13 patho

Organization and Control of Neural Function

Functions of the Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) is responsible for regulating involuntary physiological processes essential for survival. It operates largely below the level of consciousness and controls many vital functions.

• Regulates involuntary, unconscious physiological processes such as heart rate, blood pressure, digestion, breathing, and sexual arousal.

• Controls skeletal muscle movement indirectly via reflexes.

• Helps regulate cardiac and visceral smooth muscle activity.

• Enables the reception, integration, and perception of sensory information.

• Provides the substratum necessary for intelligence, anticipation, and judgment.

• Facilitates adjustment to an ever-changing external environment.

Components of the Nervous System

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

• CNS (Central Nervous System):

  • Consists of the brain and spinal cord, protected by the skull and vertebral column.

  • Responsible for computational and control functions.

• PNS (Peripheral Nervous System):

  • Located outside the CNS structures.

  • Functions as an input-output system, relaying information to the CNS and transmitting output messages to effector organs.

  • Includes the sympathetic and parasympathetic divisions.

Major Cells of the Nervous System

The nervous system is composed of two primary cell types: neurons and glial cells.

• Neurons: Functional information-processing cells responsible for transmitting electrical and chemical signals.

• Glial cells: Provide protection, support, and metabolic assistance to neurons.

Anatomy of Neurons

Neurons have specialized structures for signal transmission.

• Cell body (soma): Contains cytoplasm and the nucleus.

• Dendrites: Receive incoming signals.

• Axons: Transmit electrical impulses away from the cell body.

• Synaptic terminals: Contain synapses (electrical and chemical) for communication with other neurons.

• Myelin sheath: Insulates axons, increasing the speed of impulse transmission.

Energy Requirements of Nervous Tissue

The brain has high metabolic demands and requires a constant supply of oxygen and glucose.

• 15% cardiac output: The brain receives a significant portion of resting cardiac output.

• 20% oxygen consumption: Reflects the brain's complex functions and high metabolic rate.

• Cannot store oxygen or perform anaerobic metabolism: ATP production requires oxygen; without it, brain cells lose function within ~10 seconds, leading to loss of consciousness.

• Glucose: The major fuel source for the nervous system.

Characteristics of Neurons

Neurons transmit information via electrical impulses known as action potentials.

• Action potentials: Movement of electrical charge along the axon membrane; frequency conveys information.

• Na+ influx: In the soma, this is called depolarization.

• K+ efflux: Repolarizes the membrane.

• Phases of action potential: Depolarization, repolarization, and resting state.

• Saltatory conduction: Myelin sheath insulates axons, allowing impulses to "jump" between nodes, increasing speed.

• Damage to myelin sheath: Diseases like Multiple Sclerosis disrupt saltatory conduction, leading to symptoms such as numbness and weakness.

Types of Neurons:

• Afferent (sensory) neurons: Carry information to the CNS; located in dorsal columns.

• Efferent (motor) neurons: Carry information from the CNS to effector organs; located in ventral columns.

Structure of the Brain

The brain is organized into several regions, each with specialized functions.

• Hindbrain: Includes the medulla oblongata, pons, dorsal outgrowth, and cerebellum.

• Midbrain: Contains superior and inferior colliculi (dorsal enlargements).

• Forebrain:

  • Two hemispheres covered by the cerebral cortex.

  • Contains gray matter, basal ganglia, and diencephalon (thalamus and hypothalamus).

  • Thalamus: Acts as a relay station, helping auditory input reach the cortex; active during coma.

  • Hypothalamus: Supports survival functions (temperature, hunger, thirst, sleep cycles), but not language processing.

  • Hemispheric dominance: Typically, the left hemisphere controls the right side of the body and is associated with language, logic, and sequential thinking.

Cranial Nerves (CN)

There are twelve cranial nerves, each with unique functions and pathways.

• I. Olfactory: Sense of smell

• II. Optic: Vision

• III. Oculomotor: Eye movement, pupil control

• IV. Trochlear: Eye movement (down/inward)

• V. Trigeminal: Facial sensation, chewing

• VI. Abducens: Lateral eye movement

• VII. Facial: Facial expression

• VIII. Auditory (Vestibulocochlear): Hearing, balance

• IX. Glossopharyngeal: Taste, swallowing, salivation

• X. Vagus: Autonomic functions (heart rate, digestion, swallowing, speech)

• XI. Spinal accessory: Neck and shoulder movement

• XII. Hypoglossal: Tongue movement

Chemically Stable Environment

The brain maintains a stable chemical environment through specialized barriers.

• Selective permeability: Only water, carbon dioxide, and oxygen enter the brain easily; other substances are tightly regulated.

• Blood-brain barrier: Semi-permeable border cells allow essential nutrients (glucose, amino acids, oxygen, water) to pass, but keep plasma proteins out to prevent inflammation.

• CSF-brain barrier: Prevents harmful substances from entering the brain's fluid, protecting against pathogens.

• Both barriers work together to maintain neural function.

Neurotransmitters for the Two Divisions of the ANS

The ANS is divided into sympathetic and parasympathetic branches, each with distinct neurotransmitters and effects.

• Sympathetic Division:

  • Neurotransmitters: Norepinephrine and Epinephrine

  • Function: "Fight or flight" response; released during physical or emotional stress

  • Effects: Lung dilation, increased heart rate, blood vessel constriction, skin cooling, liver releases glucose, GI system slows, increased perspiration, pupil dilation

• Parasympathetic Division:

  • Neurotransmitter: Acetylcholine

  • Function: "Rest and digest"; dominates during quiet, non-stressful conditions

  • Effects: Constricted pupils, decreased heart rate and blood pressure, slowed breathing, increased peristalsis (digestion)

Sample Table: Comparison of Sympathetic and Parasympathetic Divisions

Division	Main Neurotransmitter	Primary Function	Key Effects
Sympathetic	Norepinephrine, Epinephrine	Fight or Flight	Increased HR, BP, pupil dilation, decreased digestion
Parasympathetic	Acetylcholine	Rest and Digest	Decreased HR, BP, pupil constriction, increased digestion

Key Equations

• ATP Production (Aerobic): $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Action Potential (Nernst Equation): $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

Example: Multiple Sclerosis

Multiple Sclerosis is a disease that damages the myelin sheath, disrupting saltatory conduction and leading to neurological symptoms such as numbness, weakness, and impaired coordination.

Additional info:

• These notes provide foundational knowledge for understanding neural function, relevant for students in physiology, neuroscience, and biomedical sciences.

• For microbiology students, understanding the nervous system is important for topics such as neurotropic infections, prion diseases, and host-pathogen interactions affecting neural tissues.