Basic Structure and Functions of the Human Nervous System

Nervous System Regions

  • The nervous system is divided into 2 main regions:

    • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS):

    • Encompasses everything outside of the CNS, which includes:

      • Peripheral nerves

      • Ganglia

      • Plexuses

Nervous Tissue Cells

  • Nervous tissue contains 2 basic types of cells:

    • Neuron:

    • Functional cells of the nervous system only c’stimulas ell that carry all information.

    • Components of a neuron include:

      • Soma (cell body):

      • Contains the nucleus and organelles.

      • Axon:

      • Extends out of the soma like a long wire that carries outgoing signals to other neurons/cells.

      • Dendrites:

      • Extend out of the soma like short wires that receive incoming signals from other neurons/cells.

    • Glial cells (glia):

    • Generic term for all the different support cells that keep the nervous system functioning properly.

Gray Matter vs White Matter

  • Gray matter:

    • Regions dominated by cell bodies and dendrites (information processing).

    • Key terms:

    • Nucleus:

      • A cluster of cell bodies in the CNS.

    • Ganglion:

      • A cluster of cell bodies in the PNS.

  • White matter:

    • Regions dominated by axons (information traveling through like wires).

    • Key terms:

    • Tract:

      • A bundle of axons in the CNS.

    • Nerve:

      • A bundle of axons in the PNS.

    • Plexus:

      • A bundle of nerves in the PNS.

Functional Divisions of the Nervous System

  • Sensation:

    • Receiving information about the environment, both inside and outside of the body.

    • Stimulus:

    • An event in the environment that is detected by the nervous system.

    • Main Senses for External Stimuli:

    • Sight

    • Smell

    • Hearing

    • Taste

    • Touch

    • There are additional senses for internal stimuli.

  • Response:

    • Motor functions (actions carried out by the body), such as:

    • Movement

    • Heart beating

    • Glandular secretions

    • Breathing

  • Integration:

    • The processing of sensory information to generate appropriate motor responses, such as:

    • Increasing heart rate for exercise

    • Dodging an incoming ball

    • Vomiting to remove ingested toxins.

Controlling the Body

  • Somatic Nervous System (SNS):

    • Involves conscious perception and voluntary motor responses, such as:

    • Using skeletal muscles to voluntarily move body parts.

    • Reflexes involving muscle contractions, e.g., pulling away from a hot stove.

  • Autonomic Nervous System (ANS):

    • Involves involuntary control of the body to maintain homeostasis or automatically respond to the environment. Examples include:

    • Adjusting heart rate and breathing rate for different activities.

    • Sweating or shivering to regulate body temperature.

    • Enteric Nervous System (ENS):

    • A subdivision of the ANS that controls gastrointestinal glands and muscles for digestion.

Neurons

  • Neurons:

    • Responsible for computation and communication within the nervous system.

    • Are electrically active and release chemical signals (neurotransmitters) to target cells.

    • Signals are transmitted across synapses (gaps between neurons).

    • The human nervous system contains trillions of neurons.

The Axon

  • Axon hillock:

    • The point where the soma tapers to form the axon.

  • Axoplasm:

    • The specialized cytoplasm of the axon hillock.

  • Trigger zone:

    • The area where action potentials are initiated in the initial segment of the axon.

  • Myelin:

    • An insulating layer that speeds up signal transmission along the axon.

  • Nodes of Ranvier:

    • Gaps in the myelin sheath that enable rapid transmission (the signal appears to jump from node to node).

  • Axon terminal:

    • The end of the axon, containing:

    • Synaptic end bulbs:

      • Bulges at the end of the terminal branches connecting to target cells at synapses.

Types of Neurons

stimulus 

  • Unipolar Neurons:

    • Often referred to as “pseudo-unipolar” neurons in humans.

    • Sensory neurons only.

    • Cell bodies are always located in ganglia.

    • Have a single process that branches, with dendrites on one end receiving a signal and sending it directly along the axon on the other end.

  • Bipolar Neurons:

    • Feature 2 processes: one axon and one dendrite.

    • Rare in humans, found only in:

    • Olfactory epithelium (responsible for smell).

    • Retina (responsible for sight).

  • Multipolar Neurons:

    • Characterized by one axon and two or more (often many more) dendrites.

    • The most common type of neuron in the human nervous system.

Glial Cells of the CNS

  • Astrocyte:

    • Star-shaped cells with multiple processes that interact with:

    • Neurons

    • Blood vessels

    • Connective tissue surrounding the brain.

    • Functions include:

    • Maintaining ion concentrations in the extracellular space.

    • Removing excess neurotransmitters.

    • Reacting to tissue damage.

    • Contributing to the blood-brain barrier.

      • Blood-brain barrier (BBB):

      • A protective barrier that blocks most substances in the blood from crossing into the CNS and brain.

  • Oligodendrocytes:

    • Cells that create the myelin sheath around axons in the CNS.

  • Microglia:

    • Small cells that act like macrophages, removing diseased or damaged cells in the CNS.

  • Ependymal Cells:

    • A single layer of cells lining the BBB that filter blood to produce cerebrospinal fluid (CSF).

    • Cerebrospinal Fluid (CSF):

    • A filtered version of blood that exchanges gases and nutrients with extracellular tissues of the CNS.

