Neurons

Neurons and Electrophysiology Overview

Functions of the Nervous System

  1. Sensory Input: Information is gathered by sensory receptors regarding changes in both internal conditions (like temperature and pH levels) and external stimuli (like light, sound, and touch). Sensory neurons transmit this information to the CNS for processing.

  2. Integration: The CNS analyzes and interprets the incoming sensory input, determining an appropriate response. This can involve complex processes such as memory recall, decision-making, and emotional response.

  3. Motor Output: Once the response is integrated, the CNS activates effector organs (such as skeletal muscles or glands) through motor neurons, leading to a physical reaction or behavioral response.

Major Divisions of the Nervous System

  • Central Nervous System (CNS): Comprises the brain and spinal cord, acting as the control center for processing and integrating information. The brain is responsible for cognitive functions, emotional regulation, and coordination of voluntary movements while the spinal cord serves as a pathway for signals between the brain and the body.

  • Peripheral Nervous System (PNS): Consists of all the nerves outside the CNS that connect the CNS to the body’s muscles, glands, sense organs, and other tissues. It is further divided into the somatic and autonomic nervous systems, with the former controlling voluntary motor functions and the latter regulating involuntary bodily functions.

Neuron and Glial Cells

  • Neuron: The fundamental unit of the nervous system that generates electrical signals known as action potentials or nerve impulses. Neurons are classified based on their function and structure into sensory, motor, and interneurons.

  • Glial Cells: Non-neuronal cells in the nervous system that provide support and protection for neurons. They do not generate nerve impulses but are crucial for transmitting nutrients, maintaining homeostasis, and supporting overall neural health.

Types of Glial Cells in the CNS

  1. Astrocytes: Star-shaped cells that regulate the extracellular fluid composition and maintain the blood-brain barrier, facilitating nutrient transport and waste removal.

  2. Microglia: The immune cells of the CNS, responsible for monitoring the health of neurons and responding to injury or disease through phagocytosis and inflammatory responses.

  3. Ependymal Cells: Line the brain's ventricles and central canal of the spinal cord, involved in the production and circulation of cerebrospinal fluid (CSF).

  4. Oligodendrocytes: Specialized cells that form the myelin sheath, which insulates axons in the CNS, thus speeding up electrical signal transmission between neurons.

Structure of a Neuron

  • Cell Body (Soma): Contains the nucleus and ribosomes, vital for producing proteins necessary for neuron function and maintaining cellular health.

  • Dendrites: Branched extensions that receive chemical signals (neurotransmitters) from other neurons, converting them into electrical impulses that are sent to the cell body.

  • Axon: Long, thin structure that conducts outgoing signals away from the cell body to target cells (like other neurons, muscles, or glands).

Axon Components

  • Initial Segment (Axon Hillock): The region where action potentials are generated; it integrates incoming signals and determines if the threshold for firing an action potential is reached.

  • Axon Terminal (Synaptic Knob): The endpoint of the axon where neurotransmitters are released into the synaptic cleft to affect neighboring neurons.

Axonal Transport

  • Axonal Transport: The process of moving materials between the neuron’s cell body and its axon terminals, essential for maintaining a neuron's health and functionality. This transport is facilitated by motor proteins that move along the axon’s cytoskeleton.

Types of Motor Proteins

  • Kinesins: Motor proteins that transport materials from the cell body to the axon terminals (anterograde transport).

  • Dyneins: Motor proteins that transport materials back from the axon terminals to the cell body (retrograde transport), potentially carrying signaling molecules or pathogens and waste materials.

Neuron Classes

  • Afferent Neurons: Transmit sensory information from peripheral receptors into the CNS; they have a unique structure with a single process that bifurcates into a peripheral and a central process.

  • Efferent Neurons: Carry motor commands from the CNS to effector cells such as muscles and glands, facilitating movement and glandular activity.

  • Interneurons: Located primarily within the CNS, these neurons integrate signals from multiple sources and account for over 99% of neurons in the human body. They play crucial roles in reflexes, network signaling, and complex processing tasks.

Neurons Development

Neural development starts with neural stem cells differentiating into neurons or glia. During development, neurons extend axons towards their targets, a process guided by neurotrophic factors. Environmental factors, including exposure to alcohol and certain viruses, can interfere with neuronal maturation, leading to developmental disorders.

Resting Membrane Potential

  • Resting Membrane Potential: The electrical potential difference across a neuron's plasma membrane when it is not actively firing, typically ranging from -40 mV to -90 mV. This difference is crucial for the neuron's ability to generate action potentials.

  • Ion Distribution: At rest, the inside of the neuron is negatively charged compared to the outside due to an abundance of negatively charged anions and a lower concentration of positively charged ions.

Ion Concentration Details

  • Intracellular Concentrations: High levels of potassium ions (K+) and organic phosphates, which contribute to the negative charge inside the cell.

  • Extracellular Concentrations: High levels of sodium ions (Na+) and chloride ions (Cl-), primarily present outside the neuron, creating an electrochemical gradient essential for action potential generation.

Membrane Potential Terminology

  • Depolarization: The process of the membrane potential becoming less negative, moving toward zero.

  • Overshoot: A point during depolarization when the inside of the neuron becomes more positive than the outside.

  • Repolarization: The return to resting potential following depolarization, restoring the negative charge inside the cell.

  • Hyperpolarization: A state in which the membrane potential becomes more negative than the resting potential.

Glossary of Potential Terms

  • Potential Difference: The voltage difference between two points due to separated charges, influencing the flow of current in neurons.

  • Graded Potential: A local change in membrane potential that varies in amplitude and does not reach a threshold.

  • Action Potential: A rapid, transient depolarization of the membrane that occurs when the threshold potential is reached, propagating along the axon.

  • Synaptic Potential: A graded potential occurring in the postsynaptic neuron as a result of neurotransmitter binding.

  • Threshold Potential: The critical level of membrane depolarization needed to trigger an action potential, typically around -55 mV in many neurons.