Human Anatomy and Physiology - Fundamentals of the Nervous System

Functions of Nervous System

• The nervous system is the body's master controlling and communicating system.
• Cells communicate via electrical and chemical signals.
• Communication is rapid and specific, usually causing almost immediate responses.
• Three overlapping functions:
  • Sensory input: Information gathered by sensory receptors about internal and external changes.
  • Integration: Processing and interpretation of sensory input.
  • Motor output: Activation of effector organs (muscles and glands) produces a response.

Divisions of the Nervous System

• Central nervous system (CNS):
  • Brain and spinal cord of dorsal body cavity.
  • Integration and control center.
  • Interprets sensory input and dictates motor output.
• Peripheral nervous system (PNS):
  • The portion of the nervous system outside the CNS.
  • Consists mainly of nerves that extend from the brain and spinal cord.
  • Spinal nerves: to and from the spinal cord.
  • Cranial nerves: to and from the brain.

Peripheral Nervous System (PNS) Divisions

• Sensory (afferent) division:
  • Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS.
  • Visceral sensory fibers: convey impulses from visceral organs to CNS.
• Motor (efferent) division:
  • Transmits impulses from CNS to effector organs (muscles and glands).
  • Two divisions:
    • Somatic nervous system.
    • Autonomic nervous system.

Somatic Nervous System

• Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle.
• Voluntary nervous system: conscious control of skeletal muscles.

Autonomic Nervous System

• Consists of visceral motor nerve fibers.
• Regulates smooth muscle, cardiac muscle, and glands.
• Involuntary nervous system.
• Two functional subdivisions:
  • Sympathetic.
  • Parasympathetic.
  • Work in opposition to each other.

Nervous Tissue Histology

• Nervous tissue consists of two principal cell types:
  • Neuroglia (glial cells): small cells that surround and wrap delicate neurons.
  • Neurons (nerve cells): excitable cells that transmit electrical signals.

Neuroglia of the CNS

• Four main neuroglia support CNS neurons:
  • Astrocytes.
  • Microglial cells.
  • Ependymal cells.
  • Oligodendrocytes.

Astrocytes

• Most abundant, versatile, and highly branched of glial cells.
• Cling to neurons, synaptic endings, and capillaries.
• Functions include:
  • Support and brace neurons.
  • Play a role in exchanges between capillaries and neurons.
  • Guide migration of young neurons.
  • Control chemical environment around neurons.
  • Respond to nerve impulses and neurotransmitters.
  • Influence neuronal functioning.
  • Participate in information processing in the brain.

Microglial Cells

• Small, ovoid cells with thorny processes that touch and monitor neurons.
• Migrate toward injured neurons.
• Can transform to phagocytize microorganisms and neuronal debris.

Ependymal Cells

• Range in shape from squamous to columnar.
• May be ciliated (cilia beat to circulate CSF).
• Line the central cavities of the brain and spinal column.
• Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells.

Oligodendrocytes

• Branched cells.
• Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers.

Neuroglia of PNS

• Two major neuroglia seen in PNS:

Satellite cells

• Surround neuron cell bodies in PNS.
• Function similar to astrocytes of CNS.

Schwann cells (neurolemmocytes)

• Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers.
• Similar function as oligodendrocytes.
• Vital to regeneration of damaged peripheral nerve fibers.

Neurons

• Neurons (nerve cells) are structural units of the nervous system.
• Large, highly specialized cells that conduct impulses.
• Special characteristics:
  • Extreme longevity (lasts a person’s lifetime).
  • Amitotic, with few exceptions.
  • High metabolic rate: requires continuous supply of oxygen and glucose.
• All have a cell body and one or more processes.

Neuron Cell Body

• Also called the perikaryon or soma.
• Biosynthetic center of neuron, synthesizes proteins, membranes, chemicals such as Rough ER (chromatophilic substance, or Nissl bodies).
• Contains spherical nucleus with nucleolus.
• Some contain pigments.
• In most, the plasma membrane is part of the receptive region that receives input info from other neurons.
• Most neuron cell bodies are located in CNS.
  • Nuclei: clusters of neuron cell bodies in CNS.
  • Ganglia: clusters of neuron cell bodies in PNS.

Neuron Processes

• Armlike processes that extend from the cell body.
• CNS contains both neuron cell bodies and their processes.
• PNS contains chiefly neuron processes.
• Tracts: Bundles of neuron processes in CNS.
• Nerves: Bundles of neuron processes in PNS.
• Two types of processes:
  • Dendrites.
  • Axon.

