Response Systems

The Human Skeleton

The skeleton is divided into two parts:

Axial Skeleton (central axis):

• Skull: protects the brain
• Vertebrae: protect the spinal cord; intervertebral discs between vertebrae act as shock absorbers and allow flexibility
• Ribs (12 pairs): protect the heart and lungs
• Sternum (breastbone): protects the heart, attachment point for ribs

Appendicular Skeleton (limbs and girdles):

• Pectoral girdle: clavicle (collarbone) + scapula (shoulder blade) - attaches arms to axial skeleton
• Pelvic girdle: supports lower body weight, protects reproductive organs
• Upper limb: humerus (upper arm) → radius + ulna (forearm)
• Lower limb: femur (thigh) → patella (kneecap) → tibia + fibula (shin)

Synovial Joints

Moveable joints with the following structures:

• Cartilage: smooth, slippery tissue on bone ends - reduces friction, absorbs shock
• Ligaments: tough, elastic connective tissue - bone to bone, holds joint together
• Tendons: strong, inelastic connective tissue - muscle to bone, transmits force
• Synovial fluid: lubricates the joint, reduces friction
• Synovial membrane: produces synovial fluid

Antagonistic Muscle Pairs

Muscles can only pull (contract), not push. Therefore they work in antagonistic pairs:

• Biceps contracts → arm bends (flexion). Triceps relaxes.
• Triceps contracts → arm straightens (extension). Biceps relaxes.

Structure of the Nervous System

Central Nervous System (CNS): brain + spinal cord - processes information and coordinates responses.
Peripheral Nervous System (PNS): all nerves outside the CNS - sensory nerves carry impulses to the CNS, motor nerves carry impulses from the CNS.

A nerve is a bundle of nerve cells (neurons) grouped together, similar to individual wires in electrical cabling. Nerve cells do not undergo mitosis - if damaged, they cannot be replaced.

Neuron Structure

Each neuron has three main parts:

• Dendrites: short branching fibres that convert chemical messages to electrical impulses and direct them toward the cell body. Branched to increase surface area for receiving impulses.
• Cell body (soma): contains the nucleus and organelles. Produces neurotransmitter chemicals.
• Axon: long fibre that conducts electrical impulses away from the cell body.
• Schwann cells: wrap around the axon to form the myelin sheath.
• Myelin sheath: white fatty insulating layer that protects and electrically insulates the axon, speeding up conduction of impulses.
• Nodes of Ranvier: gaps in the myelin sheath where ions move in/out, enabling saltatory conduction (impulse 'jumps' between nodes).
• Synaptic knob: terminal endpoint of the axon containing neurotransmitter vesicles that store chemical messengers.

Types of Neurons

• Sensory neurons: carry impulses from receptors to the CNS. Cell body is on a side-branch off the axon.
• Motor neurons: carry impulses from the CNS to effectors (muscles/glands). Cell body at the end of the axon.
• Interneurons (relay neurons): connect sensory and motor neurons within the CNS. Cell body located midway along the axon.

The Brain - Structure and Function

The brain is protected by the skull. It is surrounded by three membranes called the meninges and cerebrospinal fluid (a clear liquid that protects against shock and provides nutrients).

The CNS has two regions: white matter (axons - carries impulses) and grey matter (cell bodies - processes information). In the brain, grey matter is on the outside; in the spinal cord, grey matter forms a butterfly shape in the centre.

The Spinal Cord

Controls most reflex actions. 31 pairs of spinal nerves connect the spinal cord to specific body regions. Each spinal nerve branches into:

• Dorsal root (back): carries sensory neurons to the spinal cord. Cell bodies lie in the dorsal root ganglion.
• Ventral root (front): carries motor neurons from the spinal cord.

A central canal filled with cerebrospinal fluid runs through the centre.

Neurotransmission - Mechanism of Impulse Transfer

Nerve impulses are electro-chemical: electrical along the axon, chemical across synapses.

