Fundamentals of the Nervous System and Nervous Tissue

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Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It utilizes both electrical and chemical signals to coordinate rapid and specific responses to internal and external stimuli.

Communication: Cells communicate via electrical and chemical signals.
Functions: Sensory input, integration, and motor output.
Immediate Responses: Nervous system enables quick, targeted actions.

Functions of the Nervous System

Sensory Input: Information gathered by sensory receptors about internal and external changes.
Integration: Processing and interpretation of sensory input.
Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Organization of the Nervous System

Central and Peripheral Divisions

The nervous system is divided into two principal parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS):
- Consists of the brain and spinal cord.
- Integrates and controls sensory input and motor output.
Peripheral Nervous System (PNS):
- Consists of nerves that extend from the brain and spinal cord.
- Spinal nerves and cranial nerves link the CNS to the rest of the body.
- Divided into sensory (afferent) and motor (efferent) divisions.

Motor Division Subdivisions

Somatic Nervous System: Voluntary control of skeletal muscles.
Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into:
- Sympathetic: Mobilizes body systems during activity.
- Parasympathetic: Conserves energy, promotes housekeeping functions.

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue consists of two main types of cells: neurons and neuroglia (glial cells).

Neurons: Excitable cells that transmit electrical signals.
Neuroglia: Supporting cells that surround and support neurons.

Neuroglia of the Central Nervous System

Four main types of neuroglia support CNS neurons:

Astrocytes: Most abundant, versatile, and highly branched glial cells.
- Cling to neurons, synaptic endings, and capillaries.
- Functions: support and brace neurons, facilitate exchanges between capillaries and neurons, guide migration, influence neuronal functioning.
Microglial Cells: Small, ovoid cells with thorny processes that monitor neurons and act as phagocytes.
Ependymal Cells: Line the central cavities of the brain and spinal cord; may be ciliated to circulate cerebrospinal fluid (CSF).
Oligodendrocytes: Branched cells that form myelin sheaths in the CNS.

Neuroglia of the Peripheral Nervous System

Satellite Cells: Surround neuron cell bodies in the PNS; function similar to astrocytes.
Schwann Cells: Surround peripheral nerve fibers and form myelin sheaths; vital for regeneration of damaged peripheral nerve fibers.

Neurons

General Characteristics

Neurons are the structural units of the nervous system, specialized to conduct impulses.

Longevity: Can function for a person's lifetime.
Amitotic: Most do not divide after maturation.
High Metabolic Rate: Require continuous supply of oxygen and glucose.
All have a cell body and one or more processes.

Neuron Cell Body

Biosynthetic Center: Synthesizes proteins, membranes, and chemicals.
Nucleus: Contains the neuron's genetic material.
Location: Most neuron cell bodies are in the CNS (nuclei); clusters in the PNS are called ganglia.

Neuron Processes: Dendrites and Axons

Dendrites: Receptive regions; convey incoming messages toward the cell body as graded potentials.
Axons: Conducting region; generates and transmits nerve impulses away from the cell body.
- Each neuron has one axon, which may branch extensively.
- Axon terminals are the secretory region, releasing neurotransmitters.
Tracts: Bundles of neuron processes in the CNS.
Nerves: Bundles of neuron processes in the PNS.

Neuron Processes: Myelin Sheaths

Myelination in the PNS

Schwann Cells: Form myelin sheaths; one cell forms one segment of myelin sheath.
Nodes of Ranvier: Gaps between adjacent Schwann cells.
Nonmyelinated Fibers: Schwann cells surround but do not coil around axons.

Myelination in the CNS

Oligodendrocytes: Form myelin sheaths; each cell can myelinate multiple axons.
White Matter: Regions of the brain and spinal cord with dense collections of myelinated fibers.
Gray Matter: Mostly neuron cell bodies and nonmyelinated fibers.