Dendrites

• Motor neurons can contain 100s of these short, tapering, diffusely branched processes; contain same organelles as in cell body.
• Receptive (input) region of neuron.
• Convey incoming messages toward cell body as graded potentials (short distance signals).
• In many brain areas, finer dendrites are highly specialized to collect information, contain dendritic spines, appendages with bulbous or spiky ends.

Axon

Structure:

• Each neuron has one axon that starts at a cone-shaped area called the axon hillock.
• In some neurons, axons are short or absent; in others, the axon comprises almost the entire length of the cell.
• Long axons are called nerve fibers.
• Axons have occasional branches called axon collaterals.
• Axons branch profusely at their end (terminus) - can number as many as 10,000 terminal branches.
• Distal endings are called axon terminals or terminal boutons.

Functional characteristics:

• Axon is the conducting region of the neuron.
• Generates nerve impulses and transmits them along the axolemma (neuron cell membrane) to the axon terminal.
• Terminal: region that secretes neurotransmitters, which are released into the extracellular space; can excite or inhibit neurons it contacts.
• Carries on many conversations with different neurons at the same time.
• Axons rely on cell bodies to renew proteins and membranes and quickly decay if cut or damaged.
• Axons have efficient internal transport mechanisms; molecules and organelles are moved along axons by motor proteins and cytoskeletal elements.
• Movement occurs in both directions:
  • Anterograde: away from the cell body (examples: mitochondria, cytoskeletal elements, membrane components, enzymes).
  • Retrograde: toward the cell body (examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins).

Homeostatic Imbalance:

• Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport (examples: polio, rabies, and herpes simplex viruses, and tetanus toxin).
• Research is underway to investigate using retrograde transport to treat genetic diseases; viruses containing “corrected” genes or microRNA to suppress defective genes can enter the cell through retrograde transport.

Myelin Sheath

• Composed of myelin, a whitish, protein-lipid substance
• Function of myelin:
  • Protect and electrically insulate the axon.
  • Increase speed of nerve impulse transmission
• Myelinated fibers: segmented sheath surrounds most long or large-diameter axons.
• Nonmyelinated fibers: do not contain sheath and conduct impulses more slowly.

Myelination in the PNS

• Formed by Schwann cells.
  • Wraps around axon in jelly roll fashion and one cell forms one segment of myelin sheath.
• Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm
• Plasma membranes have less protein:
  • No channels or carriers, so good electrical insulators
  • Interlocking proteins bind adjacent myelin membranes.
• Myelin sheath gaps:
  • Gaps between adjacent Schwann cells.
  • Sites where axon collaterals can emerge.
  • Formerly called nodes of Ranvier.
• Nonmyelinated fibers:
  • Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers.

Myelin Sheaths in the CNS

• Formed by processes of oligodendrocytes, not whole cells
• Each cell can wrap up to 60 axons at once
• Myelin sheath gap is present
• No outer collar of perinuclear cytoplasm
• Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia
• White matter: regions of brain and spinal cord with dense collections of myelinated fibers (usually fiber tracts)
• Gray matter: mostly neuron cell bodies and nonmyelinated fibers

Classification of Neurons

Structural classification

• Three types grouped by the number of processes:
  • Multipolar: three or more processes (1 axon, others dendrites); most common and major neuron type in CNS.
  • Bipolar: two processes (one axon, one dendrite); rare examples: retina and olfactory mucosa.
  • Unipolar: one T-like process (two axons); also called pseudounipolar:
    • Peripheral (distal) process: associated with sensory receptor.
    • Proximal (central) process: enters CNS.

Functional classification

• Three types of neurons grouped by direction in which nerve impulse travels relative to CNS:
  • Sensory:
    • Transmit impulses from sensory receptors toward CNS.
    • Almost all are unipolar.
    • Cell bodies are located in ganglia in PNS.
  • Motor:
    • Carry impulses from CNS to effectors.
    • Multipolar.
    • Most cell bodies are located in CNS (except some autonomic neurons).
  • Interneurons:
    • Also called association neurons.
    • Lie between motor and sensory neurons.
    • Shuttle signals through CNS pathways.
    • Most are entirely within CNS.
    • 99% of the body’s neurons are interneurons.