1. At rest, the neuron membrane has a charge difference: inside negative, outside positive.
2. A threshold stimulus (minimum strength required) must be reached for an impulse to fire. Below threshold → no impulse.
3. All-or-nothing rule: once threshold is reached, the same impulse is carried regardless of stimulus strength.
4. Movement of ions reverses the charge (inside becomes positive) - this change travels along the axon to the synaptic knob.
5. At the synapse: impulse triggers neurotransmitter vesicles to release chemicals (e.g. acetylcholine) into the synaptic cleft.
6. Neurotransmitters diffuse across the cleft and lock into receptors on the post-synaptic neuron.
7. This causes an inrush of ions → new impulse generated in the next neuron.
8. Enzymes in the synaptic cleft digest the neurotransmitters, stopping the response. Products are recycled by the pre-synaptic neuron.
9. The neuron returns to its resting charges. The refractory period is the minimum time required before the neuron can carry another impulse.

The Reflex Arc

A reflex action is a rapid, involuntary, automatic response. The impulse bypasses the brain (goes only to the spinal cord) for faster protection:

1. Sensory receptor detects a stimulus (e.g. pain receptor in skin)
2. Sensory neuron carries impulse to spinal cord via dorsal root
3. Sensory neuron synapses with an interneuron in the CNS
4. Interneuron synapses with a motor neuron
5. Motor neuron carries impulse out via ventral root to an effector muscle
6. Muscle contracts → response (e.g. hand pulled away from hot object)

The brain can sometimes override a reflex (e.g. holding a hot plate to avoid dropping food) - but this must be learned.

Enzyme Inhibition at Synapses

Some medicines inhibit enzymes that break down neurotransmitters, increasing their levels and prolonging the response:

• Alzheimer's treatment: enzyme inhibitors enhance acetylcholine levels, improving nerve cell communication.
• SSRIs (for depression): block serotonin reuptake, keeping serotonin active longer.
• Nicotine: inhibits enzymes that break down neurotransmitters → overstimulation of receptors. Also raises dopamine → addiction.

Dopamine & Endorphins

Endorphins: neurotransmitters released by the pituitary gland and hypothalamus during pleasurable activities (eating, laughing, exercise). Known as the body's natural painkillers - they block nerves receiving pain signals, allowing the body to function in stressful situations.

Dopamine: the 'feel-good' neurotransmitter released following pleasurable activities. Creates a temporary 'dopamine rush' - explains why sugary foods are addictive. When the brain doesn't produce enough dopamine → Parkinson's disease (involuntary shaking), treated with L-dopa (converted to dopamine in the body).

Dopamine and endorphins work together: e.g. a runner experiences muscle pain → endorphins released to block pain → followed by a dopamine surge → 'runner's high' → encourages running again.

Addictive drugs (e.g. cocaine) interfere with normal dopamine recycling, allowing excessive dopamine to remain active.

Lifestyle factors: exercise (↑ both), dietary choices, sleep quality, social interaction all affect dopamine and endorphin levels.

Homeostasis

Homeostasis = maintaining a stable internal environment (e.g. body temperature, blood pH, water content, CO₂ levels). Uses negative feedback: change detected → response opposes change → returns to normal.

Components: receptors (detect change) → control centre (CNS/hypothalamus) → effectors (muscles/glands carry out response).

The Endocrine System

The endocrine system consists of glands that secrete hormones (chemical messengers) directly into the blood. Hormones travel via the bloodstream to target organs where they produce a response.

Major Endocrine Glands

Hormonal Manipulations

• Sport: anabolic steroids (synthetic testosterone) used illegally to build muscle - causes liver damage, heart problems
• Health: insulin injections for diabetes, HRT (hormone replacement therapy) for menopause, contraceptive pill
• Agriculture: growth hormones in livestock, plant growth regulators

Pathogens

Disease-causing organisms: prions, bacteria, protists, viruses, fungi, parasitic animals.

