Organization of the Human Nervous System

Organization of the Human Nervous System

The human nervous system is a complex network of over 100 billion cells, coordinating and regulating signals throughout the body. This system enables the body to react to external stimuli, facilitating both mental and physical actions.

Central and Peripheral Nervous Systems

There are three types of nerve cells:

  • Sensory Neurons (Afferent Neurons): Transmit sensory information from receptors to the spinal cord and brain.

  • Motor Neurons (Efferent Neurons): Transmit motor information from the brain and spinal cord to muscles and glands.

  • Interneurons: Found between other neurons, most numerous, predominantly in the brain and spinal cord, and linked to reflexive behavior.

Reflex Arcs

Neural circuits called reflex arcs control reflexive behaviors. For instance, if someone steps on a nail:

  1. Receptors in the foot detect pain and transmit the signal via sensory neurons to the spinal cord.

  2. Sensory neurons connect with interneurons in the spinal cord.

  3. Interneurons relay impulses to the brain and directly signal muscles in both legs.

  4. The individual reflexively withdraws the foot and shifts weight.

This occurs rapidly via reflex arcs, even before the brain fully processes the pain.

Overall Structure

The nervous system is divided into:

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS): Nerve tissue and fibers outside the brain and spinal cord. Includes:

    • 31 pairs of spinal nerves emanating from the spinal cord.

    • 12 pairs of cranial nerves emanating from the brain.

      • Olfactory (I) and Optic (II) nerves are structurally outgrowths of the CNS but part of the PNS.

The PNS connects the CNS to the rest of the body.

Peripheral Nervous System Subdivisions

The PNS is further divided into:

  • Somatic Nervous System: Sensory and motor neurons in skin, joints, and muscles.

    • Sensory neurons transmit information to the CNS via afferent fibers.

    • Motor impulses travel from the CNS to the body via efferent fibers.

  • Autonomic Nervous System (ANS): Regulates heartbeat, respiration, digestion, and glandular secretions.

    • Manages involuntary muscles associated with internal organs and glands.

    • Regulates body temperature through sweating or pyeloerection.

    • Functions are automatic and independent of conscious control.

Autonomic Nervous System

The ANS has two subdivisions that often act antagonistically:

  • Sympathetic Nervous System

  • Parasympathetic Nervous System

Parasympathetic Nervous System

  • Conserves energy; associated with resting and sleeping states.

  • Reduces heart rate and constricts the bronchi.

  • Manages digestion by increasing peristalsis and exocrine secretions.

  • Acetylcholine is the neurotransmitter responsible for parasympathetic responses.

Sympathetic Nervous System

  • Activated by stress (mild stressors to life-threatening emergencies).

  • Associated with rage and fear reactions (fight or flight).

  • Increases heart rate.

  • Redistributes blood to muscles of locomotion.

  • Increases blood glucose concentration.

  • Relaxes the bronchi.

  • Decreases digestion and peristalsis.

  • Dilates the eyes.

  • Releases epinephrine into the bloodstream.