Nervous Tissue: Structure, Function, and Classification (Anatomy & Physiology Study Notes)

Nervous Tissue

Introduction to the Nervous System

The nervous system is a complex network of organs and tissues responsible for receiving, processing, and responding to internal and external stimuli. It includes the brain, spinal cord, receptors in sense organs (such as the eye and ear), and nerves that connect the nervous system to other body systems. The basic functional units are neurons, specialized for intercellular communication, and neuroglia (supporting cells).

• Nervous tissue: Composed of neurons and neuroglia, along with blood vessels and connective tissues.

• Neurons: Cells specialized for communication via electrical and chemical signals.

• Neuroglia: Support, protect, and nourish neurons; maintain tissue integrity.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. These organs integrate, process, and coordinate sensory data and motor commands.

• Functions: Integration, processing, coordination of sensory data and motor commands.

• Components: Nervous tissue, blood vessels, and connective tissues.

Peripheral Nervous System (PNS)

The PNS includes all nervous tissue outside the CNS and the enteric nervous system (ENS). It delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• Nerve fibers: Bundles of axons carrying sensory and motor information.

• Peripheral nerves: Nerve fibers with associated blood vessels and connective tissues.

Functional Divisions of the Nervous System

Afferent and Efferent Divisions

The PNS is functionally divided into afferent (sensory) and efferent (motor) divisions.

• Afferent division: Brings sensory information from receptors to the CNS.

• Efferent division: Carries motor commands from the CNS to effectors (muscles, glands, adipose tissue).

Somatic and Autonomic Nervous Systems

• Somatic Nervous System (SNS): Controls voluntary and involuntary skeletal muscle contractions. Reflexes are involuntary somatic responses.

• Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, glands, and adipose tissue. Includes parasympathetic (rest and digest) and sympathetic (fight or flight) divisions.

Structure of Neurons

Neuron Components

Neurons are nerve cells specialized for communication. They have distinct structural regions:

• Cell body (soma/perikaryon): Contains the nucleus, mitochondria, ribosomes, and Nissl bodies (clusters of rough ER and ribosomes).

• Dendrites: Branched extensions that receive signals from other cells.

• Axon: Long process that propagates action potentials; contains axoplasm and is surrounded by axolemma.

• Axon hillock: Region where the axon joins the cell body.

• Telodendria: Terminal branches of the axon ending in synaptic terminals.

Axoplasmic Transport

• Anterograde flow: Movement of materials from the cell body to axon terminals.

• Retrograde flow: Movement from axon terminals to the cell body.

• Example: Rabies virus uses retrograde flow to infect the CNS.

Classification of Neurons

Structural Classification

• Anaxonic neurons: Small, many dendrites, no obvious axon; found in brain and sense organs.

• Bipolar neurons: One dendrite, one axon, cell body between; found in special sense organs.

• Unipolar (pseudounipolar) neurons: Dendrites and axon fused, cell body off to one side; most sensory neurons in PNS.

• Multipolar neurons: Many dendrites, one axon; most common in CNS and all motor neurons controlling skeletal muscle.

Functional Classification

• Sensory neurons (afferent): Carry sensory information from receptors to CNS; cell bodies in peripheral ganglia.

• Motor neurons (efferent): Carry instructions from CNS to effectors; somatic motor neurons (skeletal muscle), visceral motor neurons (smooth/cardiac muscle, glands, adipose tissue).

• Interneurons: Connect sensory and motor neurons; involved in integration, memory, planning, and learning.

Neuroglia (Supporting Cells)

Neuroglia in the CNS

• Astrocytes: Maintain blood-brain barrier, provide structural support, repair tissue, guide neuron development, regulate interstitial environment.

• Ependymal cells: Line ventricles and central canal, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Myelinate axons in CNS, provide electrical insulation.

• Microglia: Phagocytic cells, remove debris, waste, and pathogens; act as immune cells in CNS.

Neuroglia in the PNS

• Satellite cells: Surround neuron cell bodies in ganglia, regulate interstitial fluid.

• Schwann cells: Myelinate axons in PNS, form neurolemma; can myelinate one segment of one axon or enclose multiple unmyelinated axons.

Myelination and Demyelination

Myelination

• Myelin: Lipid-rich membranous wrapping that insulates axons and increases speed of action potential conduction.

• Internodes: Segments of axon covered by myelin.

• Nodes: Gaps between internodes (nodes of Ranvier).

White Matter vs. Gray Matter

• White matter: Regions with myelinated axons; appears glossy white.

• Gray matter: Regions with neuron cell bodies, dendrites, and unmyelinated axons; appears gray.

Demyelination

• Demyelination: Progressive destruction of myelin sheaths in CNS or PNS, leading to loss of sensation and motor control.

• Causes: Heavy metal poisoning, diphtheria, multiple sclerosis (MS).

• Multiple sclerosis (MS): Autoimmune disease causing demyelination in CNS; symptoms include sensory loss, motor problems, and impaired coordination.

Key Terms and Definitions

• Nissl bodies: Clusters of rough ER and ribosomes in neuron cell bodies.

• Ganglia: Collections of neuron cell bodies in the PNS.

• Sensory neuron: Neuron that transmits sensory information to the CNS.

• Motor neuron: Neuron that transmits motor commands from the CNS to effectors.

• Anterograde flow: Movement of materials from cell body to axon terminal.

• Retrograde flow: Movement from axon terminal to cell body.

• White matter: CNS regions with myelinated axons.

• Gray matter: CNS regions with neuron cell bodies and unmyelinated axons.

• Demyelination: Loss or destruction of myelin sheaths.

Table: Comparison of Neuroglia in CNS and PNS

Neuroglia Type	CNS	PNS
Structural Support	Astrocytes	Satellite cells
Myelination	Oligodendrocytes	Schwann cells
Immune Defense	Microglia	None (Additional info: Immune defense in PNS is mainly by macrophages)
CSF Production	Ependymal cells	None

Example: Action Potential Conduction

• Myelinated axons conduct action potentials faster due to saltatory conduction between nodes of Ranvier.

• Unmyelinated axons conduct action potentials more slowly via continuous conduction.

Relevant Equations

• Action potential propagation speed (simplified): $v \propto \sqrt{\frac{d}{R_m}}$ where $v$ = conduction velocity, $d$ = axon diameter, $R_m$ = membrane resistance.

Summary Table: Neuron Types

Type	Structure	Location	Function
Anaxonic	No obvious axon	Brain, sense organs	Unknown/Integration
Bipolar	One dendrite, one axon	Special sense organs	Sensory (sight, smell, hearing)
Unipolar	Dendrite and axon fused	PNS	Sensory
Multipolar	Many dendrites, one axon	CNS	Motor, integration