Welcome to the Nervous System!
Welcome to your study notes for Unit AS 7: Understanding the Physiology of Health and Illness. The nervous system is your body’s lightning-fast communication network. It allows you to sense the world around you, think, feel emotions, move your muscles, and keep vital organs working automatically without you even having to think about it.
In Health and Social Care, understanding the nervous system is not just about memorising biological names; it is about understanding how our bodies work normally, what goes wrong during neurological illness, and how these conditions impact an individual's everyday physical, intellectual, emotional, and social life. Don't worry if the biology feels a bit daunting at first—we will break down every concept step-by-step!
1. Organisation and Divisions of the Nervous System
The human nervous system is organised in a clear hierarchy. Let's look at how it is divided from the top down.
A. Central Nervous System (CNS) vs Peripheral Nervous System (PNS)
1. Central Nervous System (CNS):
• Components: Made up of the brain and the spinal cord.
• Function: Acts as the central processing unit (like a computer's CPU). It receives incoming sensory information, processes and integrates data, stores memories, coordinates responses, and directs reflex actions.
2. Peripheral Nervous System (PNS):
• Components: Consists of all the nerves extending outside the CNS throughout the body (including cranial nerves from the brain and spinal nerves from the spinal cord).
• Function: Acts as the communication cables, carrying electrical impulses back and forth between the CNS and the rest of the body (receptors and effectors).
B. Divisions of the Peripheral Nervous System
The PNS is further divided based on the direction in which signals travel:
• Sensory (Afferent) Division: Carries electrical impulses towards the CNS from sensory receptors (like your eyes, ears, or touch receptors in your skin).
Memory Trick: Afferent signals Arrive at the CNS.
• Motor (Efferent) Division: Carries electrical impulses away from the CNS out to effector organs (muscles and glands) to cause an action.
Memory Trick: Efferent signals Exit the CNS.
The Motor Division is further split into two distinct systems:
1. Somatic Nervous System (SNS): Controls voluntary movements by sending impulses to skeletal muscles (for example, choosing to raise your arm or walk across a room).
2. Autonomic Nervous System (ANS): Involuntarily regulates internal organs, cardiac muscle, smooth muscle, and glands without conscious control (for example, controlling heart rate, digestion, and pupil dilation).
C. Subdivisions of the Autonomic Nervous System
The Autonomic Nervous System has two branches that work in balance with one another:
• Sympathetic Nervous System ("Fight or Flight"): Prepares the body for action, stress, or emergency situations. It increases heart rate, dilates the airways (bronchodilation) to get more oxygen to the muscles, dilates pupils to let more light in, and inhibits non-essential functions like digestion.
• Parasympathetic Nervous System ("Rest and Digest"): Calms the body down to conserve energy and promote maintenance activities. It slows down heart rate, constricts airways (bronchoconstriction), constricts pupils, and stimulates digestive activity.
Key Takeaway: The CNS (brain and spinal cord) processes information, while the PNS carries signals to and from the body. The autonomic branch controls involuntary actions, using the sympathetic system for fight-or-flight emergencies and the parasympathetic system for rest-and-digest recovery.
2. Microscopic Structure and Cellular Transmission
Nerve cells are called neurones. They are specialised cells designed to carry rapid electrical messages called nerve impulses.
A. Structure of a Typical Neurone
• Soma (Cell Body): The central part of the neurone containing the nucleus, cytoplasm, and essential organelles that keep the cell alive.
• Dendrites: Branch-like extensions spreading out from the cell body that receive chemical signals from other neurones and convert them into electrical impulses directed toward the soma.
• Axon: A long, slender nerve fibre that conducts electrical impulses away from the cell body toward target cells or adjacent neurones.
• Myelin Sheath: A protective, fatty insulating layer wrapped around the axon (produced by Schwann cells or oligodendrocytes). It insulates the axon and dramatically speeds up the transmission of impulses via saltatory conduction (where the impulse leaps along the fibre).
