Peripheral Nervous System Overview

Peripheral Nervous System (PNS)

• The PNS consists of nerves that extend from the brain and spinal cord, serving as communication channels between sensory organs, the brain and spinal cord, and muscles/glands.
• Spinal nerves carry impulses to and from the spinal cord.
• Cranial nerves carry impulses to and from the brain.

Sensory (Afferent) Division

• This division carries information to the central nervous system.
• Somatic sensory (afferent) fibers transmit signals from skin, skeletal muscles, and joints.
• Visceral sensory (afferent) fibers convey information from internal organs.

Motor (Efferent) Division

• This division carries impulses away from the central nervous system to effector organs (muscles and glands).
• Somatic Nervous System:
  • Controls skeletal muscles voluntarily.
• Autonomic Nervous System:
  • Controls smooth and cardiac muscles, and glands involuntarily.
  • Further divided into the sympathetic and parasympathetic nervous systems.

Nervous Tissue: Support Cells

• Support cells in the CNS are collectively called neuroglia.
• These cells provide support, insulation, and protection for neurons.

Nervous Tissue: Neurons

• The basic functional unit of the nervous system.
• Neurons are specialized cells that transmit nerve impulses.
• Cell Body: The metabolic center of the neuron, containing the nucleus and other organelles.
• Processes (Fibers):
  • Dendrites: Receive impulses and carry them towards the cell body.
  • Axons: Carry impulses away from the cell body.
  • Axon Terminals: Contain neurotransmitters, released at synapses.
• Synaptic Cleft: The gap between axon terminals and the next neuron.
• Synapse: The functional junction between neurons where nerve impulses are transmitted.

Myelin Sheaths

• Myelin is a white, fatty material that covers axons, enhancing the speed of nerve impulse transmission.
• Schwann Cells: Produce myelin sheaths in the PNS.
  • Neurilemma: The outer layer of the Schwann cell.
  • Nodes of Ranvier: Gaps in the myelin sheath along the axon.
• Oligodendrocytes: Produce myelin sheaths around axons in the CNS, lacking a neurilemma.

Peripheral Nervous System (PNS)

• The PNS consists of nerves and ganglia outside the CNS.

Structure of a Nerve

• Nerves are bundles of neuron fibers found outside the CNS, with protective connective tissue coverings.
  • Endoneurium: Surrounds each individual fiber.
  • Perineurium: Wraps groups of fibers bound into a fascicle.
  • Epineurium: Binds groups of fascicles together.
• Mixed Nerves: Contain both sensory and motor fibers.
• Sensory (Afferent) Nerves: Carry impulses toward the CNS.
• Motor (Efferent) Nerves: Carry impulses away from the CNS.

Cranial Nerves

• There are 12 pairs of cranial nerves primarily serving the head and neck, with the vagus nerves extending into the thoracic and abdominal cavities.

Spinal Nerve Plexuses

• Spinal nerves form networks called plexuses, which serve specific regions of the body.
• Groups of spinal nerves can be damaged, affecting specific areas of the body.
• Cervical Plexus (C1-C5)
  • Innervates the neck, shoulders, and diaphragm.
• Brachial Plexus (C5-T1)
  • Innervates the upper limbs.
• Lumbar Plexus (L1-L4):
  • Innervates the lower abdomen, anterior and medial thigh muscles, and skin of the anteromedial leg and thigh.
• Sacral Plexus (L4-S5)
  • Innervates the buttocks, posterior thigh, and lower leg muscles.

Autonomic Nervous System

• The autonomic nervous system regulates involuntary functions, including heart rate, digestion, and breathing.
• Sympathetic Division:
  • Prepares the body for "fight or flight" responses.
  • Increases heart rate, breathing rate, and blood pressure.
  • Dilates pupils.
  • Decreases digestive activity.
• Parasympathetic Division:
  • Promotes "rest and digest" responses.
  • Decreases heart rate, breathing rate, and blood pressure.
  • Constricts pupils.
  • Increases digestive activity.