Nervous System: Organization & Function

Organization of the Nervous System

  • Functions of Nervous System

    1. ) Sensory Input—gathering information

      • Sensory receptors monitor changes (aka stimuli) occurring inside & outside of body

    2. ) Integration

      • Nervous system processes & interprets sensory input and decides whether action is needed

    3. ) Motor Output

      • A response/effect activates muscles or glands

  • Nervous System Classifications

    • Structural Classification: Based on the structures of the nervous system

      • Central Nervous System (CNS)

      • Peripheral Nervous System (PNS)

    • Functional Classification: Based on activities of the nervous system

      • Afferent (Sensory) Division: Nerve fibers that carry information to CNS

        • Somatic Sensory Fibers: Carry info. from skin, skeletal muscles, joints

        • Visceral Sensory Fibers: Carry info. from visceral organs

      • Efferent (Motor) Division: Nerve fibers that carry impulses away from CNS to effector organs

        • Somatic Nervous System: Voluntary; consciously controls skeletal muscles

        • Autonomic Nervous System: Involuntary; automatically controls smooth & cardiac muscles and glands

        • Subdivision: Sympathetic & Parasympathetic

Structural Classification

  • Central Nervous System (CNS)

    • Organs:

    • Brain

    • Spinal Cord

    • Function:

    • Integration; command center

    • Interprets incoming sensory information

    • Issues outgoing instructions

  • Peripheral Nervous System (PNS)

    • Nerves: Extending from brain and spinal cord

    • Spinal Nerves: Carry impulses to/from spinal cord

    • Cranial Nerves: Carry impulses to/from brain

    • Functions:

    • Serve as communication lines among sensory organs, the brain & spinal cord, glands/muscles

Nervous Tissue: Structure & Function

  • Nervous Tissue Composed of Two Types of Cells:

    • Neurons

    • Supporting Cells (Neuroglia)

Support Cells in CNS (Neuroglia)

  • General Functions:

    • Support

    • Insulate

    • Protect Neurons

  • Types of CNS Glial Cells:

    • Astrocytes:

      • Abundant, star-shaped

      • Brace & anchor neurons to blood capillaries

      • Control permeability and exchanges b/w blood capillaries & neurons

      • Protect neurons from harmful substances

      • Control the chemical environment of the brain

    • Microglia:

      • Spiderlike phagocytes

      • Monitor health of nearby neurons

      • Dispose of debris

    • Ependymal Cells:

      • Line brain and spinal cord cavities

      • Cilia assist in circulation of cerebrospinal fluid

    • Oligodendrocytes:

      • Wrap around nerves in CNS

      • Produce myelin sheaths

  • PNS Glial Cells:

    • Schwann Cells:

      • Form myelin sheath around nerve fibers in the PNS

    • Satellite Cells:

      • Protect and cushion neuron cell bodies

Nervous Tissue: Neurons

  • Definition: Neurons = nerve cells specialized to transmit messages (nerve impulses)

Major Regions of Neurons

  • Cell Body:

    • Nucleus and metabolic center of the cell

    • Contains Nissl bodies and neurofibrils

  • Processes (Fibers): Extend from cell body

    • Dendrites: Conduct impulses toward the cell body

    • Axons: Conduct impulses away from the cell body

      • Neurons have only one axon arising at the axon hillock

      • End in axon terminals with neurotransmitter vesicles

Synaptic Structures

  • Synaptic Cleft: Gap between axon terminals and next neuron

  • Synapse: Functional junction between nerves where a nerve impulse is transmitted

Myelin Sheaths

  • Definition: Myelin is a white, fatty material covering axons

  • Function:

    • Protects and insulates fibers

    • Speeds up nerve impulse transmission

  • Schwann Cells: Wrap axons in a jelly roll-like fashion (PNS) to form myelin sheaths

  • Neurilemma: Part of Schwann cell external to myelin sheath

  • Nodes of Ranvier: Gaps in myelin sheaths without neurilemma, important for regeneration

Terminology

  • Nuclei: Clusters of cell bodies in CNS

  • Ganglia: Collections of cell bodies outside CNS in PNS

  • Tracts: Bundles of nerve fibers in CNS

  • Nerves (axons): Bundles of nerve fibers in PNS

  • White Matter: Collections of myelinated fibers (tracts)

  • Gray Matter: Mostly unmyelinated fibers and cell bodies

Functional Classification

  • Sensory (Afferent) Neurons:

    • Carry impulses from sensory receptors to CNS

    • Receptors Include:

    • Cutaneous: Sense organs in skin that detect pain, temperature, touch, and pressure

    • Proprioceptors: in muscles and tendons detect stretch

  • Functional