The Nervous System

The Nervous System Overview

• Purpose:

  • Designed to receive, interpret, and react to stimuli from the external environment.

  • Plays a significant role in regulating and maintaining homeostasis of the internal environment.

Divisions of the Nervous System

• Main Divisions:

  • Divided into Central Nervous System (CNS) and Peripheral Nervous System (PNS).

  • Both systems have additional subdivisions that further classify their functions.

Neurons

• Basic Functional Unit:

  • Comprised of a soma (body), nucleus, and axons.

  • Axons:

    • Extensions of the soma that connect to other cells.

    • Connections made by axons are termed synapses.

Types of Neurons

• Sensory Neurons:

  • Contain specialized sensory receptors to transmit information to the CNS.

• Interneurons:

  • Primarily located in the CNS, input is received from sensory neurons and other interneurons, processing and relaying information to motor neurons.

• Motor Neurons:

  • Convey messages from the CNS to effector muscles, responsible for reactions in the body.

Neuroglia

• Supporting Cells:

  • Various cells that support neuron function and combat infection.

  • Outnumber neurons by a ratio of about 10 to 1.

Nerve Impulses

• Electrical Phenomenon:

  • Result from differences in electrical charges along neuronal membranes.

  • Sodium-Potassium Pump:

    • Aids in maintaining charge differences across the cell membrane, crucial for impulse transmission.

Action Potential

• Process:

  • Upon signaling, the soma end of the axon becomes depolarized.

  • Depolarization spreads down the axon, while the first membrane section begins to repolarize due to inactivation of Na+ channels and activation of K+ channels.

  • Continues along the axon till the end, triggering synaptic communication.

Synaptic Transmission

• Stages:

  1. Action potentials reach the axon terminal.

  2. Voltage-gated Ca2+ channels open, allowing Ca2+ to enter.

  3. Calcium signals induce synaptic vesicles to release neurotransmitters via exocytosis.

  4. Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell.

Reflex Arc

• Functionality:

  • A crucial neurological pathway that mediates reflex actions, allowing for quick responses to stimuli.

Breakdown of the CNS

• Components:

  • Cerebrum/Cerebral Cortex

  • Medulla Oblongata

  • Midbrain

  • Limbic System (including the Amygdala and Hypothalamus)

  • Spinal Cord

Cerebral Lobes

• Structure:

  • Composed of gray matter about 2-3 mm thick with numerous folds leading to the formation of cerebral convolutions, furrows, and cissures reflecting specialized functions.

  • Four main lobes: Frontal, Parietal, Temporal, Occipital.

  • Notable fissures:

    • Rolando Fissure: Separates frontal and temporal lobes.

    • Silvian Fissure: Divides the temporal lobe from other lobes.

    • Parieto-occipital Fissure: Distinguishes parietal from occipital lobe.

PNS: Cranial Nerves

• Characteristics:

  • 12 pairs of nerves arising directly from the brain, functioning without passing through the spinal cord.

  • Can include sensory, motor, or mixed types.

Autonomic Nervous System (ANS)

• Components:

  • Parasympathetic: Regulates maintenance functions across various organ systems.

  • Sympathetic: Prepares the body for stressful situations, initiating the fight-or-flight response.

PNS: Spinal Nerves and Dermatomes

• Functions:

  • Nerves originating from the spinal column, providing both sensory and motor functions.

  • Dermatomes: Skin areas innervated by specific spinal nerves, responsible for sensory information relay.

CNS: Blood Flow

• Major Arteries:

  • Includes cerebral anterior, middle, and posterior arteries, with the middle cerebral artery being most frequently involved in strokes.

Diseases of the Nervous System

• Strokes:

  • Affect brain through disrupted blood flow, with impacts on bodily function and sensation.

• Epilepsy:

  • Characterized by recurrent seizures due to irregular electrical discharges in the brain.

  • Symptoms range from loss of consciousness to myoclonic movements.

• Encephalitis:

  • Inflammation of the brain typically caused by viral infections, requiring hospital care and treatment (antiviral, antibiotic, antifungal).

• Dementia:

  • Disorders leading to progressive decline in higher cerebral functions (memory, language, reasoning), with Alzheimer’s being a common type.

Questions for Review

• What are the main divisions of the nervous system?

• What is a synapse? Explain how a signal travels through a synapse at a neuromuscular junction.

• How does a reflex arc work?

• What are the divisions of the ANS? What is the fight-or-flight response?

• What is a somatic reflex action? How does it differ from a normal reaction?