The Nervous System

Right now your body is measuring the salt in your blood, the pressure in your arteries, the angle of your knees and the temperature of the air. You are aware of almost none of it. The nervous system is doing two jobs at once: giving you a conscious picture of the world, and quietly running the parts you never think about.

The brain as an integration organ

The brain is billions of interconnected neurones, and different regions do different jobs. You are not expected to know fine detail — you are expected to match a region to a function without hesitating.

• Cerebral cortex — the outer layer, divided into two hemispheres and highly folded. Handles the higher-order processes: intelligence, memory, consciousness, personality.
• Cerebellum — sits underneath the cortex. Responsible for balance, muscle coordination and movement.
• Brainstem — relays messages between the cortex, the cerebellum and the spinal cord. The key part is the medulla, which controls unconscious activities such as heart rate and breathing.
• Hypothalamus — regulates body temperature, and produces hormones that control the pituitary gland.
• Pituitary gland — produces many hormones, including FSH and LH for the menstrual cycle.

Input: what the brain is listening to

Before the brain can coordinate anything it needs information. That comes from receptors, and it is worth splitting them by whether you are aware of them.

The spinal cord as an integration centre

The spinal cord is part of the CNS, and most of the time it acts as a cable between the body and the brain. But it can also process information on its own, without the brain, and that is what makes it an integration centre.

Sensory information arrives along a sensory neurone, is processed immediately, and leaves along a motor neurone. That pathway is a reflex arc. The brain is not involved in the decision, so this is unconscious control directed by the spinal cord alone.

There are two tissue types inside it:

• White matter — mainly the axons of neurones carrying information to and from the brain.
• Grey matter — the cell bodies, relay neurones and synapses that do the integrating and produce the reflex response.

If information is processed at the conscious level instead, the cerebrum is involved. That is the difference between flinching and deciding.

Input through sensory neurones

Every neural pathway begins with a receptor: a specialised cell that detects a change in the environment. Receptor cells are transducers. They take energy in one form — light, heat, sound, a chemical — and convert it into an electrical impulse in a sensory neurone.

Not all receptors are the same kind of thing. Some, such as photoreceptors in the eye and chemoreceptors in the taste buds, are separate specialised cells that influence a nearby sensory neurone. Others, such as some touch receptors, are simply the bare endings of the sensory neurone itself.

Stimulating a receptor

When a receptor cell is stimulated it becomes depolarised. Whether anything happens next depends on how strong the stimulus was:

• Weak stimulus — the cell is not sufficiently depolarised, and no impulse is sent.
• Strong enough stimulus — an action potential is initiated in the sensory neurone and the impulse travels to the CNS.

The taste bud example

This is the sequence IB uses, so learn it in order. The tongue is covered in bumps called papillae, each covered in taste buds, each containing chemoreceptors covered with receptor proteins. Different receptor proteins detect different chemicals; the salt ones respond directly to sodium ions.

• Sodium ions diffuse through highly selective channel proteins in the membranes of the microvilli of the chemoreceptor.
• The chemoreceptor membrane depolarises. The rise in positive charge inside is called the receptor potential.
• If the receptor potential is large enough, voltage-gated calcium ion channels open.
• Calcium ions enter the cytoplasm and trigger exocytosis of vesicles of neurotransmitter from the basal membrane.
• The neurotransmitter stimulates an action potential in the sensory neurone, which carries the impulse to the brain.

Output through motor neurones

Once the CNS has processed the information, the cerebrum uses it to plan movement — the region responsible is the motor cortex. Motor neurones then carry action potentials to muscles.

A motor neurone ends at a neuromuscular junction (also called a motor end plate), between the terminal branches of the neurone and a muscle cell. There are many of them spread across the muscle fibres, and they work in almost exactly the same way as a synapse.

Worked examples