Divisions of the Nervous System and Neural Pathways

Our nervous system consists of interconnected networks that allow for information processing and response coordination:

• The nervous system is organised hierarchically:
  • Central Nervous System
    • Brain - processes complex information
    • Spinal cord - relays messages between brain and body
  • Peripheral Nervous System
    • Somatic system - controls voluntary actions and conscious sensation
    • Autonomic system - regulates automatic functions through:
      • Sympathetic division - mobilises energy during stress
      • Parasympathetic division - conserves energy during rest

The sympathetic system prepares us for action by:

• Increasing heart rate and breathing
• Releasing glucose into the bloodstream
• Decreasing digestive activity

Meanwhile, the parasympathetic system promotes recovery by:

• Slowing heart rate and breathing
• Increasing digestive activity
• Conserving energy resources

Key Concept: Information Flow Neural pathways determine how signals travel through the nervous system. Converging pathways allow multiple inputs to affect a single output, like when different sensory signals contribute to a single perception. Diverging pathways enable one signal to trigger multiple responses. Reverberating pathways create feedback loops that can sustain activity even after the initial stimulus ends.

The Cerebral Cortex

The cerebral cortex forms the outermost layer of the brain and serves as the command centre for our most sophisticated mental abilities. It is responsible for conscious thought, memory storage and retrieval, and behaviour modification based on experience.

Brain function localisation shows that specific regions handle different tasks:

• Sensory areas process incoming information:

  • Receive signals from eyes, ears, skin and other sensory organs
  • Transform sensory impulses into meaningful perceptions
  • Alert us to changes in our environment

• Motor areas control movement:

  • Send instructions to muscles and glands
  • Coordinate voluntary actions
  • Act on information processed by association areas

• Association areas perform advanced mental processing:

  • Language comprehension and production
  • Personality development
  • Imagination and creativity
  • Logical reasoning and intelligence

Key Concept: Brain Hemispheric Specialisation The brain demonstrates contralateral organisation, with each hemisphere primarily controlling the opposite side of the body. The left hemisphere receives input from the right visual field and controls the right side of the body, while the right hemisphere does the opposite. The corpus callosum serves as the communication bridge between hemispheres, integrating information and allowing the brain to function as a coordinated whole.

Memory

Memory involves three fundamental cognitive processes that work together to store and access information:

• Encoding: Converting sensory information into a form the brain can store
• Storage: Maintaining encoded information over time
• Retrieval: Accessing stored information when needed

Our memory system works through a series of stages:

• Sensory memory:

  • Holds raw sensory impressions (visual, auditory)
  • Very brief duration $1-2 seconds$
  • Large capacity but most information is filtered out

• Short-term memory (STM):

  • Limited capacity ("memory span")
  • Temporary storage $15-30 seconds without rehearsal$
  • Processes information actively as working memory

• Long-term memory (LTM):

  • Potentially unlimited capacity
  • Can store information permanently
  • Strengthened through rehearsal, organisation and meaning-making

Information is lost from STM through:

• Displacement: When new information pushes out existing information
• Decay: When memory traces fade naturally over time

Key Concept: Memory Organisation The brain organises information into categories to make retrieval more efficient. Contextual cues play a crucial role in memory retrieval—these are environmental, emotional, or situational triggers associated with the original memory formation. This explains why smells, songs, or returning to specific locations can suddenly trigger detailed memories from the past.

The Cells of the Nervous System and Neurotransmitters at the Synapses

Neurons are specialised cells that form the communication network of the nervous system. Each neuron consists of:

• Dendrites: Receive signals from other neurons
• Cell body: Contains the nucleus and processes information
• Axon: Conducts electrical impulses away from the cell body

The myelin sheath is a fatty insulating layer that surrounds axons, significantly increasing the speed of nerve impulse transmission. This myelin is produced by glial cells, which provide structural and functional support to neurons.