• Nodes of Ranvier: Small, unmyelinated gaps along the axon between neighbouring sections of myelin where the electrical impulse "jumps" forward.
• Axon Terminals / Synaptic Knobs: The swollen ends of the axon branches that house tiny sacs (synaptic vesicles) containing chemical messengers called neurotransmitters.
B. Three Functional Classes of Neurones
1. Sensory Neurone: Transports impulses from sensory receptors (e.g., in skin, eyes, nose) into the central nervous system.
2. Relay Neurone (Interneurone): Found entirely within the CNS. It acts as a connector, linking sensory neurones to motor neurones and processing information.
3. Motor Neurone: Carries impulses from the CNS to effector organs, such as muscles (causing contraction) or glands (causing hormone/enzyme secretion).
C. Synaptic Transmission (Chemical Signalling)
Neurones do not physically touch each other. The microscopic gap between two neurones is called the synapse (or synaptic cleft). Passing a signal across this gap happens chemically through these clear steps:
Step 1: Arrival of Action Potential
An electrical impulse (action potential) travels down the axon and reaches the pre-synaptic terminal (synaptic knob).
Step 2: Calcium Influx & Vesicle Fusion
The electrical charge causes voltage-gated calcium channels to open. Calcium enters the pre-synaptic bulb, causing synaptic vesicles to move toward and fuse with the pre-synaptic membrane.
Step 3: Neurotransmitter Release
The vesicles release chemical messengers called neurotransmitters (such as acetylcholine or dopamine) into the synaptic cleft by exocytosis.
Step 4: Diffusion & Receptor Binding
Neurotransmitter molecules diffuse across the fluid-filled synaptic cleft and bind specifically to matching receptor sites on the post-synaptic membrane (like a key fitting into a lock).
Step 5: Generation of a New Impulse & Inactivation
Binding triggers the opening of channels in the post-synaptic membrane, generating a new electrical impulse. Finally, the neurotransmitter is rapidly broken down by enzymes or reabsorbed by the pre-synaptic neurone (reuptake) to prevent continuous, uncontrolled stimulation.
Key Takeaway: Signals travel electrically along the axon of a neurone (accelerated by the fatty myelin sheath) and chemically across the synaptic cleft using neurotransmitters.
3. The Reflex Arc and Homeostasis
A reflex is a rapid, automatic, and involuntary response to a stimulus. Reflexes protect the body from immediate injury (such as instantly pulling your hand away from a hot stove) without waiting for the conscious brain to make a decision.
The Reflex Arc Pathway
The physical route taken by nerve impulses during a reflex is known as the reflex arc:
\(\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Sensory Neurone} \rightarrow \text{Relay Neurone (Spinal Cord)} \rightarrow \text{Motor Neurone} \rightarrow \text{Effector} \rightarrow \text{Response}\)
• Stimulus: A change in the environment (e.g., sharp pin or high heat).
• Receptor: Specialised sensory cells detect the stimulus and initiate an impulse.
• Sensory Neurone: Passes the impulse from the receptor into the spinal cord (CNS).
• Relay Neurone: Connects the sensory neurone directly to a motor neurone within the spinal cord.
• Motor Neurone: Passes the impulse out of the spinal cord to the target effector.
• Effector: A muscle or gland that carries out the physical action (e.g., arm muscle contracts).
• Response: The rapid protective action (e.g., withdrawing your hand).
Key Takeaway: Because reflex arcs pass through the spinal cord (bypassing conscious brain processing), the response is virtually instantaneous, keeping the body safe from harm.
4. Neurological Disorders and Physiological Malfunction
When the structure of neurones or the chemical balance of the nervous system is damaged, severe neurological disorders occur. Under Unit AS 7, you must understand both the underlying biology and the resulting symptoms.