Classification of Neurons

Structural Classification

Multipolar: Three or more processes (one axon, others dendrites); most common type.
Bipolar: Two processes (one axon, one dendrite); rare, found in sensory organs.
Unipolar: Single short process; divides into peripheral and central processes.

Functional Classification

Sensory (Afferent): Transmit impulses from sensory receptors toward the CNS.
Motor (Efferent): Carry impulses from the CNS to effectors.
Interneurons: Shuttle signals through CNS pathways; most are multipolar.

Principles of Electricity in Neurons

Basic Electrical Concepts

Voltage (Potential Difference): Charge differences across the plasma membrane create potential energy.
Current: Flow of electrical charge between two points.
Resistance: Hindrance to charge flow.

Ohm's Law:

Relationship between voltage, current, and resistance:

I: Current (amperes)
V: Voltage (volts)
R: Resistance (ohms)

Role of Membrane Channels

Types of Membrane Channels

Leakage Channels: Always open; allow ions to pass through continuously.
Gated Channels: Open and close in response to specific signals.
- Chemically Gated: Open with binding of a specific chemical.
- Voltage Gated: Open and close in response to changes in membrane potential.
- Mechanically Gated: Open in response to physical deformation of the receptor.

When gated channels are open, ions diffuse quickly across the membrane, creating electrical currents and voltage changes.

Ohm's Law equation:

Generating a Resting Potential

Mechanisms of Resting Membrane Potential

Generated by differences in ionic composition of intracellular fluid (ICF) and extracellular fluid (ECF).
Differences in plasma membrane permeability to ions.
Sodium-Potassium Pump: Maintains resting potential by pumping 3 Na+ out and 2 K+ in.

Resting membrane potential: Typically -70 mV in neurons.

Changing the Resting Potential

Membrane Potential Changes

Occurs when concentrations of ions across the membrane change.
Membrane permeability to ions changes.
Signals are classified as graded potentials (short distances) or action potentials (long distances).

Depolarization: Inside of membrane becomes less negative.

Hyperpolarization: Inside of membrane becomes more negative.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential, triggered by stimulus strength and location.

Can be depolarizations or hyperpolarizations.
Current flows but dissipates quickly and decays with distance.

Action Potentials

Mechanism and Properties

Principal way neurons send signals; occur only in muscle cells and axons of neurons.
Brief reversal of membrane potential with a change in voltage of ~100 mV.
Do not decay with distance; all-or-none phenomenon.
Involves opening and closing of voltage-gated channels.

Action Potential Phases:

Resting state: All gated Na+ and K+ channels are closed.
Depolarization: Na+ channels open, Na+ enters the cell.
Repolarization: Na+ channels inactivate, K+ channels open, K+ exits the cell.
Hyperpolarization: Some K+ channels remain open, Na+ channels reset.

Voltage-gated Na+ channels: Have two gates (activation and inactivation) and alternate between closed and open states.

Summary Table: Neuroglia of the Nervous System

Type	Location	Main Functions
Astrocytes	CNS	Support neurons, regulate exchanges, guide migration, influence synaptic transmission
Microglial Cells	CNS	Monitor neurons, phagocytosis of debris and microorganisms
Ependymal Cells	CNS	Line cavities, circulate cerebrospinal fluid
Oligodendrocytes	CNS	Form myelin sheaths
Satellite Cells	PNS	Surround neuron cell bodies, support function
Schwann Cells	PNS	Form myelin sheaths, aid regeneration

Summary Table: Structural Classification of Neurons

Type	Number of Processes	Location/Function
Multipolar	Three or more	Most common; major type in CNS
Bipolar	Two	Rare; found in sensory organs (e.g., retina)
Unipolar	One (divides)	Mainly in PNS; sensory neurons

Summary Table: Functional Classification of Neurons

Type	Direction of Impulse	Location
Sensory (Afferent)	Toward CNS	Cell bodies in ganglia (PNS)
Motor (Efferent)	Away from CNS	Cell bodies in CNS
Interneurons	Within CNS	Between sensory and motor neurons; 99% of neurons