Anatomy & Physiology: The Nervous System – Structure, Function, and Neural Communication

System

Nervous System Overview

The nervous system is responsible for integrating and coordinating all human capabilities, including perception, thought, learning, memory, and action. It is composed of specialized cells and structures that enable rapid communication throughout the body.

• Nervous System Function: Enables perception, thinking, learning, remembering, and acting through integrated activity of nervous system cells.

• Myasthenia Gravis: An autoimmune disease where the immune system attacks proteins in the nervous system, disrupting cell communication.

• Symptoms and Diagnosis: Symptoms include fatigue and muscle weakness. Diagnosis involves understanding the structure and function of nervous system cells.

• Treatment: Treatments are available, but research is ongoing to improve patient quality of life.

• Cell Communication: Understanding how nervous system cells communicate is key to understanding nervous system function and disease.

Cells of the Nervous System

General Features

• The human brain contains approximately 86 billion neurons.

• There is a similar number of non-neuronal cells in the brain.

• The nervous system is divided into two basic divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

LO 2.1 Contrast features of the central and peripheral nervous systems.

Central vs. Peripheral Nervous System

The nervous system is divided into the CNS and PNS, each with distinct structures and functions.

• Central Nervous System (CNS): Comprises the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes nerves and sensory organs outside the CNS.

• Peripheral Nerves: Bundles of axons and glial cells wrapped in a protective membrane, similar to electrical cables with many wires.

• Sensory Neurons: Specialized cells in the PNS that gather information from the environment (light, sound, odors, contact, taste).

• Motor Neurons: Control muscle contractions and are part of the PNS.

• Interneurons: Located entirely within the CNS, they connect sensory and motor neurons.

Types of Neuronal Circuits

• Local Interneurons: Form circuits with nearby neurons to analyze small pieces of information.

• Relay Interneurons: Connect circuits of local interneurons in different brain regions, facilitating complex tasks like learning, remembering, and decision-making.

Supporting Cells

• Neurons and Supporting Cells: Essential components of the nervous system.

• Blood–Brain Barrier: Chemically isolates cells in the CNS from the rest of the body.

LO 2.2 Distinguish among the structures of a neuron.

Neuron Structure

Neurons are specialized cells for communication, with unique structures for receiving and transmitting information.

• Soma (Cell Body): Contains the nucleus and machinery for cell life processes.

• Dendrites: Tree-like structures that receive messages from other neurons across synapses; function like antennae.

• Axon: Long, thin tube that carries information from the soma to terminal buttons via electrical signals (action potentials). Axons transport materials within the neuron and can be covered with myelin for faster conduction.

• Terminal Buttons: Knobs at the end of axons that release neurotransmitters to communicate with other neurons.

• Cell Membrane: Defines the neuron’s boundary, controls substance entry/exit, and provides structural information.

• Cytoskeleton: Protein framework giving the neuron its shape, with microtubules transporting materials within the cell.

• Organelles: Structures inside the neuron, including mitochondria, ribosomes, and nucleus.

• Nucleus: Contains chromosomes with DNA, which holds the recipes for protein synthesis.

• Mitochondria: Power plants of the neuron, producing ATP from nutrients to supply energy for cellular functions.

LO 2.3 Compare supporting cells in the central and peripheral nervous systems.

Supporting Cells (Glia)

Supporting cells, or glia, are crucial for neuron survival and function. They provide physical and chemical support, regulate the environment, and assist in growth, repair, and synaptic communication.

• Astrocytes:

  • Provide physical and chemical support to neurons in the brain.

  • Control the chemical environment around neurons by regulating substance movement.

  • Surround and isolate synapses to limit neurotransmitter dispersion.

  • Perform phagocytosis to clear dead neurons and form scar tissue in damaged areas.

• Oligodendrocytes:

  • Support axons and produce the myelin sheath in the CNS.

  • Myelin is composed of 80% lipid and 20% protein, forming segmented tubes around axons.

• Microglia:

  • Small cells acting as phagocytes to remove dead neurons.

  • Part of the brain’s immune system, protecting against microorganisms and involved in inflammatory responses.

• Schwann Cells (PNS):

  • Perform similar functions to oligodendrocytes but in the peripheral nervous system.

  • Each Schwann cell myelinates a single axon segment and aids in regeneration by guiding regrowth of axons after injury.

Differences in Regeneration

• Schwann cells in the PNS facilitate axon regrowth and reconnection with targets after injury.

• In the CNS, astrocytes form scar tissue that inhibits axon regrowth, and lack the guiding function of Schwann cells.

Multiple Sclerosis

• An autoimmune disorder where the immune system attacks myelin produced by oligodendrocytes in the CNS.

