Nervous Tissue Characteristics

Nervous Tissue Characteristics

• Made up of two types of cells: nerve cells (neurons) and neuroglial cells (glia)
• Neurons are the structural and functional units of nervous tissue, responsible for receiving, transmitting, and processing stimuli
• Neuroglial cells support, protect, and nourish neurons

Neuron Structure

• Most neurons consist of three parts: cell body, dendrite, and axon
• Cell body (perikaryon or soma): serves as the trophic center, variable in size and shape
• Dendrite: numerous elongated processes that receive stimuli from other neurons
• Axon: a single long process that generates and conducts nerve impulses to other cells

Neuron Classification

• Based on the number of processes:
  • Anaxonic neurons: many dendrites but no true axon
  • Bipolar neurons: one dendrite and one axon
  • Unipolar or pseudounipolar neurons: single process that soon divides into two
  • Multipolar neurons: two or more dendrites
• Based on functional roles:
  • Sensory neurons (afferent): receive sensory stimuli
  • Motor neurons (efferent): send impulses to effector organs
  • Interneurons: establish relationships among other neurons

Neuron Components

• Perikaryon:
  • Nucleus: large, spherical, and pale-staining, with a prominent nucleolus
  • Cytoplasm: contains coarse or fine basophilic particles (Nissl body), microtubules, actin filaments, and intermediate filaments
• Dendrite:
  • Dendritic spines: initial processing sites for synaptic signals
  • Usually short and thick, devoid of Golgi complex
• Axon:
  • Axon hillock: a conical region
  • Axolemma and axoplasm
  • Branches in axon: terminal arborization and collaterals
  • Organelles in axon: very few, including polyribosomes or cisternae of RER

Nerve Impulse

• An electrochemical process initiated at the axon hillock
• A wave of membrane depolarization, involving voltage-gated Na+ and K+ channels

Synapses

• Sites where nerve impulses are transmitted from one neuron to another or from neurons to other effector cells
• Transmission is unidirectional
• Types of synapses:
  • Chemical synapse: uses a chemical mediator (neurotransmitter) to transmit impulses
  • Electrical synapse: permits direct flow of electrical current between two neurons (gap junction)

Synapse Structure

• Presynaptic axon terminal (terminal bouton):
  • Contains mitochondria and numerous synaptic vesicles
  • Releases neurotransmitter by exocytosis
• Synaptic cleft: extracellular space between pre- and postsynaptic membranes, about 20-30 nm wide
• Postsynaptic cell membrane: contains receptors for the neurotransmitter and ion channels or other mechanisms to initiate a new impulse

Neuroglial Cells

• Astrocytes:
  • Most numerous glial cells
  • Have a large number of long, radiating, branching processes
  • Contain bundles of intermediate filaments made of glial fibrillary acid protein (GFAP)
  • Have processes with expanded perivascular feet that cover capillary endothelial cells
• Oligodendrocytes:
  • Fewer processes
  • Myelinate and insulate CNS axons
• Microglia:
  • Small cells with actively mobile processes
  • Protect the CNS by engulfing infectious agents and other harmful substances
• Ependymal cells:
  • Low columnar epithelial cells with cilia and processes
  • Assist in the production and circulation of cerebrospinal fluid (CSF)

Peripheral Nervous System

• Schwann cells:
  • Surround and insulate PNS axons
  • Myelinate those having large diameters
• Satellite cells:
  • Form a layer around cell bodies of neurons in the ganglia
  • Electrically insulate PNS cell bodies and regulate nutrient and waste exchange
• Peripheral nerves:
  • Bundles of nerve fibers (axons) individually surrounded by Schwann cells and connective tissue
  • Establish communication between CNS and sense organs and effectors

Nerve Endings

• Terminal portion of dendrites and axons
• Classified into sensory and motor nerve endings
• Free nerve endings:
  • Devoid of myelin sheath
  • Distribute in skin, cornea, hair follicle, and connective tissue
  • Responsible for temperature and pain
• Encapsulated nerve endings:
  • A special connective tissue capsule surrounds the actual nerve endings
  • Tactile corpuscles, lamellar corpuscles, and muscular spindles

Motor Nerve Endings

• Terminal structure of the axon of motor neuron
• Supply muscular tissue and gland
• Form effectors: somatic motor nerve endings and visceral motor nerve endings
• Somatic motor nerve ending:
  • Motor end plate or neuromuscular junction
  • A motor neuron innervates from one to more than a thousand muscle fibers
  • Each branch of axon ends on a skeletal muscle fiber