Nervous, Endocrine & Skeletal Systems

Your body is a huge country, and it needs a fast postal service to carry orders from one place to another. That service is the nervous system. When you touch a hot pan and pull your hand back, a message has travelled from your finger to your spinal cord and back to your muscle in a fraction of a second. The wire that carries this message is the neuron (nerve cell), the structural and functional unit of the nervous system.

The impulse inside a neuron always travels one way: dendrite, cell body, axon, axon terminal. It never runs backwards, because the next neuron's dendrite can only receive and the axon terminal can only release. Between two neurons there is a tiny gap called the synapse (synaptic cleft). The electrical impulse cannot jump this gap, so it is changed into a chemical message: the neurotransmitter is released from the presynaptic terminal, crosses the gap and starts a new impulse in the next cell.

The nervous system has two main parts. The Central Nervous System (CNS) is the brain plus the spinal cord; the heart is not part of it. The Peripheral Nervous System (PNS) is all the nerves that run between the CNS and the rest of the body: 12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord. A part of the PNS called the autonomic nervous system runs the organs without our will; its sympathetic division prepares the body for fight or flight (heart beats faster, pupils widen) and its parasympathetic division calms the body down (rest and digest).

The spinal cord runs inside the vertebral column from the medulla down to the lumbar region, and in an adult it is about 45 cm long. Its inner core is grey matter shaped like a butterfly (H) and the outer layer is white matter; in the cerebrum it is the other way round. The spinal cord does two jobs: it carries messages between the body and the brain, and it is the control centre for reflex actions. Both brain and spinal cord are wrapped in three membranes called meninges (from outside to inside: dura mater, arachnoid, pia mater), and the space inside holds cerebrospinal fluid (CSF), which acts as a shock absorber for the brain.

A reflex action is a sudden, automatic response to a stimulus without any thought. It is fast because the message takes a short cut through the reflex arc and does not wait to reach the brain. Examples are the knee jerk, pulling the hand away from fire, blinking and sneezing. The brain is informed a moment later, which is why you feel the pain after you have already moved your hand. In a spinal reflex the spinal cord is the centre; in blinking and sneezing the centre is in the brain (cranial reflex).

The brain is the control room of the body. An adult brain weighs about 1.3 to 1.4 kg and is protected three times over: by the hard skull (cranium), by the three meninges, and by the cerebrospinal fluid that floats it like a cushion. To learn it without confusion, remember that the brain has three big divisions: forebrain, midbrain and hindbrain. Every question on the brain is really a question about which small part sits in which division and what it does.

The cerebrum is divided into two hemispheres joined by the corpus callosum. The left hemisphere controls the right side of the body and the right hemisphere controls the left side, because the nerves cross over. The folded outer layer, the cerebral cortex, is grey matter, and its folds increase the surface area so that more neurons fit inside the skull. Its four lobes are frontal (thinking, speech, voluntary movement), parietal (touch and pain), temporal (hearing and memory) and occipital (vision).

Grey matter and white matter are another favourite. Grey matter is made of neuron cell bodies, dendrites and synapses, so it is where information is processed (thinking, memory, decisions). White matter is made of myelinated axons, so it is the cabling that carries signals from one region to another. Injury, stroke or a tumour in a particular part of the brain therefore takes away exactly that part's function, which is how doctors map the brain.

The brain is blind and deaf on its own. It knows about the world only because sense organs turn light, sound, touch, taste and smell into nerve impulses. Each sense organ has its own receptors: photoreceptors for light, mechanoreceptors for touch and sound, chemoreceptors for taste and smell. Learn each organ as a story of a signal travelling inward.

The ear does two jobs: hearing and balance. Sound waves are caught by the outer ear (pinna and ear canal) and hit the eardrum (tympanic membrane). In the middle ear three tiny bones, the malleus (hammer), incus (anvil) and stapes (stirrup), pass the vibrations on and make them stronger. The Eustachian tube equalises air pressure on both sides of the eardrum. In the inner ear the snail-shaped cochlea turns vibrations into nerve impulses (the sensory part is the organ of Corti), and the semicircular canals and vestibule keep the body balanced. The stapes is the smallest bone of the human body. The human ear hears frequencies from 20 Hz to 20,000 Hz; below that is infrasound and above is ultrasound. How the brain reads the frequency of a sound as high or low is called pitch.

Every sense uses a receptor that matches the stimulus: rods and cones for light, hair cells of the cochlea for sound, taste buds (gustatory receptors) for taste, olfactory receptors for smell and touch receptors in the skin. The sense of smell is the only sense whose signals reach the cerebrum without first passing through the thalamus.

Nerves are the fast telephone lines of the body; hormones are the slow but long-lasting letters sent by post. A hormone is a chemical messenger made by a ductless (endocrine) gland and poured straight into the blood, which carries it to the target organ. Because there is no duct, the glands are called ductless. Hormones are needed in tiny amounts, yet too little or too much of any of them produces a disease, and that is exactly what examiners ask: gland, hormone, function, disorder.

The hypothalamus is the real boss: it sends releasing hormones to the pituitary, and the pituitary (the 'master gland') then orders the thyroid, adrenal cortex and gonads. Hormone levels are held steady by negative feedback: when the thyroid hormone in the blood is high, the pituitary makes less TSH, so the thyroid slows down. Similarly, when blood sugar rises the pancreas releases insulin, and when it falls the pancreas releases glucagon.

The skeleton is the frame of the body, like the steel frame of a building. It gives shape, protects soft organs (skull for brain, rib cage for heart and lungs, vertebral column for spinal cord), lets us move together with muscles, stores calcium and phosphorus, and its red bone marrow makes blood cells. A newborn has around 300 soft bones, many of them partly cartilage; as the child grows, several fuse and cartilage turns to bone, and an adult has 206 bones.

Bone is living tissue, hard because it holds calcium phosphate, and it is made and broken down all life long. Osteoblasts build bone, osteoclasts dissolve it, and osteocytes are the mature cells. Cartilage is softer and flexible (ear flap, nose tip, rings of the trachea, ends of bones) and has no calcium salts like bone. Ligaments join bone to bone; tendons join muscle to bone. Ossification means the deposition of calcium in cartilage to make bone, and because it follows a known pattern in age, an 'ossification test' of bones is used to estimate a person's age.

Muscles pull but never push, so they work in antagonistic pairs: when the biceps contracts the arm bends, and the triceps relaxes; for straightening the arm the roles swap. Inside the muscle fibre the contractile unit is the sarcomere, where actin and myosin filaments slide past each other using ATP and calcium. When muscles work hard without enough oxygen, lactic acid builds up and causes cramps and fatigue. The largest muscle of the body is the gluteus maximus (buttocks), and the smallest is the stapedius in the ear. There are more than 600 muscles in the human body.