Glial Cells of the PNS

  • Satellite Cells:

    • Located in sensory and autonomic ganglia.

    • Function as the PNS equivalent of astrocytes, maintaining ion concentrations, removing excess neurotransmitters, and reacting to tissue damage.

  • Schwann Cells:

    • Responsible for creating the myelin sheath in the PNS.

    • Each Schwann cell is involved in the myelin sheath around a single segment of the axon, unlike oligodendrocytes, which myelinate multiple segments simultaneously.

Learning Intent

  • To explore excitable membranes and the types of ion channels that contribute to their excitability.

Do Now Activity

  • Explanation of an excitable membrane:

    • An excitable membrane can generate electrical signals due to the movement of ions across it.

    • This ability is crucial for neuron function, enabling communication in the nervous system.

  • Function of ion channels:

    • Ion channels facilitate the passage of ions across the cell membrane, crucial for generating membrane potentials and action potentials.

Excitable Membranes

  • Definition:

    • Neurons possess an excitable cell membrane capable of producing electricity by ion movement, creating a charge difference (membrane potential).

  • Membrane Potential:

    • Measurement of the electrical charge inside the membrane relative to the outside at a specific location.

    • Resting Membrane Potential (RMP):

    • The electrical charge of a neuron at rest, typically around −70mV−70mV.

Mechanisms of Ion Movement

  • Ions and Membrane Permeability:

    • Ions cannot pass freely through the cell membrane and depend on ion channels being opened to traverse along their concentration gradients.

  • Sodium/Potassium Pump:

    • An active transport mechanism that utilizes ATPATP to maintain ion concentrations:

    • Sodium (Na+Na+) remains higher outside the cell.

    • Potassium (K+K+) stays higher inside the cell when ion channels are closed.

    • This mechanism is crucial for preserving the resting membrane potential of neurons.

Ion Channels

  • Types of Ion Channels:

    • Electrochemical Exclusion:

    • Channels can selectively allow either positive (cations) or negative ions (anions) through, but not both.

    • Size Exclusion:

    • Some channels restrict passage based on the size of the ions or molecules.

    • Nonspecific Channels:

    • These channels are selective for charge but not for size.

    • Gated Channels:

    • Require a specific signal (ligand, voltage change, or mechanical force) to open.

Gated Ion Channels

  • Types of Gated Ion Channels:

    • Ligand-gated (Ionotropic) Channels:

    • Open when a signaling molecule (ligand) binds to the channel (e.g., neurotransmitters, hormones).

    • Mechanically Gated Channels:

    • Open in response to physical distortion of the membrane (e.g., pressure or stretching).

    • Voltage-gated Channels:

    • Open in response to changes in membrane voltage; these play a crucial role in action potential generation.

    • Leakage Channels:

    • These channels randomly open and close, contributing to maintenance of the resting membrane potential.

Action Potential

  • Phases of Action Potential:

    1. Depolarization:

    • Triggered by Na+Na+ channels opening due to a stimulus, allowing Na+Na+ to rush into the cell along its concentration gradient.

    • This results in the interior of the membrane becoming more positive.

    • Na+Na+ channels close at peak depolarization (+30mV+30mV).

    1. Repolarization:

    • Following peak depolarization, K+K+ channels open, allowing K+K+ to exit the cell following its concentration gradient, restoring a negative charge.

    1. Hyperpolarization:

    • K+K+ channels close slowly, causing temporary overshooting of the resting membrane potential to below −70mV−70mV.

    1. Recovery:

    • After the action potential, both Na+Na+ and K+K+ channels close; the Na+/K+Na+/K+ pump restores the resting membrane potential to −70mV−70mV.

  • Threshold Voltage:

    • The critical membrane potential (−55mV−55mV) that must be reached for an action potential to occur.

    • At threshold, a large number of voltage-gated Na+Na+ channels open, leading to action potential generation.

  • All-or-none Principle:

    • Action potentials occur fully or not at all; once the threshold is reached, the action potential will proceed to completion.

  • Refractory Periods:

    • Absolute Refractory Period:

    • No new action potential can be generated as Na+Na+ channels remain inactive until the membrane potential drops below −55mV−55mV.

    • Relative Refractory Period:

    • Another action potential can be initiated but requires a stimulus stronger than usual until the membrane is stabilized back at −70mV−70mV.

Local Potentials

  • Characteristics:

    • Local potentials can trigger an action potential if strong enough:

    • Graded: The more Na+Na+ channels activated, the stronger the potential.

    • Decremental: Local potentials decrease in strength as they spread from the initial site.

    • Reversible: If the threshold is not reached, they do not generate an action potential.

Propagation of the Action Potential

  • Initiation and Propagation:

    • Action potentials begin at the axon hillock, where there is a high density of voltage-gated Na+Na+ channels.

    • When depolarization spreads, more voltage-gated Na+Na+ channels open, propagating the action potential down the axon.

  • Direction of Propagation:

    • Action potentials move forward only due to the refractory period preventing backward triggering.

  • Conduction Types:

    • Continuous Conduction:

    • Occurs in unmyelinated neurons; slower as each segment has Na+Na+ channels opening sequentially.

    • Saltatory Conduction:

    • Happens in myelinated neurons; the signal jumps between nodes of Ranvier, speeding up conduction without opening every Na+Na+ channel in the membrane.