Nervous System Overview

Organization of the Nervous System

• General Functions of the Nervous System:
  • Sensory: Detect changes or stimuli (approx. 10 million).
  • Integrative: Evaluate stimuli and initiate response (approx. 20 billion; called interneurons or association neurons).
  • Motor: Carry out the response (approx. 0.5 million).

Major Anatomical Divisions

1. Central Nervous System (CNS): Comprises the brain and spinal cord.
2. Peripheral Nervous System (PNS): Involves spinal and cranial nerves connecting the CNS to various body parts.

Functional Divisions

• Afferent (Sensory) Pathways: Neural pathways that direct toward the CNS.
• Efferent (Motor) Pathways: Neural pathways that operate away from the CNS.

Afferent/Sensory Division Subdivisions

1. Somatic Sensory System: Receives nerve signals from skeletal muscles, skin, bones, and joints.
2. Visceral Sensory System: Receives signals from involuntary muscles (heart, lungs, stomach, bladder); known as the Autonomic Nervous System (ANS).

Efferent/Motor Division Subdivisions

1. Somatic Motor Division: Controls voluntary muscle movement.
2. Visceral Motor Division: Controls involuntary movements, which is further subdivided into:
   • Sympathetic Division: Activates the body’s 'fight or flight' response.
   • Parasympathetic Division: Manages 'rest and digest' activities including healing and repair.

Nervous System Cells

Glial Cells

• Glial Cells (or Neuroglia): Support neural function and continue dividing throughout life.
• Types of Glial Cells:
  1. Astrocytes:
     • Nourish, protect, and repair neurons.
     • Provide nutrients, oxygen, ions; form the Blood-Brain Barrier (BBB).
     • Perform phagocytosis on debris and remove excess neurotransmitters.
  2. Microglia: Immune cells that perform phagocytosis.
  3. Ependymal cells: Produce and circulate cerebrospinal fluid (CSF).
  4. Oligodendrocytes: Form myelin sheath in CNS.
  5. Schwann Cells: Myelinate axons in the PNS; assist in axonal regeneration.
  6. Satellite Cells: Protect neuron cell bodies in the PNS.

Neurons

• Neurons: The excitable cells of the nervous system.
• Features:
  • Cannot regenerate/divide after adolescence.
  • Require oxygen and glucose; high metabolic rate.
  • Include components like:
  1. Perikaryon: Cell body and axon hillock.
  2. Dendrites: Tree-like receptor branches.
  3. Axons: Long projections that transmit signals, may have collaterals.
  4. Teledendria: Branches at the axon’s end with synaptic knobs.
• Types of Neurons:
  1. Multipolar: Many dendrites (most common in CNS).
  2. Bipolar: One axon and one elongated dendrite (found in sensory organs).
  3. Unipolar: Single process from the cell body (usually sensory neurons).
  4. Anaxonic: Lacks axon; mainly in the brain and sensory organs.

Nerves and Tracts

• Nerves: Bundles of peripheral nerve fibers held by connective tissue. Connect CNS to sensory receptors, muscles, glands.
• Tracts: Bundles of nerve fibers within the CNS.
• White Matter: Myelinated axons (CNS tracts); appear white.
• Gray Matter: Unmyelinated axons, cell bodies, and dendrites; appear gray.

Reflex Arcs

• Pathways connecting sensory input to effector responses.
• Simple 2-neuron arc includes afferent and efferent neurons; 3-neuron arc adds interneurons.

Neuron Repair

• Neurons can regenerate if the cell body and neurilemma remain intact.
• Repair is more challenging in the CNS due to lack of neurilemma and scarring by astrocytes.

Nerve Impulse

• A signal comprising electrical fluctuations traveling through the neuron's membrane.
• Resting Membrane Potential: Neurons maintain a polarized state (approx. -70mV) via ion exchange and sodium-potassium pumps.
• Action Potential: A sudden increase in membrane potential that propagates along the axon.
  • Triggered by a stimulus opening sodium channels resulting in depolarization.
  • Followed by repolarization through potassium channels.
  • Refractory Periods:
  • Absolute: no stimulus can trigger an action potential.
  • Relative: only a strong stimulus can trigger an action potential.

Neurotransmitters

• Chemicals used for communication at synapses.

Major Neurotransmitters

1. Acetylcholine: Excitatory or inhibitory based on target tissue.
2. Serotonin: Primarily inhibitory; regulates mood and sleep; linked to depression.
3. Dopamine: Involved in pleasure and motor control; deficiency linked to Parkinson’s.
4. GABA: The main inhibitory neurotransmitter in the CNS.
5. Glutamate: Major excitatory neurotransmitter; deficiency associated with cognitive decline.
6. Endorphins: Natural painkillers.

Brain and Spinal Cord

Meningeal Layers

1. Dura Mater: Tough outer layer.
2. Arachnoid Mater: Cobweb-like middle layer.
3. Pia Mater: Delicate inner layer in contact with CNS surface.

Cerebrospinal Fluid (CSF)

• Protects the brain and spinal cord, removes waste, and circulates nutrients.

Spinal Cord Structure

• Conducts impulses to/from brain and integrates spinal reflexes.
• Contains dorsal roots (sensory) and ventral roots (motor).

Functional Areas of the Brain

• Cerebellum: Coordinates movements and maintains posture.
• Cerebrum: Higher brain functions like thought and action; divided into lobes (frontal, parietal, temporal, occipital).
• Brainstem: Controls vital reflexes and connects to spinal cord.

Autonomic Nervous System

• Sympathetic Division: Preps for 'fight or flight'.
• Parasympathetic Division: Promotes 'rest and digest' functions.

Conclusion

The nervous system is a complex network vital for communication, reflexes, movement, and information processing in the human body. Understanding its organization, cells, impulses, and functions is essential for grasping neurobiology and pathology.