• The chemical composition of myelin differs between the CNS and PNS, influencing disease pathology.

LO 2.4 Assess the function of the blood–brain barrier.

Blood–Brain Barrier (BBB)

The blood–brain barrier is a selective barrier that protects the brain from harmful substances in the blood, maintaining the precise environment needed for neural function.

• Function: Regulates the composition of the extracellular fluid surrounding neurons, crucial for proper brain function. Also protects the brain from harmful chemicals.

• Selective Permeability: The BBB allows some substances to cross into the CNS while blocking others. Capillaries in the CNS lack gaps, unlike those in the rest of the body.

• Active Transport: Substances can move into the brain via active transport, catalyzed by proteins in the capillary walls.

• Non-uniformity: The BBB is more permeable in certain areas, such as the area postrema, which controls vomiting and allows detection of toxins in the blood.

Communication Within a Neuron

Electrical Signaling

• Neurons communicate through electrical signals: Establish a membrane potential and generate an action potential.

• Action potential initiation: Begins at the axon hillock and travels along the axon to the terminal buttons.

LO 2.5 Explain the process of neural communication in a reflex.

Neural Communication in Reflexes

• Sensory neurons: Detect painful stimuli and send messages via axons to terminal buttons in the spinal cord.

• Terminal buttons: Release neurotransmitters that excite interneurons, which then excite motor neurons.

• Motor neurons: Release neurotransmitters that excite motor neurons, causing muscle contractions and withdrawal from the painful stimulus.

Inhibitory Synapses and Reflex Control

• Inhibitory interneurons release neurotransmitters that decrease motor neuron activity, blocking the withdrawal reflex.

• Reflexes involve thousands of neurons, including sensory neurons, interneurons, and motor neurons.

LO 2.6 Describe membrane potential, resting potential, hyperpolarization, depolarization, and the action potential.

Membrane Potentials

• Resting Potential: The axon at rest is negatively charged inside, typically around –70 mV.

• Membrane Potential: The difference in electrical charge across the membrane, representing stored electrical energy.

• Hyperpolarization: The inside of the axon becomes more negative relative to the outside, making it less likely to send an electrical message.

• Depolarization: The inside of the axon becomes more positive relative to the outside, making it more likely to send an electrical message.

• Threshold of Excitation: A specific point of depolarization needed to trigger an action potential.

LO 2.7 Summarize how diffusion, electrostatic pressure, and the sodium–potassium pump help establish membrane potential.

Establishing Membrane Potential

• Diffusion: Molecules move from regions of high concentration to low concentration due to random motion.

• Electrostatic Pressure: Ions with like charges repel and opposite charges attract, influencing ion movement across the membrane.

• Intracellular and Extracellular Fluids:

  • Intracellular fluid: Contains organic anions (A–) and potassium ions (K+).

  • Extracellular fluid: Contains sodium ions (Na+) and chloride ions (Cl–).

• Ion Distribution:

  • Organic anions (A–) are trapped inside the cell due to membrane impermeability.

  • Potassium ions (K+) are pushed out by diffusion but pulled in by electrostatic pressure.

  • Chloride ions (Cl–) are pushed in by diffusion but pushed out by electrostatic pressure.

  • Sodium ions (Na+) are pushed in by both diffusion and electrostatic pressure.

• Sodium–Potassium Pump: Exchanges three Na+ ions out for every two K+ ions in, using ATP for energy. Maintains low Na+ and high K+ inside the cell, contributing to the resting membrane potential.

Equation for the Nernst Potential:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

LO 2.8 Summarize the series of ion movements during the action potential.

Action Potential Phases

• Initial Phase: Brief increase in membrane permeability to Na+ allows these ions to enter the cell.

• Subsequent Phase: Followed by an increase in permeability to K+, allowing these ions to exit the cell.

• Depolarization: Na+ influx causes the inside of the cell to become more positive.

• Repolarization: K+ efflux restores the negative membrane potential.

• Hyperpolarization: Membrane potential temporarily becomes more negative than the resting potential before returning to baseline.

Action Potential Graph: The action potential is an all-or-none event, with a rapid rise and fall in membrane potential.

LO 2.9 Describe conduction of the action potential.

Action Potential Conduction

• All-or-None Law: Action potentials occur fully or not at all; their size and speed are constant along the axon.

• Saltatory Conduction: In myelinated axons, action potentials jump from node to node (nodes of Ranvier), increasing conduction speed and efficiency.

• Unmyelinated Axons: Action potentials propagate continuously along the axon.

Additional info:

Some details, such as the Nernst equation and the specific roles of glial cells, have been expanded for academic completeness and clarity.