Innate Immunity (Non-Specific)

Immediate, does not improve with repeated exposure:

• Physical barriers: skin, mucus membranes, stomach acid, tears (lysozyme)
• Phagocytes (e.g. monocytes): engulf and destroy pathogens by phagocytosis
• Natural killer cells: destroy virus-infected cells and cancer cells
• Inflammation: increased blood flow brings white blood cells to infection site

Adaptive Immunity (Specific)

Slower to develop but has memory for faster future responses:

• B lymphocytes: produce antibodies (Y-shaped proteins) that bind to specific antigens on the pathogen surface → neutralise them
• T lymphocytes:
  • Helper T cells: activate B cells and other immune cells
  • Killer T cells: destroy infected cells directly
  • Suppressor T cells: switch off immune response when infection cleared
  • Memory T cells: provide long-term immunity

Acquired Immunity

• Active immunity: body makes its own antibodies - by infection or vaccination (weakened/inactive pathogen triggers memory cell production)
• Passive immunity: antibodies received from another source - breast milk (colostrum), antibody injections

Viral Replication

Viruses cannot reproduce on their own - they hijack host cells:

1. Virus attaches to host cell surface
2. Injects genetic material (DNA or RNA) into cell
3. Host cell machinery copies viral genes and makes viral proteins
4. New virus particles assembled
5. Host cell bursts (lysis), releasing new viruses

Infectious Disease Spread

Emergence and spread depends on: persistence in host, mutations, antibiotic resistance, population immunity, mobility of populations, R₀ (reproduction number - average number of people one infected person infects).

A stimulus is any change in the environment. A receptor detects it, the CNS processes the information, and an effector (muscle or gland) carries out the response. The nervous system works with the hormonal system to maintain homeostasis.

Two divisions

• Central Nervous System (CNS) - brain and spinal cord
• Peripheral Nervous System (PNS) - all the nerves that carry impulses to and from the CNS

Neurons (nerve cells)

Made of dendrites (receive impulses), a cell body with the nucleus, and a long axon (carries the impulse). The axon is often insulated by a myelin sheath broken by nodes of Ranvier, which lets impulses jump and travel faster. Mature neurons do not divide by mitosis, so damage is often permanent.

Impulse & synapse

At rest the inside of an axon is negatively charged. A stimulus at or above the threshold lets Na⁺ ions rush in, flipping the charge and creating a nerve impulse that travels along the axon. The refractory period is the short time before the neuron can fire again.

Where two neurons meet is a synapse. The impulse cannot jump the gap electrically, so neurotransmitter chemicals stored in vesicles are released into the synaptic cleft. They bind to receptors on the next neuron and start a new impulse. Enzymes then break the neurotransmitter down so the signal stops. Examples of neurotransmitters: dopamine (reward, movement) and endorphins (block pain signals - the body's natural painkiller).

Reflex arc

A very fast, automatic response that does not need the brain (e.g. pulling your hand off a hot plate). Pathway: receptor → sensory neuron → inter-neuron in spinal cord → motor neuron → effector. The brain is informed a fraction later.

The brain

The CNS is protected by bone (skull, vertebrae), three meninges membranes and cerebrospinal fluid that cushions and feeds it. Grey matter = cell bodies; white matter = myelinated axons.

An emerging infectious disease is one that has newly appeared in a population, or that already existed but is rapidly increasing in incidence or spreading into new geographic areas. A re-emerging disease is an old one returning, often because control measures slipped or resistance developed. Examples include COVID-19, Ebola, Zika, drug-resistant tuberculosis and Lyme disease.

Factors behind emergence

Zoonotic spillover: most new human pathogens come from animals. Clearing land, wildlife trade and intensive farming increase contact between people and animal reservoirs.

Travel and trade: an infected person can cross continents inside an incubation period, so a local outbreak becomes global quickly.

Population density: crowded cities, refugee camps and poor sanitation give a pathogen many close contacts.

Climate change: warmer conditions extend the range of vectors such as mosquitoes and ticks into new regions, including parts of Europe.

Antimicrobial resistance: overuse of antibiotics in medicine and agriculture selects for resistant strains, so treatable infections become dangerous again.

Pathogen mutation: viruses with RNA genomes mutate rapidly, producing variants that spread more easily or evade immunity.

Why spread is fast

Spread depends on how easily the pathogen transmits, how long a person is infectious before showing symptoms, how many contacts each person has, and how much immunity exists in the population. A pathogen that spreads before symptoms appear is far harder to contain, because carriers do not know to isolate.