A. Multiple Sclerosis (MS)
• Biological Cause: Multiple Sclerosis is a chronic autoimmune disorder. The body’s own immune system mistakenly attacks and destroys the protective myelin sheath surrounding axons in the central nervous system (brain and spinal cord).
• Physiological Malfunction: Without the insulating myelin, patches of scar tissue (known as sclerosis or plaques) form along the axons. This disrupts, slows down, or completely blocks the transmission of electrical nerve impulses.
• Physical Symptoms: Muscle weakness, spasticity (stiffness), loss of balance and motor coordination, chronic fatigue, numbness or tingling sensations, and visual disturbances (e.g., blurred vision or optic neuritis).
B. Parkinson’s Disease
• Biological Cause: A progressive neurodegenerative condition caused by the gradual loss and death of dopamine-producing neurones in a specific area of the brain called the substantia nigra (part of the basal ganglia).
• Physiological Malfunction: Dopamine is a vital neurotransmitter responsible for coordinating smooth, controlled body movements. As dopamine levels fall, the brain cannot properly regulate muscle activity.
• Physical Symptoms: Resting tremors (involuntary shaking, often starting in the hands), bradykinesia (extreme slowness of movement), muscle rigidity (stiffness), and postural instability (poor balance leading to falls).
Key Takeaway: Multiple Sclerosis involves immune destruction of the myelin sheath in the CNS, slowing impulse conduction. Parkinson's disease involves the loss of dopamine-producing neurones in the substantia nigra, leading to movement and coordination failure.
5. Holistic Impact on the Individual (The PIES Framework)
In CCEA Health and Social Care, you are expected to look beyond the medical biology and evaluate the wider impact of chronic neurological conditions on the whole person using the PIES framework (Physical, Intellectual, Emotional, and Social factors).
1. Physical Impacts:
• Progressive loss of mobility and gross/fine motor skills, leading to reliance on walking aids or wheelchairs.
• Severe, overwhelming fatigue that limits daily activities.
• Chronic pain, muscle spasms, and risk of incontinence.
• Increased risk of injury from falls due to poor balance or tremors.
2. Intellectual / Employment Impacts:
• Disruption of cognitive functioning, including slowed information processing, memory lapses, or difficulty concentrating.
• Inability to maintain full-time employment, leading to early retirement, loss of professional status, and significant financial strain/loss of income.
3. Emotional / Psychological Impacts:
• Elevated risk of clinical depression and anxiety regarding disease progression.
• Frustration, grief, and anger over the loss of physical independence and bodily control.
• Lowered self-esteem and altered self-image as physical appearance and abilities change.
4. Social and Relationship Impacts:
• Social isolation and withdrawal due to fatigue, mobility challenges, or embarrassment over symptoms (such as tremors or slurred speech).
• Strained family relationships and shifts in dynamic when a partner or child becomes an informal carer.
• Increased dependence on statutory care services (e.g., occupational therapists, district nurses, social workers) and voluntary support groups.
• Restricted access to leisure and recreational activities due to inaccessible venues or transport barriers.
6. Exam Pitfalls and Revision Checklist
Make sure you avoid these common mistakes highlighted in CCEA examiner reports:
• Don't mix up Sympathetic and Parasympathetic: Remember that Sympathetic creates Stress responses (increases heart rate, inhibits digestion), while Parasympathetic creates Peaceful responses (lowers heart rate, promotes digestion).
• Don't reverse Afferent and Efferent: Sensory (afferent) goes in to the CNS; Motor (efferent) goes out from the CNS.
• Give the biological cause, not just symptoms: If asked about Multiple Sclerosis, clearly state that it is an autoimmune attack on the myelin sheath. If asked about Parkinson's, state that it is the degeneration of dopamine-producing neurones in the substantia nigra.
• Always use PIES for extended writing: Whenever an exam question asks about the impact of a condition on the individual, balance your answer across Physical, Intellectual, Emotional, and Social effects to achieve top-band marks.