Active, fast, and highly coordinated, the human nervous system is your body's rapid-response communication network. It allows you to sense changes in your surroundings, process this information, and react in a fraction of a second.

In this section, we will break down how this complex electrical system is organized, examine the specialized cells that make it up, and explore how we study and treat the most complex organ of all: the brain.

What you'll learn

• How the nervous system coordinates rapid, automatic reflex responses to protect your body.
• The structures of sensory, relay, and motor neurones, and how electrical impulses travel across synapses.
• The structures of the brain and how modern scanning techniques (CT and PET scans) help us study it. (Triple Biology)
• The physical and biological limitations of treating damage and disease in the nervous system. (Triple Biology / Higher Tier)

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The Nervous System: Organised for Action

The human nervous system is divided into two primary parts:

1. The Central Nervous System (CNS): Composed of the brain and the spinal cord. The CNS is the control centre—it receives sensory information, processes it, and coordinates a response.
2. The Peripheral Nervous System (PNS): Made up of all the nerves that lie outside the CNS. These nerves connect the CNS to the rest of the body.

For the nervous system to coordinate a response, a clear pathway must be followed:

$$Stimulus⟶Receptor⟶Coordination\; (CNS)⟶Effector⟶Response$$

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Specialised Cells: The Three Types of Neurones

Electrical signals in the nervous system are carried by highly specialised cells called neurones.

You need to know the structure and function of three main types of neurones:

1. Sensory Neurones

Sensory neurones carry electrical impulses from receptor cells to the Central Nervous System.

• Structure: They have a long dendron that carries the impulse from the receptor to the cell body, and a short axon that carries the impulse from the cell body to the spinal cord (CNS).
• Cell Body: The cell body sits on a side branch halfway along the neurone.

2. Relay Neurones

Relay neurones are located entirely within the CNS (the spinal cord and brain). They act as connectors, linking sensory neurones to motor neurones.

• Structure: They have short dendrites that carry impulses to the cell body, and a short axon.

3. Motor Neurones

Motor neurones carry electrical impulses from the CNS to effectors (muscles or glands).

• Structure: They have short dendrites on the cell body inside the CNS, and a long axon that carries the impulse all the way to the effector.

Key Anatomical Features of Neurones

To understand how these cells transmit signals so fast, we need to look at three structural features:

• Dendrons / Dendrites: Branching fibres that receive electrical signals and carry them towards the cell body.
• Axons: Long, thin fibres that carry electrical impulses away from the cell body.
• Myelin Sheath: A thick, fatty layer that wraps around the axon and dendron of many neurones. It acts as an electrical insulator, preventing the signal from leaking out, and significantly increases the speed at which electrical impulses travel.

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Synapses: The Chemical Gaps

Neurones do not actually touch each other. Where two neurones meet, there is a tiny physical gap called a synapse.

Because electrical impulses cannot jump across this gap, the nervous system converts the electrical signal into a chemical one using chemical messengers called neurotransmitters.

How neurotransmission works at a synapse:

1. An electrical impulse travels along the first neurone (the presynaptic neurone) until it reaches the end of the axon.
2. This electrical signal triggers tiny storage sacs called vesicles to release chemical neurotransmitters into the synaptic gap.
3. The neurotransmitter molecules diffuse across the tiny gap down a concentration gradient.
4. The neurotransmitters bind to specific, complementary receptor molecules on the membrane of the second neurone (the postsynaptic neurone).
5. The binding of neurotransmitters to these receptors triggers a brand-new electrical impulse, which then travels down the second neurone.

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The Reflex Arc: Speed and Protection

A reflex is a rapid, automatic, and involuntary response to a stimulus. It does not require conscious thought, which makes it extremely fast. This speed helps protect the body from immediate harm (such as pulling your hand away from a hot stove).

The pathway that controls a reflex response is called a reflex arc. It bypasses the conscious parts of your brain:

1. A receptor detects a stimulus (e.g., pain or high temperature).
2. An electrical impulse is generated and travels along a sensory neurone.
3. The impulse reaches a synapse in the spinal cord, triggering the release of neurotransmitters.
4. These diffuse across the gap and trigger an impulse in a relay neurone inside the Central Nervous System.
5. The impulse crosses a second synapse to trigger an impulse in a motor neurone.
6. The motor neurone carries the impulse out of the CNS to an effector (a muscle or a gland).
7. The effector carries out the response (e.g., the bicep muscle contracts to pull your hand away).