The process of myelination (developing myelin sheaths) continues from birth through adolescence, explaining why:

• Nervous system control gradually improves with age
• Young children have slower responses to stimuli
• Coordination becomes more refined during development

Key Concept: Neurological Disorders Several diseases affect the myelin sheath, including multiple sclerosis (MS), polio, and Tay Sachs disease. When myelin is damaged, nerve impulses travel more slowly or may be blocked entirely, resulting in symptoms like loss of coordination, muscle weakness, and sensory disturbances.

Neurotransmitters are chemical messengers that:

• Are stored in vesicles within neurons
• Cross the synaptic cleft between neurons
• Bind to specific receptors on the receiving neuron
• Must be quickly removed after signalling through enzyme breakdown or reabsorption

The effects of neurotransmitters depend on the receptors they activate, producing either:

• Excitatory responses: Increasing the likelihood of generating an action potential
• Inhibitory responses: Decreasing the likelihood of generating an action potential

Weak stimuli typically fail to release sufficient neurotransmitters to reach the threshold needed for signal transmission, effectively filtering out low-level or unimportant stimuli.

Neurotransmitters and Drug Effects

Summation occurs when multiple axons release neurotransmitters simultaneously, collectively reaching the threshold required for impulse transmission. This allows a series of weak stimuli to trigger a response when individual stimuli cannot.

Important neurotransmitters in the brain include:

• Endorphins:

  • Natural painkillers produced by the body
  • Reduce pain intensity by stimulating specific neurons
  • Production increases during:
    • Intense physical injury
    • Sustained exercise
    • Emotional or physical stress
    • Consumption of certain foods (chocolate, chilli peppers)

• Dopamine:

  • Creates feelings of pleasure and satisfaction
  • Reinforces beneficial behaviours through the reward pathway
  • Critical for motivation and learning

Key Concept: Pharmacological Actions Drugs affecting the nervous system work through three primary mechanisms:

• Agonists bind to receptors and activate them, mimicking natural neurotransmitters
• Antagonists bind to receptors and block them, preventing natural neurotransmitter action
• Inhibitors prevent neurotransmitter breakdown or reuptake, prolonging their effects

These mechanisms explain two important phenomena:

• Drug addiction results from antagonists blocking receptors, causing sensitisation (increased receptor numbers and sensitivity), leading to cravings and dependence

• Drug tolerance develops from agonists repeatedly stimulating receptors, causing desensitisation (decreased receptor numbers and sensitivity), requiring escalating doses

Recreational drugs typically affect neurotransmission in the brain's reward centre, explaining their addictive potential and why their effects often diminish with continued use.

Non-specific Defence Body Defences

The body's non-specific defences act as the first line of protection against all types of pathogens, regardless of whether they've been encountered before.

Epithelial cells provide crucial initial protection:

• Form continuous physical barriers across body surfaces
• Prevent pathogens from accessing internal tissues
• Produce protective secretions including:
  • Tears - wash away irritants and contain lysozyme enzyme
  • Saliva - contains antimicrobial compounds
  • Mucus - traps particles and microorganisms
  • Stomach acid - creates a hostile environment for many pathogens

When pathogens breach these barriers, the inflammatory response is triggered:

• Causes swelling, redness, heat and pain at the affected site
• Initiated by mast cells releasing inflammatory chemicals
• Helps isolate and eliminate the infection

Key Concept: Cellular Defence Phagocytes are specialised white blood cells that patrol the body searching for invaders. When they encounter pathogens, they engage in phagocytosis - a process where they surround, engulf and destroy the pathogen using powerful digestive enzymes contained in cellular structures called lysosomes.

During infection, phagocytes release cytokines that:

• Act as chemical signals in the immune system
• Attract more phagocytes to the infection site
• Direct other immune cells to the area
• Help coordinate the overall immune response

These non-specific mechanisms work constantly to protect against invasion, buying time for the more powerful specific immune response to develop if needed.

Specific Cellular Defences

The specific immune response targets particular pathogens through specialised white blood cells called lymphocytes.