Break the route of transmission

Hand washing, respiratory hygiene and face coverings interrupt droplet and contact spread. Clean water, sewage treatment and safe food handling stop faecal-oral spread. Vector control, such as removing standing water or using treated bed nets, cuts insect-borne disease. Sterilising equipment and disinfecting surfaces protect hospital patients.

Reduce contact between people

Isolation separates people who are infected; quarantine separates people who may have been exposed. Restricting travel and closing crowded settings buys time, but has social and economic costs, so measures are usually proportionate to the threat.

Protect the host

Vaccination gives active artificial immunity: memory cells respond rapidly on later exposure. Once enough of the population is immune, herd immunity protects those who cannot be vaccinated because the pathogen cannot find enough susceptible hosts. Good nutrition supports immune function, and antivirals or antibiotics treat those infected - antibiotics only work on bacteria, and the full course must be finished to limit resistance.

Find and track cases

Testing, contact tracing, surveillance of case numbers and compulsory notification of certain diseases let public health teams see an outbreak early and target measures where they will work. Public education matters because most of these measures depend on individual cooperation, and international bodies such as the WHO coordinate the response when a disease crosses borders.

Dopamine: the reward signal

Dopamine is released in the brain's reward pathway when we do something the brain treats as beneficial, and it also plays a role in motivation, focus and controlled movement. Exercise, achieving a goal, sleep and a balanced diet supply the raw materials and conditions for healthy dopamine signalling.

Nicotine, alcohol, cocaine and other drugs cause an unnaturally large dopamine release. The brain responds by reducing the number of receptors, so more of the substance is needed for the same effect (tolerance) and ordinary rewards feel flat. This is the biological basis of addiction, and the same pattern can occur with gambling and compulsive gaming or social media use. Long-term loss of dopamine-producing neurons is also what causes the tremor and stiffness of Parkinson's disease.

Endorphins: natural painkillers

Endorphins are released by the pituitary gland and hypothalamus. They bind to opioid receptors, reducing the perception of pain and producing a feeling of wellbeing, the "runner's high". Sustained exercise, laughter, music, massage and eating spicy or dark chocolate all raise endorphin levels.

Opioid drugs such as morphine and heroin fit the same receptors. With repeated use the body makes fewer of its own endorphins, so pain feels worse without the drug and withdrawal is severe.

The take-home message

Regular exercise, adequate sleep, good nutrition, social contact and managing stress support both systems naturally, while substance misuse hijacks and then depletes them.

A synapse is the junction between two neurons. The neurons do not touch: there is a tiny gap, the synaptic cleft, so the electrical impulse must be carried across chemically.

How transmission works

1. The impulse arrives at the end of the presynaptic neuron.
2. Calcium ions enter, and vesicles of neurotransmitter move to the membrane and release their contents into the cleft.
3. The neurotransmitter diffuses across the gap.
4. It binds to specific receptor proteins on the postsynaptic membrane, and a new impulse begins in the next neuron.
5. The neurotransmitter is then broken down by an enzyme or taken back up into the first neuron, so the synapse resets and the signal does not repeat endlessly.

Why synapses matter

Receptors are only on the postsynaptic side, so transmission is one way. Many neurons feed into one synapse, so signals can be added together or cancelled out, which is the basis of decision making and learning. Synapses are also the point where drugs and toxins act.

When transmission is disrupted

Because every step depends on a specific molecule fitting a specific receptor, a synapse is easily interfered with:

• Blocking reuptake or the breakdown enzyme leaves transmitter in the cleft, so the signal is stronger and lasts longer. Many antidepressants and stimulant drugs act this way.
• Blocking the receptor means the transmitter cannot bind and the signal is weakened or stopped. Some nerve toxins and anaesthetics work like this.
• Mimicking the transmitter: a drug with a similar shape binds to the receptor and triggers a response the body did not ask for, as opiates do at endorphin receptors.
• Too little transmitter: loss of the neurons that release dopamine in one region of the brain causes the tremor and stiffness of Parkinson's disease.

Repeated interference causes the brain to adjust, reducing its own receptor numbers or transmitter output, which is the biological basis of tolerance and dependence.