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The Structure of the Brain (Triple Biology Only)

The brain is the major coordinating organ of the Central Nervous System, containing billions of interconnected neurones. For your exam, you must be able to identify and state the functions of three main regions of the brain:

1. Cerebral Hemispheres (Cerebrum):

   • Location: The highly folded outer surface making up the vast majority of the brain. It is split into two halves: the left and right cerebral hemispheres.
   • Function: Controls conscious thoughts, memory, language, intelligence, senses, and voluntary movements. Note that the left hemisphere controls the muscles on the right side of the body, and vice versa.

2. Cerebellum:

   • Location: Found at the lower back of the brain, below the cerebrum. It looks like a small, highly folded cauliflower.
   • Function: Coordinates muscle contraction, balance, posture, and the fine-tuning of precise physical movements.

3. Medulla Oblongata:

   • Location: Found in the brainstem, at the very base of the brain where it directly connects to the spinal cord.
   • Function: Controls unconscious, involuntary processes that keep us alive, such as heart rate, breathing rate, and automatic reflexes like swallowing, coughing, and sneezing.

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Investigating Brain Function (Triple Biology / Higher Tier Only)

Studying the brain is incredibly challenging because it is safely encased inside a thick, hard skull, and the tissue itself is delicate and easily damaged. Historically, scientists had to wait until a patient died to perform an autopsy or study patients who had suffered specific brain injuries.

To overcome these physical barriers, modern medicine uses non-invasive scanning techniques to investigate brain structure and function in living patients:

1. CT (Computerised Tomography) Scans

• How they work: CT scanners use a series of X-ray beams rotated around the head. A computer processes these X-rays to generate highly detailed, 3D cross-sectional images of the brain's physical structure.
• What they show: They show the physical, anatomical features of the brain. They are excellent for identifying bleeding (haemorrhages), blood clots, physical brain damage, or structural tumours.
• Limitation: A CT scan only shows structure, not active brain function. If a patient has a condition that does not alter physical brain structure (like some forms of epilepsy), a CT scan will look completely normal.

2. PET (Positron Emission Tomography) Scans

• How they work: The patient is injected with a small, harmless amount of a radioactive tracer (typically radioactive glucose). Active, respiring brain cells take up glucose rapidly. The scanner detects the radiation emitted by the tracer.
• What they show: PET scans show brain activity in real-time. Areas of high metabolic activity (where brain cells are working hard, or dividing rapidly like a growing tumour) take up more tracer and show up as bright "hotspots" on the screen.
• Advantage: PET scans allow doctors to see which parts of the brain are active while a patient performs specific tasks (such as speaking or reading) and can detect abnormal functional activity even when the physical structure of the brain looks perfect.

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Limitations in Treating Nervous System Damage (Triple Biology / Higher Tier Only)

Treating damage and disease in the brain and spinal cord is exceptionally difficult, and many injuries result in permanent disability. Doctors face several fundamental limitations:

• No Neurone Regeneration: Unlike skin, bone, or muscle cells, adult neurones in the Central Nervous System (brain and spinal cord) cannot easily replicate or repair themselves. If these neurones die or are severed, they do not grow back.
• Surgical Accessibility: The brain is shielded by the skull and the spinal cord is encased in the bony vertebral column. Navigating a scalpel through these protective bones to operate on delicate tissue is physically difficult and highly risky.
• The Blood-Brain Barrier: The capillaries supplying the brain are lined with highly selective membranes that prevent most chemicals and drugs in the blood from entering brain tissue. This barrier makes it incredibly difficult to deliver chemotherapy drugs to treat brain tumours.
• Risk of Collateral Damage: The brain is packed so densely with vital neurones that any surgical attempt to remove a tumour or repair tissue carries a massive risk of accidentally damaging healthy tissue, which could lead to permanent paralysis, personality changes, or blindness.

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