5.7 Main Components of Nervous System

The human nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, both of which are hollow and protected by bone and membranes.

• Protection:
  • Bone: The brain is enclosed by the cranium, and the spinal cord is enclosed by the vertebral column.
  • Meninges: Three protective membranes cover the CNS:
    1. Dura mater: The tough outer layer, next to the bone.
    2. Arachnoid mater: The middle, web-like membrane.
    3. Pia mater: The delicate inner layer, adhering to the surface of the brain and spinal cord.
  • Cerebrospinal Fluid (CSF): Found between the arachnoid and pia mater, this fluid cushions the CNS from physical shock.

The Brain

The brain is the primary control center of the nervous system and is divided into three main parts.

1. Forebrain The largest and most complex part of the brain, consisting of the cerebrum, thalamus, and limbic system.

• Cerebrum:
  • Structure: The largest part of the brain, divided into two cerebral hemispheres connected by the corpus callosum. The surface, or cerebral cortex, is highly folded into ridges (gyri) and grooves (sulci).
  • Lobes: Each hemisphere has four lobes: frontal, parietal, temporal, and occipital.
  • Functional Areas:
    • Sensory Area: Receives impulses from the body.
    • Association Area: Interprets and analyzes information.
    • Motor Area: Controls voluntary responses.
  • Higher Functions: Responsible for memory, reasoning, judgment, thoughts, and dreams.
• Thalamus:
  • Function: Acts as a relay station for all sensory impulses (except smell), directing them to the appropriate regions of the cerebrum.
• Limbic System:
  • A set of structures involved in emotion, memory, and homeostasis.
  • Hypothalamus: Regulates homeostasis (hunger, thirst, body temperature, blood pressure) and controls the pituitary gland, linking the nervous and endocrine systems. Blood Pressure→
  • Amygdala: Controls emotions such as love, hate, anger, fear, and sexual arousal.
  • Hippocampus: Crucial for converting short-term memory into long-term memory.

2. Midbrain

• Function: Acts as a relay station for nerve tracts between the cerebrum, spinal cord, and cerebellum. It contains the reticular formation, which connects the hindbrain with the forebrain.

3. Hindbrain Located at the base of the brain, it controls vital bodily functions.

• Cerebellum: Controls equilibrium (balance), body position, and coordinates complex muscle actions like walking and running. It is also involved in learning and memory for behaviors.
• Pons: Acts as a bridge connecting the cerebellum, medulla, and cerebrum. It helps control the rate and pattern of heartbeat and breathing. Mechanism of Ventilation→
• Medulla Oblongata: Controls automatic functions such as heartbeat, blood pressure, respiration, and swallowing.

The Spinal Cord

A vital link that extends from the medulla oblongata down the vertebral column.

• Function: Transmits nerve impulses between the brain and the rest of the body.
• Structure:
  • A cross-section reveals an outer region of white matter (myelinated nerve fibers) and a central, H-shaped region of grey matter (neuron cell bodies and unmyelinated fibers). Neuron Structure→
  • The arrangement of white and grey matter is opposite to that in the brain.
  • A tiny central canal filled with CSF runs through the center.
  • Spinal nerves emerge from the dorsal (back) and ventral (front) roots of the grey matter.

Peripheral Nervous System (PNS)

The PNS consists of all the nerves that branch out from the CNS, connecting it to the rest of the body. It includes cranial and spinal nerves.

Cranial Nerves

• There are 12 pairs of cranial nerves that arise directly from the brain.
• They transmit signals between the brain and the head, neck, and torso, controlling functions like smell, sight, taste, hearing, and facial movement.

Spinal Nerves

• There are 31 pairs of spinal nerves that emerge from the spinal cord.
• Each spinal nerve is a mixed nerve, containing both sensory and motor fibers.
• They are categorized by the region of the vertebral column they emerge from:
  • 8 Cervical pairs
  • 12 Thoracic pairs
  • 5 Lumbar pairs
  • 5 Sacral pairs
  • 1 Coccygeal pair

Subdivisions of the PNS

The PNS is further divided based on function.

1. Somatic Nervous System:
   • Controls conscious and voluntary activities.
   • Connects the CNS to the skin and skeletal muscles.
2. Autonomic Nervous System (ANS):
   • Controls unconscious and involuntary activities.
   • Connects the CNS to visceral organs (heart, stomach, glands).
   • It has two opposing divisions:

Sensory Receptors and their Working

Specialized cells that detect stimuli from the internal and external environment. Receptors as Transducers→

• Olfactory Receptors (Smell): Chemoreceptors in the upper nasal cavity that are stimulated by dissolved chemicals (gases).
• Taste Receptors (Gustation): Taste buds on the tongue's papillae contain taste cells that detect four primary sensations: sweet, sour, salty, and bitter.
• Sensory Receptors in Skin:
  • Mechanoreceptors (Touch/Pressure):
    • Meissner's corpuscles & Merkel disks: Detect light touch.
    • Pacinian corpuscles: Detect deep pressure.
  • Thermoreceptors: Detect changes in temperature (cold and heat).
  • Nociceptors (Pain): Free nerve endings that respond to tissue damage or extreme stimuli.

Application: Nerve Gas and Acetylcholinesterase

• Nerve gases like sarin are potent inhibitors of the enzyme acetylcholinesterase.
• Normally, this enzyme breaks down the neurotransmitter acetylcholine in muscle synapses, allowing muscles to relax after contraction.
• By blocking the enzyme, nerve gas causes acetylcholine to build up, leading to continuous muscle contraction.
• If this affects the diaphragm, breathing becomes impossible, leading to death.