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.