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 (such as those in the eyes and ears), and nerves that connect the nervous system to other body systems.

• Nervous tissue is the main tissue type in nervous system organs, supported by blood vessels and connective tissues.

• The basic functional units are neurons, specialized for intercellular communication.

• Neuroglia (glial cells) support, protect, and nourish neurons.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

The nervous system is divided into two main anatomical regions: the CNS and the PNS.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS and the enteric nervous system (ENS). Delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• Nerve fibers: Bundles of axons in the PNS that carry sensory and motor information.

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

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.

• Receptors: Structures that detect environmental changes, ranging from cellular extensions to complex organs.

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

• Effectors: Peripheral tissues that respond to CNS commands.

Somatic vs. Autonomic Nervous System

The efferent division is further subdivided:

• 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. Operates at the subconscious level and includes:

  • Parasympathetic Division (PANS): Slows heart rate and promotes 'rest and digest' functions.

  • Sympathetic Division (SANS): Accelerates heart rate and supports 'fight or flight' responses.

Structure of Neurons

Components of a Typical Neuron

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

• Cell Body (Perikaryon): Contains the nucleus, mitochondria, ribosomes, and rough endoplasmic reticulum (RER). Clusters of RER and ribosomes are called Nissl bodies.

• Dendrites: Slender, branched processes that receive signals from other cells.

• Axon: Long process that propagates action potentials. The axon hillock is the initial segment connecting the cell body to the axon.

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

Axoplasmic transport moves materials between the cell body and axon terminals:

• Anterograde flow: From cell body to axon terminal.

• Retrograde flow: From axon terminal to cell body.

Example: Rabies virus uses retrograde flow to reach the CNS from peripheral tissues.

Classification of Neurons

Structural Classification

Neurons are classified by the arrangement of dendrites, axon, and cell body:

• Anaxonic Neurons: Small, with many 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, inner ear).

• Unipolar (Pseudounipolar) Neurons: Dendrites and axon are fused; cell body off to one side. Most sensory neurons in the PNS are unipolar.

• Multipolar Neurons: Many dendrites and one axon. Most common in the CNS; all motor neurons controlling skeletal muscles are multipolar.

Functional Classification

Neurons are also classified by function:

• Sensory Neurons (Afferent): Carry sensory information from receptors to the CNS. Cell bodies are located in peripheral sensory ganglia.

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

• Interneurons: Located mostly in the CNS; connect sensory and motor neurons, integrate information, and coordinate responses. Involved in higher functions such as memory and learning.

Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, cardiovascular, etc.) and sensations like stretch and pain.

• Exteroceptors: Provide information about the external environment (touch, temperature, pressure, taste, smell, sight, hearing).

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

Neuroglia (Glial Cells)

Introduction and Functions

Neuroglia are supporting cells that protect, nourish, and maintain neurons. They outnumber neurons and make up about half the volume of the nervous system.

• Provide structural support

• Act as phagocytes (defense)

• Regulate interstitial fluid composition

Neuroglia in the CNS

• Astrocytes: Largest and most numerous. Functions include maintaining the blood-brain barrier (BBB), providing structural support, repairing tissue, guiding neuron development, and regulating the interstitial environment.

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

• Oligodendrocytes: Myelinate axons in the CNS, providing electrical insulation. One oligodendrocyte can myelinate multiple axons.

• Microglia: Least numerous; act as phagocytic immune cells, removing debris and pathogens.

Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate interstitial fluid.

• Schwann Cells (Neurolemmocytes): Myelinate axons in the PNS; each Schwann cell myelinates one segment of one axon. Also support unmyelinated axons.

Myelination and Nervous Tissue Organization

Myelinated vs. Unmyelinated Axons

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

• Internodes: Segments of axon covered by myelin.

• Nodes (Nodes of Ranvier): Gaps between myelinated segments.

• White Matter: Regions of CNS with myelinated axons; appears glossy white.

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

Demyelination

Definition and Clinical Relevance

Demyelination is the progressive destruction of myelin sheaths in the CNS or PNS, leading to loss of sensation and motor control.

• Causes include heavy metal poisoning (arsenic, lead, mercury), bacterial infections (diphtheria), and autoimmune diseases (multiple sclerosis).

• Multiple Sclerosis (MS): Most common demyelinating disease; characterized by partial loss of vision, speech problems, motor coordination issues, and loss of bladder/bowel control. Involves autoimmune attack on myelin in white matter, producing multiple scars.

Key Terms and Definitions

• Nissl bodies: Clusters of rough ER and ribosomes in neuron cell bodies, involved in 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 of materials from the axon terminal to the cell body.

• White matter: CNS regions with myelinated axons.

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

• Demyelination: Loss or destruction of myelin sheaths.

Comparison Table: Neuroglia of CNS vs. PNS

Example Equations

• Action Potential Propagation:

• Rate of Nerve Conduction (simplified):

Additional info:

• Some content inferred for completeness, such as the role of myelin in conduction velocity and the autoimmune nature of MS.

• Table entries and equations added for academic context.