (2 neurons) or polysynaptic (more than 2).
Action Potential
Definition: Electrical signal in neurons; crucial for communication.
Nernst Equation: Predicts membrane potential for a single ion, influenced by concentration gradients and membrane permeability.
Goldman Equation: Calculates resting membrane potential based on several ion concentrations.
Ionic Roles:
Sodium (Na+): Primarily extracellular; contributes to positive charge when influxing during action potential.
Potassium (K+): Primarily intracellular; affects resting membrane potential.
Phases of Action Potential
Resting Membrane Potential:
Neuron is at approximately -70 mV.
Neuron is negative compared to the outside.
Depolarization Phase:
Triggered when voltage reaches -55 mV (threshold).
Opening of voltage-gated sodium channels allows Na+ influx, making the neuron more positive.
Overshoot:
Membrane potential reaches maximum around +30 mV.
Repolarization Phase:
Closing of sodium channels and opening of potassium channels allows K+ to exit.
Hyperpolarization:
Membrane potential drops below resting potential (
approx. -90 mV).
Refractory Periods:
Absolute Refractory Period: No new action potential can be initiated.
Relative Refractory Period: A stronger stimulus can initiate an action potential.
Neuronal Support Cells
Glial Cells: Support neuronal functions, classified as either CNS or PNS.
CNS Glial Cells:
Astrocytes: Form blood-brain barrier, regulate blood flow and nutrient supply.
Oligodendrocytes: Myelinate several axons.
Microglia: Immune function in CNS.
Ependymal Cells: Line brain cavities, involved in cerebrospinal fluid production.
PNS Glial Cells:
Schwann Cells: Myelinate single axons.
Satellite Cells: Support neuronal cell bodies in PNS.
Neurotransmitter System
Types: Neurotransmitters, neuromodulators, neurohormones.
Ionotropic Receptors: Direct (channel-linked) receptors; allow ions to flow across membranes.
Metabotropic Receptors: G-protein coupled receptors; indirectly affect neuron activity.
Neurotransmitter Release: Triggered by action potential arriving at axon terminals, leading to calcium influx and exocytosis of vesicles.
Graded Potentials vs Action Potentials
Graded Potentials:
Vary in strength based on stimulus; can summate (temporal or spatial summation).
Initiated at dendrites and cell body.
Action Potentials:
All-or-none response, uniform amplitude; occur at axon hillock after reaching threshold.
Cannot summate.
Summary
Action potential generation and propagation is crucial for neuronal signaling and communication, influenced by various ionic movements and neuronal structures. Understanding the intricacies of these processes is fundamental for grasping physiology and neurology concepts.
Organization: The nervous system is primarily organized into two main components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
Central Nervous System (CNS): This integral component comprises the brain, which serves as the control center for processing information and coordinating responses, and the spinal cord, which acts as a conduit for signals between the brain and the rest of the body, also managing reflex actions.
Peripheral Nervous System (PNS): This network contains sensory neurons (afferent neurons) responsible for carrying information from sensory receptors to the CNS and motor neurons (efferent neurons) that transmit signals from the CNS to effectors such as muscles and glands.
Motor Neurons
Autonomic Division: This division regulates involuntary physiological functions and is subdivided into:
Sympathetic Division: Prepares the body for stressful or emergency situations by initiating the 'fight or flight' response.
Parasympathetic Division: Promotes relaxation and recovery after stress, managing 'rest and digest' activities.
Somatic Division: Controls voluntary movements of skeletal muscles, allowing for conscious motor functions.
Emergent Properties
Emergent properties refer to characteristics that arise from the interactions of individual components within the nervous system, illustrating the complexity of neuronal networks and their roles in behavior, cognition, and perception.
Neurons and Neural Communication
Neurons: The primary functional units of the nervous system, classified by structure and function:
Multipolar Neurons: Most abundant type, typically involved in motor and interneuron functions, featuring multiple dendrites extending from the cell body.
Bipolar Neurons: Found mainly in sensory pathways, such as in the retina; have one axon and one dendrite.
Pseudounipolar Neurons: Consist of a single process that bifurcates into two branches, commonly serving sensory functions.
Anaxonic Neurons: Lack an axon and are involved in local communication in the brain and retina.
Neuronal Structure
Dendrites: Highly branched structures that receive signals from other neurons, playing a crucial role in synaptic transmission.
Cell Body (Soma): Contains the nucleus and organelles, crucial for maintaining neuronal function and integrating synaptic inputs.
Axon: Conducts action potentials away from the cell body, often myelinated, which enhances signal conduction speed.
Synapses: Specialized junctions between neurons where neurotransmitter release occurs; can be monosynaptic (involving two neurons) or polysynaptic (involving multiple interconnected neurons).