The two main types of lymphocytes are:

• B-lymphocytes:

  • Produce antibodies - Y-shaped proteins that bind to specific antigens
  • Each B-lymphocyte produces antibodies for just one type of antigen
  • Help neutralise pathogens circulating in body fluids

• T-lymphocytes:

  • Possess surface proteins that recognise self vs. non-self antigens
  • Destroy infected body cells by triggering apoptosis (programmed cell death)
  • Coordinate the broader immune response

Antigens are protein molecules found on cell surfaces that serve as identification markers. The immune system uses these to distinguish between:

• The body's own cells ("self")
• Foreign invaders $"non-self"$

Immune system malfunctions can lead to several conditions:

• Allergies: B-lymphocytes become hypersensitive to normally harmless substances (pollen, dust, certain foods)

• Autoimmune diseases: T-lymphocytes incorrectly identify self-antigens as threats and attack the body's own tissues (examples: rheumatoid arthritis, Type 1 diabetes)

Key Concept: Immunological Memory After fighting an infection, some B and T-lymphocytes become memory cells that remain in the body long-term. If the same pathogen invades again, these memory cells recognise it immediately and trigger a secondary immunological response that is both faster and stronger than the primary response. This forms the basis of immunity to diseases we've previously encountered.

HIV specifically targets and destroys T-lymphocytes, gradually weakening the immune system and leading to AIDS, where the body becomes vulnerable to opportunistic infections that normally wouldn't affect healthy individuals.

Immunisation

Immunisation is a preventive health strategy that develops immunity through vaccination using antigens from inactivated or weakened pathogens.

Vaccines often contain adjuvants, substances that:

• Enhance vaccine effectiveness
• Strengthen the immune response
• Allow for smaller antigen doses
• Improve vaccine stability

Herd immunity occurs when a sufficient percentage of a population is immunised against a disease, providing protection beyond just vaccinated individuals:

• Reduces pathogen circulation in the community
• Protects vulnerable individuals who cannot be vaccinated
• Prevents disease outbreaks and epidemics

The herd immunity threshold is the critical percentage of the population that must be immunised to prevent disease spread. This threshold varies based on:

• The disease's basic reproduction number (R₀)
• The vaccine's efficacy
• Population density and interaction patterns

Key Concept: Vaccination Barriers Multiple factors can hinder effective vaccination programmes:

• Economic barriers in developing countries prevent access to vaccines
• Vaccine hesitancy in developed countries reduces coverage rates
• Pathogen evolution can reduce vaccine effectiveness over time

Some pathogens employ antigenic variation as a survival strategy, changing their surface proteins to evade immune recognition. The influenza virus demonstrates this challenge effectively:

• Regularly changes its surface antigens
• Requires annual vaccine reformulation
• Necessitates yearly vaccination for vulnerable populations
• Remains a persistent public health concern despite vaccination efforts

This ability to change antigens explains why some diseases remain problematic despite extensive vaccination programmes.

Clinical Trials of Vaccines and Drugs

Vaccines and drugs must undergo systematic clinical trials to establish their safety and effectiveness before receiving regulatory approval. These trials follow specific scientific protocols to ensure reliable results.

Clinical trials employ three essential methodological elements:

• Randomised design:

  • Participants are assigned to groups using random selection
  • Prevents biased distribution of participant characteristics
  • Creates comparable groups that differ only in their treatment

• Double-blind methodology:

  • Neither researchers nor participants know who receives the treatment versus placebo
  • Eliminates conscious and unconscious bias in assessment
  • Ensures objective evaluation of outcomes

• Placebo control:

  • Treatment group receives the active substance
  • Control group receives an identical-appearing inactive substance
  • Provides a baseline for comparing true treatment effects

Key Concept: Statistical Analysis Clinical trials require properly sized participant groups to ensure statistical validity. Larger sample sizes reduce the impact of random variation and individual differences, making results more reliable. After the trial, researchers conduct statistical analysis to determine whether differences between groups are statistically significant - meaning the observed effects are unlikely to be due to chance alone.

This rigorous approach to testing is essential for protecting public health by ensuring that only treatments with demonstrated benefits and acceptable safety profiles reach the market.