Body membranes and neuron structure

Two meanings of membrane

A cell membrane belongs to one cell. It is a phospholipid bilayer with proteins that controls what enters and leaves, and it is measured in nanometres. A body membrane is a macroscopic sheet of tissue, often several cell layers and their matrix, and it covers an organ, lines a cavity or forms a partition. The two are related — every body membrane is made of cells, and every cell has a cell membrane — but they answer different questions. When a clinician speaks of the mucous membrane of the nose, they mean a sheet of epithelium and connective tissue, not the boundary of a single cell. Keeping the two meanings separate prevents confusion when you read or write about membranes, and it makes the names of the body membranes easier to place.

For example: The cell membrane of one epithelial cell is invisible without a microscope. The mucous membrane lining the mouth is a visible sheet that you can touch and that renews itself continuously.

Common misconception: In anatomy, “membrane” does not always mean the boundary of a cell. Most named body membranes are sheets of tissue made of many cells.

Epithelial membranes

Epithelial membranes consist of an epithelium attached to an underlying layer of connective tissue, and there are three kinds. A mucous membrane, or mucosa, lines body passages that open to the exterior, such as the mouth, nose, digestive tract, airways and reproductive passages. It secretes mucus, which keeps the surface moist and traps particles. A serous membrane lines the closed cavities of the body and covers the organs inside them; it has a parietal layer on the cavity wall and a visceral layer on the organ, with lubricating serous fluid between them, as described in section 1.6. The pleura, pericardium and peritoneum are examples. The cutaneous membrane is the skin, which is exposed to the outside world; its epithelium is stratified and dry, and it protects the body rather than secreting mucus.

For example: The inside of your cheek is a mucous membrane: wet, renewing quickly and able to heal fast. The pleura around your lungs is a serous membrane: thin, lubricated and gliding with every breath.

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Connective-tissue membranes

Some membranes are made mainly of connective tissue rather than epithelium. A synovial membrane lines the cavity of a freely movable joint. It secretes synovial fluid, a slippery liquid that lubricates the joint surfaces, nourishes the cartilage and absorbs shock. Unlike epithelial membranes, it does not have a continuous epithelium covering its surface. The meninges are three connective-tissue membranes that cover the brain and spinal cord and enclose the cerebrospinal fluid. They protect the central nervous system, carry its blood supply in part and separate it from the bones around it. Fascia is a further connective-tissue sheet that wraps muscles, organs and nerves and holds them together. These membranes do not secrete mucus or serous fluid; their job is mechanical support, protection and lubrication of a different kind.

For example: A knee joint contains synovial fluid so the surfaces glide without friction; when the synovial membrane is inflamed, the joint becomes painful and swollen.

The parts of a neuron

A neuron has three main parts. The cell body, also called the soma, contains the nucleus and most of the cell’s organelles; it is the neuron’s control centre. Dendrites are short, branched extensions that receive signals from other neurons and carry them towards the cell body. The axon is a single long extension that carries the signal away from the cell body towards its target; it may be wrapped in a fatty myelin sheath that speeds conduction, with gaps called nodes of Ranvier between the wrapped segments. At its end, the axon branches into terminals that release chemical messengers onto the next cell. Because dendrites receive and the axon transmits, signals normally travel in one direction: dendrite → cell body → axon → terminal. This one-way arrangement is what allows nervous tissue to form reliable circuits.

For example: In a motor neuron, dendrites receive commands from the spinal cord, the cell body integrates them, and the axon carries the final signal all the way to a muscle fibre.

Types of neurons

Neurons are classified by the direction in which they carry information. Sensory neurons, also called afferent neurons, carry signals from receptors towards the central nervous system; they tell the brain and spinal cord what is happening. Motor neurons, or efferent neurons, carry signals from the central nervous system to muscles and glands; they produce action. Interneurons, or association neurons, lie within the central nervous system and connect sensory to motor pathways, allowing processing, comparison and coordination. This three-part arrangement explains the structure of a reflex: a sensory neuron detects the stimulus, an interneuron in the spinal cord processes it, and a motor neuron produces the response. Neurons can also be described by shape, but the functional classification is the more useful one in clinical practice because it predicts what a damaged neuron will affect.

For example: Touching a hot pan triggers a sensory neuron, an interneuron in the spinal cord and a motor neuron that pulls the hand away — all before the brain has consciously felt the pain.