Nervous Tissue Overview

Nervous Tissue

• Nervous tissue is a communication network in the body, responsible for controlling and integrating body functions within limits that maintain life.
• It's composed of closely associated cells that perform related functions.
• There are four main types of tissue: nervous, epithelial, muscle, connective.
• Nervous tissue originates from ectoderm.

Learning Objectives

• Understand the general characteristics of nervous tissue.
• Identify the components of nervous tissue.
• Identify central nervous system (CNS) organs.
• Identify peripheral nervous system (PNS) organs.

Divisions of Nervous System

• Nervous system is divided into central (CNS) and peripheral (PNS).
• CNS includes the brain and spinal cord.
• PNS includes cranial nerves, spinal nerves, and ganglia.
• The PNS further splits into somatic and autonomic nervous systems. Somatic controls voluntary movement. Autonomic controls involuntary processes (e.g., heart rate). Autonomic further divides into sympathetic (activating) and parasympathetic (calming).

General Organization

• CNS includes the brain and spinal cord.
• PNS includes cranial, spinal nerves, and ganglia.
• Histology of CNS: neurons, astrocytes, oligodendrocytes, microglial cells, ependymal cells. Histology of PNS: Neurons, satellite cells, Schwann cells.
• CNS tissue contains neurons and neuroglial cells (supportive, non-transmitting). PNS contains neurons and supporting neuroglial cells, involved in protection, nutrition, and structure support.

Neurons or Nerve Cells

• Structural unit of nervous tissue.
• Conducts impulses.
• Characteristics: longevity, amitotic, high metabolic rate (requiring O2 and glucose).
• Structure: cell body, perikaryon or soma, processes or neurites. The cell body is the metabolic center of the neuron.
• Cell body contains, Golgi apparatus, Rough ER (Nissl bodies), smooth ER, mitochondria, and other cellular components (cytoplasmic inclusions).
• Processes or neurites extend from the cell body include dendrites (receive stimuli) and axons (transmit impulses).

Cell Body

• Plasma membrane: receives stimuli, integrates them, and generates nerve impulses, contains ion channels and the Na+-K+ pump.
• Cytoplasm: contains Nissl bodies (rough endoplasmic reticulum), Golgi apparatus, mitochondria.

Processes or Neurites

• Dendrites: short, branched extensions that receive stimuli from other nerve cells, many, branched, irregularly shaped.
• Axon: single long extension that transmits impulses to other neurons or effector cells, uniform diameter, branched at their ends (forming synaptic knobs) with neurotransmitters, no Nissl's granules.

Classification of Neuron

• Function: sensory (afferent), interneurons, motor (efferent).
• Number of processes: unipolar, bipolar, pseudounipolar, multipolar.

Synapses

• Functional connection between a nerve fiber and a target cell.
• Types: chemical, electrical. Chemical synapses are unidirectional and utilize neurotransmitters to signal. Electrical synapses provide direct electrical coupling.

Neuroglial Cells

• 10 times more numerous than neurons in the nervous system.
• Support, nourish, and protect neurons.
• Types in CNS: astrocytes, oligodendrocytes, microglia, ependymal cells.
• Types in PNS: satellite cells, Schwann cells.

Astrocytes

• Star-shaped, abundant cytoplasm with intermediate filaments (GFAP)
• Processes contact blood vessels and neurons.
• Functions: maintain blood-brain barrier, regulate tissue fluid composition, structural support and organization, stimulate scar tissue formation.

Oligodendrocytes

• Smaller than astrocytes, fewer processes.
• Form myelin sheaths in the CNS.

Microglia

• Smallest glial cells, originate from monocytes.
• Phagocytic functions in the CNS for defense and debris removal.

Ependymal Cells

• Low columnar or cuboidal epithelial cells.
• Line ventricles of the brain and central canal of the spinal cord.
• Form cerebrospinal fluid, facilitate its movement.

Satellite Cells

• Small, round, spindle-shaped cells, surround neuron cell bodies in ganglia of PNS.
• Functions: protect cell bodies, regulate nutrient supply for cell bodies.

Schwann Cells

• Flattened cells, encase axons of peripheral nerves with a membrane.
• Myelinate PNS axons
• Allow faster nerve impulse conduction.

Nerve Fibers

• Myelinated: enclosed in a myelin sheath for faster conduction.
• Unmyelinated: no myelin sheath.

Myelin Sheath

• Composed of phospholipids, glycolipids and cholesterol
• Function: insulates the axons, increases speed of nerve impulses.

Myelination

• Invagination of a single nerve axon into a Schwann cell forms myelin sheath.

Organization of Nervous Tissue in CNS

• Grey matter: soma, dendrites, unmyelinated fibers, glial cells.
• White matter: axons, myelinated fibers, network of neural tissue.
• Meninges (connective tissue): dura mater, arachnoid mater, pia mater.

Spinal Cord

• Ventral horns: motor neurons.
• Dorsal horns: sensory neurons.

Cerebellum

• Grey matter comprises the cortex, white matter comprises the medulla.

Organization of Nervous Tissue in PNS

• Consists of peripheral nerves, ganglia, and sensory receptors.

Peripheral Nerves

• Nerve fibers bundled together into elongated fascicles.
• Includes epineurium, perineurium, endoneurium.

Peripheral Nerve Regeneration

• Stages: injury, chromatolysis, atrophy, scarring, successful/unsuccessful regeneration.

Ganglia

• Associated with cranial nerves (pseudounipolar cell bodies).
• Multipolar neurons.

Sensory Receptors

• Receptors detect stimuli.
• Types: mechanoreceptors (touch, hearing), thermoreceptors (temperature), nociceptors (pain), photoreceptors (light), chemoreceptors (smell, taste).
• Classification based on stimulus source (exteroceptors, interoceptors, proprioceptors).
• Senses: vision, hearing, smell, taste, touch.