Nervous System & Nervous Tissue: Organization, Cells, and Structure (Chapter 11 Study Notes)

Nervous System & Nervous Tissue

Overview and Organization

The nervous system is a complex network responsible for internal communication, control, and integration throughout the body. It is divided into two main anatomical divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integrative and control center.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves; connects the CNS to the rest of the body.

• Cranial nerves: Carry signals to and from the brain.

• Spinal nerves: Carry signals to and from the spinal cord.

Functional Organization

The nervous system performs three essential functions: sensory input, integration, and motor output.

• Sensory input: Sensory receptors detect internal and external changes and send information to the CNS.

• Integration: The CNS processes and interprets sensory information.

• Motor output: The CNS sends signals to effector organs (muscles and glands) to elicit a response.

Divisions of the Peripheral Nervous System

• Sensory (afferent) division: Conducts impulses from receptors to the CNS.

  • Somatic sensory fibers: Carry impulses from skin, skeletal muscles, and joints.

  • Visceral sensory fibers: Carry impulses from internal organs (viscera).

• Motor (efferent) division: Conducts impulses from the CNS to effector organs.

  • Somatic nervous system: Voluntary control of skeletal muscles.

  • Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

    • Sympathetic division: Increases activity in the body ("fight or flight").

    • Parasympathetic division: Decreases activity in the body ("rest and digest").

Nervous Tissue

Types of Tissue

Nervous tissue is one of the four major tissue types in the body, specialized for internal communication and control.

• Major function: Control and internal communication.

• Location: Brain, spinal cord, nerves.

Cell Types in Nervous Tissue

Nervous tissue contains two main types of specialized cells:

• Neurons (nerve cells): Excitable cells that transmit electrical impulses (action potentials).

• Neuroglia (glial cells): Small cells that support, insulate, and protect neurons.

  • Schwann cells: Example of neuroglia in the PNS.

Neuroglia (Glial Cells)

Neuroglia of the CNS

There are four main types of neuroglia in the CNS:

• Astrocytes: Abundant, versatile, and highly branched; cling to neurons, synaptic endings, and capillaries.

• Microglial cells: Thorny processes touch and monitor neurons; migrate toward injured neurons and can phagocytize microorganisms and debris.

• Ependymal cells: Often ciliated; cilia beat to circulate cerebrospinal fluid (CSF); line central cavities (ventricles) in CNS filled with CSF.

• Oligodendrocytes: Branched; processes wrap CNS nerve fibers, forming myelin sheaths.

Neuroglia of the PNS

• Satellite cells: Surround neuron cell bodies in the PNS.

• Schwann cells: Surround neuron axons and form myelin sheaths; vital for regeneration of damaged peripheral nerve fibers.

Neurons

General Structure and Function

Neurons are large, specialized cells that send electrical impulses (action potentials) and are characterized by:

• Cell body (soma): Contains nucleus and organelles; site of protein synthesis.

• Processes: Extensions from the cell body, including dendrites and axons.

• Special characteristics: Extreme longevity, amitotic (do not divide), high metabolic rate (require oxygen and glucose).

Neuron Cell Body

• Also called: Perikaryon or soma.

• Functions: Makes proteins via rough ER (chromatophilic substance or Nissl bodies); part of the receptive region for signals from other neurons.

• Nuclei: Clusters of neuron cell bodies in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

Neuron Processes

• Dendrites: Short, tapering, branching processes; receptive (input) region; convey incoming messages to cell body; may contain hundreds per neuron; specialized dendritic spines in the brain.

• Axon: Each neuron has one axon; cone-shaped start at axon hillock; can be short or over 1 meter long (nerve fibers); branch profusely at their end (up to 10,000 terminal branches); axon terminals are distal endings.

Axon Function

• Conducting region: Generates and sends nerve impulses along the axolemma (neuron membrane) to axon terminals.

• Terminals (secretory region): Secrete neurotransmitters.

Myelin Sheath

• Made of: Myelin, a protein-lipid substance.

• Functions: Protects and electrically insulates axons, increasing speed of nerve impulses.

• Myelinated fibers: Axons with segmented sheath; conduct impulses rapidly.

• Nonmyelinated fibers: Axons without sheath; conduct impulses more slowly.

Myelin Sheath in PNS

• Schwann cells: Wrap around axon like a jelly roll; each cell forms one segment of myelin sheath.

• Outer collar of perinuclear cytoplasm (neurilemma): Contains nucleus and most cytoplasm.

Myelin Sheath in CNS

• Oligodendrocytes: Processes, not whole cells, form myelin sheaths; each can wrap up to 60 axons; no outer collar of perinuclear cytoplasm.

• White matter: Myelinated fibers (axons).

• Gray matter: Mostly neuron cell bodies and nonmyelinated processes.

Classification of Neurons

Structural Classifications

Neurons are classified by the number of processes extending from the cell body:

• Multipolar: 3 or more processes (1 axon, 2 or more dendrites); most common in CNS.

• Bipolar: 2 processes (1 axon, 1 dendrite); rare (e.g., retina, olfactory epithelium).

• Unipolar (pseudounipolar): 1 T-like process (2 axons); found mainly in PNS.

Neuron Type	Processes	Location
Multipolar	Many dendrites, one axon	Most abundant; major neuron type in CNS
Bipolar	One dendrite, one axon	Rare; retina, olfactory epithelium
Unipolar	One T-like process (two axons)	Mainly in PNS; dorsal root ganglia, sensory ganglia of cranial nerves

Functional Classifications

• Sensory neurons: Send impulses from sensory receptors to CNS; most are unipolar.

• Interneurons: Between motor and sensory neurons; usually in CNS; send signals through CNS pathways; 99% of body's neurons; most are multipolar.

• Motor neurons: Send impulses from CNS to effectors; multipolar (with cell bodies in CNS).

Functional Class	Direction of Impulse	Structure
Sensory Neurons	From receptors to CNS	Mostly unipolar
Interneurons	Between sensory and motor neurons	Multipolar
Motor Neurons	From CNS to effectors	Multipolar

Examples and Applications

• Example: Sensory neurons in the skin detect temperature changes and send signals to the CNS for processing.

• Application: Motor neurons transmit signals from the CNS to skeletal muscles, enabling voluntary movement.

Additional info: The notes above expand on the original content by providing definitions, examples, and tables for structural and functional classifications of neurons, as well as a more detailed explanation of neuroglia types and nervous system organization.