Action Potential
Definition: An action potential is a rapid change in the electrical charge of a neuron, which is essential for the transmission of signals along the nerve fiber, enabling communication throughout the nervous system.
Nernst Equation: This equation predicts the equilibrium potential for a single ion based on its concentration gradient across the cell membrane, providing insight into ionic behavior during action potentials.
Goldman Equation: A more complex formula that calculates resting membrane potential considering multiple ion concentrations, delivering a comprehensive understanding of resting and action potentials.
Ionic Roles
Sodium (Na+): Mostly found outside the neuron; it plays a pivotal role in generating depolarization of the membrane during action potentials as it influxes through voltage-gated sodium channels.
Potassium (K+): Primarily located inside the neuron; its efflux is critical for repolarization and helps establish the resting membrane potential that allows the neuron to reset for subsequent action potentials.
Phases of Action Potential
Resting Membrane Potential: The neuron is at a stable voltage of approximately -70 mV, indicating it is negatively charged relative to the outside environment due to the unequal distribution of ions.
Depolarization Phase: Initiated when the membrane potential reaches a threshold value of -55 mV, leading to the opening of voltage-gated sodium channels and a swift influx of Na+, drastically increasing the internal positive charge.
Overshoot: At its peak, the membrane potential can exceed +30 mV, marking the maximum depolarization of the neuron.
Repolarization Phase: Occurs as sodium channels close and voltage-gated potassium channels open, allowing K+ to exit, returning the membrane potential back toward a negative value.
Hyperpolarization: Following repolarization, the membrane potential may drop further to approximately -90 mV as K+ channels remain open longer than necessary.
Refractory Periods: Encompasses two phases:
Absolute Refractory Period: A phase during which a new action potential cannot be initiated, regardless of stimulus strength, ensuring unidirectional signal propagation.
Relative Refractory Period: A phase during which a stronger-than-normal stimulus is needed to initiate another action potential, allowing the neuron to adjust its excitability.
Neuronal Support Cells
Glial Cells: Essential support cells which maintain homeostasis, form myelin, and provide support and protection for neurons, distinguished into:
CNS Glial Cells:
Astrocytes: Provide structural support, contribute to the blood-brain barrier, and regulate nutrient supply and ion balance in the CNS.
Oligodendrocytes: Form myelin sheaths around several axons, facilitating rapid conduction of electrical signals.
Microglia: Act as immune cells in the CNS, clearing debris and modulating inflammation.
Ependymal Cells: Line the ventricles of the brain and spinal canal, involved in the production and circulation of cerebrospinal fluid (CSF).
PNS Glial Cells:
Schwann Cells: Myelinate single axons in the PNS, aiding in the rapid transmission of signals.
Satellite Cells: Provide structural support and regulate the microenvironment around neuronal cell bodies within ganglia in the PNS.
Neurotransmitter System
Types: The communication in the nervous system employs various types of chemical messengers:
Neurotransmitters: Chemicals that transmit signals across a synapse from one neuron to another.
Neuromodulators: Substances that modify the strength or efficacy of neurotransmission.
Neurohormones: Hormones released by neurons that enter the bloodstream and act on distant targets.
Receptor Types:
Ionotropic Receptors: Fast-acting, channel-linked receptors that open ion channels directly, altering ion flow across the membrane for swift neurotransmission.
Metabotropic Receptors: G-protein coupled receptors that activate intracellular signaling cascades, indirectly affecting neuron excitability and synaptic plasticity.
Neurotransmitter Release: The arrival of an action potential at axon terminals causes voltage-gated calcium channels to open, leading to an influx of Ca2+, which triggers the exocytosis of neurotransmitter-containing vesicles into the synaptic cleft.
Graded Potentials vs Action Potentials
Graded Potentials:
Vary in strength and magnitude depending on the stimulus, can depolarize or hyperpolarize the membrane, and can summate either temporally (over time) or spatially (over space).
Initiated at the dendrites and cell body, serving as the initial signal input to the neuron before reaching the axon hillock.
Action Potentials:
Display an all-or-none response with a uniform amplitude, generated when the accumulated depolarization at the axon hillock reaches a specific threshold.
Cannot summate; each action potential is fully propagated along the axon regardless of the intensity of the stimulus.
Summary
The generation and propagation of action potentials are fundamental for neuronal signaling and communication within the nervous system. This process is influenced by various ionic movements, neuronal structures, and the complex interplay of neurotransmitter systems. A comprehensive understanding of these mechanisms is essential for grasping physiological and neurological concepts, as they form the basis for all nervous system functions and interactions.