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

Nervous Tissue

Introduction to the Nervous System

The nervous system is a complex network responsible for receiving, processing, and responding to internal and external stimuli. It is composed of specialized cells called neurons and supporting cells known as neuroglia. The nervous system integrates sensory input and coordinates motor output, ensuring the proper functioning of the body.

• Neurons: Specialized for intercellular communication; basic functional units of the nervous system.

• Neuroglia (glial cells): Support, protect, and nourish neurons; maintain the environment around neurons.

• Nervous tissue: Includes neurons, neuroglia, blood vessels, and connective tissues.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. These organs are responsible for integrating, processing, and coordinating sensory data and motor commands.

• Functions: Integration of sensory information, coordination of motor output.

• Components: Brain, spinal cord, blood vessels, and connective tissues.

Peripheral Nervous System (PNS)

The PNS includes all neural tissue outside the CNS and the enteric nervous system (ENS). It connects the CNS to limbs and organs.

• Functions: Delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• Components: Nerves (bundles of nerve fibers), ganglia.

• Nerve fibers: Axons in the PNS, often bundled with blood vessels and connective tissue as peripheral nerves.

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 (reflex) contractions of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, glands, and adipose tissue at the subconscious level. Includes:

  • Sympathetic division (SANS): Prepares the body for 'fight or flight' responses.

  • Parasympathetic division (PANS): Promotes 'rest and digest' activities.

Structure of Neurons

Basic Components

Neurons are highly specialized cells with unique structures for communication.

• Cell body (soma): Contains the nucleus and organelles; cytoplasm around the nucleus is called the perikaryon.

• Nissl bodies: Clusters of rough endoplasmic reticulum and ribosomes involved in protein synthesis.

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

• Axon: Long process that transmits action potentials away from the cell body.

• Axon hillock: Cone-shaped region where the axon originates from 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 the axon terminal.

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

• Clinical relevance: Certain viruses (e.g., rabies) use retrograde transport to infect the CNS.

Classification of Neurons

Structural Classification

• Anaxonic neurons: Small, with numerous dendrites and no obvious axon; found in the brain and special sense organs.

• Bipolar neurons: One dendrite and one axon with the cell body between them; found in special sense organs (retina, olfactory epithelium).

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

• Multipolar neurons: Two or more dendrites and a single axon; most common type in the CNS; all motor neurons controlling skeletal muscles are multipolar.

Functional Classification

• Sensory (afferent) neurons: Transmit sensory information from receptors to the CNS; cell bodies located in sensory ganglia; most are unipolar.

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

• Interneurons (association neurons): Located within the CNS; connect sensory and motor neurons; involved in higher functions such as memory, planning, and learning.

Types of Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) and internal senses (stretch, pressure, pain).

• Exteroceptors: Provide information about the external environment (touch, temperature, pressure, special senses).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neuroglia (Glial Cells)

Overview

Neuroglia are supporting cells that maintain the environment around neurons, provide structural support, and participate in defense and repair. They outnumber neurons and are essential for nervous system function.

Neuroglia in the CNS

• Astrocytes: Star-shaped, most numerous; maintain the blood-brain barrier, provide structural support, repair tissue, guide neuron development, and regulate the interstitial environment.

• Ependymal cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Myelinate axons in the CNS; a single cell can myelinate multiple axons; myelin increases the speed of action potential conduction.

• Microglia: Smallest and least numerous; act as phagocytes, removing debris, waste, and pathogens; the resident immune cells of the CNS.

Neuroglia in the PNS

• Satellite cells: Surround neuron cell bodies in ganglia; regulate the environment around neurons, similar to astrocytes in the CNS.

• Schwann cells (neurolemmocytes): Myelinate axons in the PNS; each cell myelinates a single segment of one axon; also support unmyelinated axons.

Myelination and Nervous Tissue Organization

Myelin Sheath

• Myelin: Lipid-rich, insulating layer around axons; increases speed of electrical conduction.

• Nodes of Ranvier: Gaps between myelinated segments; facilitate rapid signal transmission.

• Internodes: Myelinated segments between nodes.

White Matter vs. Gray Matter

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

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

Demyelination and Clinical Relevance

Demyelination

• Demyelination: Progressive destruction of myelin sheaths in the CNS and/or PNS; leads to loss of sensation and motor control.

• Causes: Heavy metal poisoning (arsenic, lead, mercury), infections (diphtheria), autoimmune diseases (multiple sclerosis).

• Multiple Sclerosis (MS): Most common demyelinating disease; characterized by loss of myelin in the CNS, leading to neurological deficits.

Key Terms and Definitions

• Nissl bodies: Clusters of rough ER and ribosomes in neuron cell bodies; site of protein synthesis.

• 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 the cell body to the axon terminal.

• Retrograde flow: Movement from the axon terminal to the 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.

Comparison Table: Neuroglia of CNS vs. PNS

Neuroglia Type	CNS	PNS
Astrocytes	Yes	No
Ependymal cells	Yes	No
Oligodendrocytes	Yes	No
Microglia	Yes	No
Satellite cells	No	Yes
Schwann cells	No	Yes

Example: Action Potential Conduction

• Myelinated axons conduct action potentials more rapidly due to saltatory conduction, where the action potential jumps from node to node.

• Unmyelinated axons conduct impulses more slowly via continuous conduction.

Key Equations

• Ohm's Law (applied to neurons):

$V = IR$

• Where V is voltage (potential difference), I is current, and R is resistance.

Summary

• The nervous system is divided anatomically (CNS and PNS) and functionally (afferent/efferent, somatic/autonomic).

• Neurons are classified by structure and function; neuroglia support and protect neurons.

• Myelination is essential for rapid signal conduction; demyelination leads to neurological disorders.