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Neurophysiology: The Nervous System

The nervous system is the master control and communication system of the body. It provides higher mental function and emotional expression, maintains homeostasis, and regulates activities of muscles and glands.

Structural Divisions of the Nervous System

  1. Central Nervous System (CNS):
  • Brain
  • Spinal Cord
  1. Peripheral Nervous System (PNS):
  • Cranial Nerves
  • Spinal Nerves

Humans have 31 pairs of spinal nerves:

  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal

Functional Divisions of the Nervous System

  1. Sensory (Afferent) Functions:
  • Collect information about the external environment and internal conditions of the body.
  • Subcategories:
    • Special Sensory: Vision, hearing, equilibrium, smell, taste.
    • Somatic Sensory: Touch, temperature, joint position, pain.
    • Visceral Sensory: Conditions of various internal organs.
  1. Integration Function:
  • Analyze sensory input, process it, and decide on the appropriate response.
  1. Motor (Efferent) Functions:
  • Carry out instructions resulting from integration by sending impulses to effector organs (muscles and glands).
  • Subcategories:
    • Somatic (Voluntary) Motor: Controls skeletal muscles.
    • Autonomic (Involuntary) Motor: Regulates activities of cardiac and smooth muscle and glands.
      • Sympathetic System
      • Parasympathetic System

Cells of the Nervous System

  1. Neurons:
  • Excitable cells that do not normally undergo cell division after birth.
  • Parts:
    • Cell Body: Control center with nucleus and organelles, carries out metabolic functions.
    • Processes:
      • Dendrites: Convey impulses to the cell body.
      • Axon: Convey impulses away from the cell body, ending in axon terminals that form synapses with other cells.
  • Myelin Sheath: Whitish, fatty insulating material that protects nerve fibers and increases transmission rate of nerve impulses.
  • Nodes of Ranvier: Gaps in the myelin sheath along the axon.
  1. Supporting Cells (Neuroglia):
  • Provide support and protection to neurons.
  • Types in the CNS: Ependymal cells, astrocytes, microglial cells, oligodendrocytes.

Reflexes

Definition:

  • Rapid, predictable, involuntary response to a stimulus.
  • Occur through neural pathways involving both the PNS and CNS (reflex arc).

Components of Reflex Arc:

  1. Receptor: Reacts to the stimulus.
  2. Sensory Neuron
  3. Interneuron
  4. Motor Neuron
  5. Effector: Muscle or gland being stimulated.

Types of Reflexes:

  • Somatic Reflexes: Involves skeletal muscles (e.g., withdrawal reflexes).
  • Autonomic Reflexes: Regulate activities of smooth muscles (e.g., pupil diameter control, salivary secretion).

The Brain

  1. Cerebrum:
  • Lobes:
    • Frontal Lobe: Primary Motor Cortex, Broca’s Area, Prefrontal Cortex.
    • Parietal Lobe: Primary Somatosensory Cortex.
    • Temporal Lobe: Primary Auditory Cortex, Hippocampus, Wernicke’s Area, Olfactory Area.
    • Occipital Lobe: Visual Cortex.
  1. Diencephalon:
  • Thalamus: Relays sensory impulses.
  • Hypothalamus: Regulates hormonal output, integrates autonomic functions.
  1. Brain Stem:
  • Divided into Midbrain, Pons, and Medulla Oblongata.
  • Plays a role in regulation of cardiac and respiratory function.
  1. Cerebellum:
  • Coordinates voluntary movement, balance, and equilibrium.

The Spinal Cord

Functions:

  1. Connects the PNS to the brain, transmitting sensory information to the brain and motor signals from the brain.
  2. Acts as a minor coordinating center for reflexes like the withdrawal reflex.

Peripheral Nervous System (PNS)

Anatomical Division:

  • Cranial Nerves: 12 pairs.
  • Spinal Nerves: 31 pairs.

Functional Division:

  • Sensory (Afferent) Nervous System:
    • Transmits sensory impulses from the periphery to the CNS.
    • Parts: Visceral sensory, somatic sensory, special sensory.
  • Motor (Efferent) Nervous System:
    • Transmits motor impulses from the CNS to the periphery.
    • Parts: Somatic nervous system, autonomic nervous system (sympathetic and parasympathetic).

Autonomic Nervous System

Functions:

  • Controls involuntary subconscious functions.
  • Maintains homeostasis by regulating internal environment.

Sympathetic Nervous System:

  • Activates the body’s “fight or flight” response during stress.

Parasympathetic Nervous System:

  • Activates the body’s “rest and digest” functions during relaxation.
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