Chapter 11: Nervous Tissue

Preliminary Concepts

This chapter introduces the structure and function of nervous tissue, focusing on the cellular and electrical properties that underlie neural signaling.

• Nervous tissue shares similarities with muscle tissue but uses dopamine as a neurotransmitter.

• Key ions: Na+ (sodium) and K+ (potassium).

• The type of chemically gated channel determines the outcome of neural signaling (e.g., depolarization or hyperpolarization).

• Dopamine-gated potassium channels play a role in neural excitability.

Ionic Concentrations and Gradients

Ionic Distributions

• Cations (positively charged): Sodium (Na+), Potassium (K+).

• Anions (negatively charged): Chloride (Cl-), Proteins (–).

• Sodium: Mostly found in extracellular fluid.

• Potassium: Mostly found inside the cell.

• Chloride: More outside than inside the cell.

Creating an Electrical Gradient

• Typical resting membrane potential: ~ -40 mV (more positive charges outside than inside).

• Electrical gradients are established by the distribution of ions across the membrane.

Electrochemical Gradients at Rest

• Potassium and sodium gradients are maintained at rest, contributing to the resting membrane potential.

• Electrochemical gradients drive the movement of ions when channels open.

Essential Concepts in Physiology

Seven core concepts are emphasized throughout physiology:

• Cell Membrane

• Cell-to-Cell Communication

• Interdependence

• Flow-Down Gradients

• Energy

• Structure/Function

• Homeostasis

Each concept should be considered in the context of every physiological system.

Neuroanatomy

Types of Cells in the Nervous System

• Neuroglia (Glial Cells): Support and protect neurons.

• Neurons: The primary signaling cells.

Central Nervous System (CNS)

• Brain and Spinal Cord

• Astrocytes: Manage extracellular potassium, highly organized.

• Microglia: Immune cells, remove debris, can become phagocytic.

• Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths in CNS, insulate axons, increase speed of action potentials.

Peripheral Nervous System (PNS)

• Satellite Cells: Surround neuron cell bodies, regulate environment.

• Schwann Cells: Form myelin sheaths in PNS, each cell myelinates a single axon segment.

Characteristics of Neurons

Functional Units

• Long-lived (up to 100 years), mostly amitotic (do not divide after maturation).

• High metabolic rate, require continuous oxygen and glucose.

Basic Structure

• Plasma Membrane

• Dendrites: Receive signals.

• Cell Body (Soma): Contains nucleus and organelles.

• Axon Hillock: Initiates action potentials.

• Axon: Conducts impulses away from cell body.

• Axon Collaterals: Branches of axon.

• Myelin Sheath: Insulates axon, increases speed of conduction.

• Nodes of Ranvier: Gaps in myelin sheath, facilitate saltatory conduction.

Resting Membrane Potential (RMP)

• Similar to skeletal muscle.

• Maintained by ion gradients and selective permeability.

• Typical value: -70 mV.

Information Flow Through Neurons

• Dendrites: Collect incoming signals.

• Cell Body: Integrates signals.

• Axon: Transmits outgoing signals to other neurons or effectors.

Synaptic Transmission

• Presynaptic neuron: Sends signal.

• Postsynaptic neuron: Receives signal.

• EPSP (Excitatory Postsynaptic Potential): Increases likelihood of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Decreases likelihood of action potential.

Axonal Transport

• Microtubules: Provide tracks for movement of materials.

• Neurofilaments: Structural support.

• Axonal Transport: Bidirectional movement of organelles and proteins.

• Anterograde: Away from cell body (by kinesin).

• Retrograde: Toward cell body (by dynein).

Clinical Note: Some viruses (e.g., rabies) and toxins hijack axonal transport mechanisms.

Structural Classes of Neurons

Functional Regions of Neurons

• Receptive region: Receives stimuli (dendrites, cell body).

• Conducting region: Generates and transmits action potentials (axon).

• Secretory region: Releases neurotransmitters (axon terminals).

Abundance and Variation

• Multipolar neurons are the most abundant in the CNS.

• Bipolar neurons are rare, found in special senses (e.g., retina, olfactory epithelium).

• Unipolar neurons are common in the PNS (sensory neurons).

Direction of Impulse Conduction

• Multipolar: Conducts action potentials from brain to body (efferent).

• Bipolar: Afferent, sends sensory information up the chain of command.

Demyelination Disorders

• Multiple Sclerosis (MS): CNS demyelination, symptoms come and go, autoimmune.

• Guillain-Barré Syndrome (GBS): PNS demyelination, often follows viral infection, usually acute and reversible.

Functions of the Nervous System

• Sensation: Sensory input via afferent neurons.

• Integration: Processing and interpretation of sensory input.

• Response: Motor output via efferent neurons.

Principles of Electricity in Neurons

• More positive charges outside the cell than inside at rest.

• Energy is required to separate opposite charges across a membrane.

• When charges move toward each other, energy is released.

Key Electrical Terms

• Voltage (V): Potential difference between two points. In neurons, measured in millivolts (mV).

• Current (I): Flow of electrical charge between two points.

• Resistance (R): Hindrance to charge flow (e.g., myelin increases resistance).

Ohm's Law

• Relationship:

• Current is directly proportional to voltage and inversely proportional to resistance.

Coulomb's Law

• Like charges repel, opposite charges attract.

Excitable Membranes

• Neurons have leakage Na+ channels with inactivation gates, contributing to resting membrane potential and excitability.

Additional info: The notes reference several graphics and tables; students are encouraged to review these for visual reinforcement of concepts such as neuron structure, axonal transport, and demyelination disorders.