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Chapter 7: The Nervous System

Functions of the Nervous System

  • Sensory Input

    • Gathering information from internal and external environments.
    • Monitors changes (stimuli) around and inside the body.

  • Integration

    • Processing and interpreting sensory input.
    • Deciding whether action is required based on processed information.

  • Motor Output

    • Activation of muscles or glands as a response to stimuli.

Structural Classification of the Nervous System

  • Central Nervous System (CNS)

    • Components: Brain and spinal cord.
    • Functions:
    • Serves as command center.
    • Interprets sensory information and sends out instructions.

  • Peripheral Nervous System (PNS)

    • Components: Nerves extending from the brain and spinal cord, including spinal and cranial nerves.
    • Functions:
    • Communication pathways among sensory organs, the CNS, and muscles or glands.

Functional Classification of the Peripheral Nervous System

  • Sensory (Afferent) Division

    • Carries information to the CNS.

  • Motor (Efferent) Division

    • Carries impulses away from the CNS.
    • Subdivisions:
    • Somatic Nervous System:
      • Voluntary control over skeletal muscles.
    • Autonomic Nervous System:
      • Involuntary control over smooth and cardiac muscles and glands.
      • Further divided into:
      • Sympathetic Nervous System: Activates body for 'fight or flight.'
      • Parasympathetic Nervous System: Maintains 'rest and digest' functions.

Nervous Tissue: Support Cells

  • Neuroglia (glial cells)
    • Support, insulate, and protect neurons.
  • Types of Neuroglia:
    • Astrocytes:
    • Star-shaped, provide support, form blood-brain barrier, control chemical environment.
    • Microglia:
    • Act as phagocytes, clear debris in CNS.
    • Ependymal Cells:
    • Line cavities of brain & spinal cord; assist in cerebrospinal fluid (CSF) circulation.
    • Oligodendrocytes:
    • Produce myelin sheaths around CNS fibers.
    • Satellite Cells:
    • Protect neuron cell bodies in PNS.
    • Schwann Cells:
    • Form myelin in the PNS.

Nervous Tissue: Neurons

  • Neurons

    • Specialized cells for transmitting signals.
    • Regions of Neurons:
    • Cell Body: Contains nucleus and metabolic center.
    • Processes:
      • Dendrites: Conduct impulses toward cell body. Many per neuron.
      • Axons: Conduct impulses away from cell body. Only one per neuron, arising from axon hillock.

  • Myelin Sheath

    • Fatty layer covering some axons, speeds up nerve impulse transmission.
    • Nodes of Ranvier: Gaps in myelin sheath.

Functional Classification of Neurons

  • Sensory (Afferent) Neurons

    • Carry impulses from sensory receptors to CNS.

  • Motor (Efferent) Neurons

    • Carry impulses from CNS to muscles/glands.

  • Interneurons

    • Connect sensory and motor neurons, found in CNS.

Nerve Impulses

  • Resting Neuron:

    • Plasma membrane is polarized, more Na⁺ outside and more K⁺ inside.

  • Depolarization:

    • Stimulus opens Na⁺ channels, allowing Na⁺ to flow in, making inside of cell more positive.

  • Action Potential:

    • Rapid change in membrane potential initiated if stimulus is strong enough.

  • Propagation of the Action Potential:

    • Action potentials propagate down the axon when sufficient Na⁺ enters the cell.
    • Speed is increased in myelinated fibers.

  • Repolarization:

    • K⁺ rushes out of the neuron, restoring negative internal charge.
    • Requires sodium-potassium pump to restore ionic conditions post-action potential.

Transmission of a Signal at Synapses

  • Upon arrival of action potential, calcium channels open at axon terminal.
  • Calcium influx causes release of neurotransmitters from vesicles into the synaptic cleft.
  • Neurotransmitters bind to receptors on the postsynaptic neuron, potentially generating a new action potential if sufficient ligand-gated Na⁺ channels open.

Reflex Arc

  • Components of a Reflex Arc:

    • Sensory Receptor: Detects stimulus.
    • Sensory Neuron: Carries message to integration center.
    • Integration Center (CNS): Processes information.
    • Motor Neuron: Carries message to effector.
    • Effector Organ: Muscle or gland that responds.

  • Types of Reflexes:

    • Somatic Reflexes: Involuntary reactions such as pulling away from heat.
    • Autonomic Reflexes: Control smooth muscle, heart, etc. (e.g., regulating blood pressure).

Spinal Cord

  • Extends from the foramen magnum to first or second lumbar vertebra, providing communication pathway to and from the brain.

  • Contains 31 pairs of spinal nerves, with a collection of nerves (cauda equina) at the end.

  • Spinal Cord Anatomy:

    • Central gray matter surrounds the central canal filled with CSF.
    • White matter consists of conduction tracts.

Peripheral Nervous System (PNS)

  • Nerves and ganglia outside of CNS with both sensory and motor functions.
  • Cranial Nerves: 12 pairs serving primarily head and neck.
  • Spinal Nerves: 31 pairs categorized into cervical, thoracic, lumbar, sacral, and coccygeal.

Autonomic Nervous System (ANS)

  • Has sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.
  • Regulates involuntary functions in cardiac, smooth muscle, and glands.

Development Aspects of the Nervous System

  • The nervous system is formed during early embryonic development.
  • Maternal infections can impact development.
  • Neurons do not regenerate once formed, but growth continues for several years after birth.