Introduction to the Nervous System
Welcome to your study notes for The Nervous System, an essential topic in Unit AS 7: Understanding the Physiology of Health and Illness for CCEA A Level Health and Social Care. Don't worry if physiology feels daunting at first! Think of the nervous system as your body's master electrical communication network. It constantly gathers information from the world around you, makes sense of it, and instructs your body on how to respond.
In Health and Social Care, understanding this system is vital because neurological conditions have profound physical, emotional, and social impacts on individuals and their care needs. Let's break everything down step-by-step!
1. Organisation of the Nervous System
The nervous system is divided into two primary structural components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
A. The Central Nervous System (CNS)
The CNS acts as the central command hub of the entire body. It consists of two main parts:
• The Brain: The main processing centre responsible for coordinating thoughts, memory, emotions, sensory input, and complex actions.
• The Spinal Cord: A long, tube-like structure of nerves that functions as a relay pathway, transmitting messages back and forth between the brain and the rest of the body.
B. The Peripheral Nervous System (PNS)
The PNS consists of all the nerves branching outside the CNS. It connects the central command hub to your limbs, skin, and organs. The PNS is subdivided into two functional systems:
1. Somatic Nervous System:
This system controls voluntary movements. For example, when you decide to lift a cup of tea or wave to a friend, your somatic nervous system carries the instructions to your skeletal muscles.
2. Autonomic Nervous System (ANS):
This system controls involuntary functions—automatic processes that keep you alive without conscious thought, such as your heart rate, breathing rate, and digestion. The ANS is further split into two opposing divisions:
• Sympathetic Division ("Fight or Flight"): Prepares your body for action and stress. It speeds up your heart rate, dilates airways, and elevates blood pressure.
• Parasympathetic Division ("Rest and Digest"): Calms the body down after stress. It slows the heart rate, lowers blood pressure, and aids digestion.
Memory Trick:
• Sympathetic = Stress & Speed (Fight or Flight)
• Parasympathetic = Peace & digestion (Rest and Digest)
Quick Review — Organisation of the Nervous System:
• CNS: Brain (processing) + Spinal Cord (relay pathway).
• PNS: All other nerves (Somatic = voluntary muscle control; Autonomic = involuntary organ control).
• Autonomic branches: Sympathetic (speeds up) vs. Parasympathetic (slows down).
2. Nerve Cells and Communication
How does information travel across this vast network? It happens through specialised cells called neurons (neurones).
A. Neurons
Neurons are the basic functional units of the nervous system. They carry information in the form of tiny electrical impulses.
Exam Focus: The axon (the long fiber of a neuron) is protected by a fatty layer called the myelin sheath. A common exam trap is saying that myelin "carries" the signal. Remember: myelin insulates the nerve fibre to speed up electrical transmission.
B. The Three Types of Neurons
Information travels along a specific chain involving three types of neurons:
1. Sensory Neurons (Afferent): Carry electrical impulses from receptors (e.g., in your skin, eyes, or ears) towards the CNS.
2. Relay Neurons (Interneurons / Association Neurons): Located entirely within the CNS (brain and spinal cord). They connect sensory neurons to motor neurons and process incoming signals.
3. Motor Neurons (Efferent): Carry electrical impulses away from the CNS to effectors (such as muscles or glands) to produce a response.
Direction of Impulses:
Receptor → Sensory Neuron → Relay Neuron (CNS) → Motor Neuron → Effector (Muscle/Gland)
Common Pitfall to Avoid:
Examiners frequently report students reversing the directions! Always remember: Sensory goes IN to the CNS; Motor goes OUT to effectors.
C. Synaptic Transmission
Neurons do not physically touch one another. Between the end of one neuron and the start of the next is a microscopic gap called a synapse.
How signals cross the synapse:
1. An electrical impulse reaches the end of the first neuron.
2. This triggers the release of chemical messengers known as neurotransmitters into the synaptic gap.
3. The neurotransmitters diffuse across the gap and bind to receptor sites on the next neuron.
4. This binding initiates a new electrical impulse in the receiving neuron.
Key Takeaway: Communication along a neuron is electrical, while communication between neurons across the synapse is chemical.
3. Neurological Disorders
For Unit AS 7, you must be able to explain the physiological causes of specific neurological disorders and describe their symptoms and effects on an individual.
A. Multiple Sclerosis (MS)
• Physiological Cause: Multiple Sclerosis is an autoimmune condition where the body's immune system mistakenly attacks and damages the protective myelin sheath covering nerve fibres in the CNS.
• Effect: This breakdown (demyelination) leaves scar tissue known as sclerosis. Without healthy myelin insulation, electrical signals become slowed, distorted, or blocked entirely.
• Common Symptoms: Chronic fatigue, numbness or tingling sensations in limbs, blurred or impaired vision, and mobility issues (such as unsteady gait or muscle weakness).
B. Parkinson’s Disease
• Physiological Cause: A progressive loss (degeneration) of neurons in a specific region of the brain called the substantia nigra. These neurons are responsible for producing the neurotransmitter dopamine, which coordinates smooth muscle movement.
• Effect: Low dopamine levels mean the brain cannot send clear, smooth instructions to the muscles.
• Common Symptoms: Resting tremors (shaking, often in the hands), bradykinesia (slowness of movement), and rigid muscles (muscle stiffness and reduced facial expressions).
C. Alzheimer’s Disease
• Physiological Cause: A progressive neurodegenerative condition characterised by an abnormal buildup of proteins in the brain, forming amyloid plaques and tau tangles.
• Effect: These abnormal protein structures damage brain cells and sever connections between neurons, leading to widespread loss of brain tissue.
• Common Symptoms: Short-term and progressive memory loss, confusion about time and place, and overall cognitive decline affecting decision-making and communication.
D. Stroke
• Physiological Cause: Occurs when the blood supply to part of the brain is interrupted. Strokes fall into two main categories:
1. Ischaemic Stroke: A blood clot blocks an artery supplying blood to the brain.
2. Haemorrhagic Stroke: A weakened blood vessel bursts, bleeding into surrounding brain tissue.
• Effect: Deprived of oxygen and essential nutrients, brain cells in the affected area rapidly die.
• Impact on Health: Depending on which side and area of the brain is affected, strokes can cause sudden weakness or paralysis (often down one side of the body), slurred speech, and loss of coordination.
4. Summary and Exam Checklist
To score top marks in your AS 7 examination, make sure you can confidently answer the following:
• Distinguish between CNS and PNS: CNS = Brain and spinal cord; PNS = Somatic (voluntary) and Autonomic (involuntary).
• Explain the two Autonomic divisions: Sympathetic (fight or flight) vs. Parasympathetic (rest and digest).
• Trace the reflex pathway: Sensory neuron → Relay neuron (in CNS) → Motor neuron → Effector.
• Define the synapse: The microscopic gap where neurotransmitters pass chemical signals between neurons.
• Clarify myelin's true role: It provides fatty insulation to accelerate electrical transmission.
• Summarise the 4 key disorders:
• Multiple Sclerosis: Autoimmune destruction of myelin (sclerosis); causes fatigue, tingling, vision loss, mobility problems.
• Parkinson's Disease: Loss of dopamine neurons in the substantia nigra; causes tremors, bradykinesia, rigidity.
• Alzheimer's Disease: Buildup of amyloid plaques and tau tangles; leads to memory loss, confusion, cognitive decline.
• Stroke: Interruption of blood flow (ischaemic clot or haemorrhagic bleed); causes brain cell death due to lack of oxygen.