Your Knee Kicks Out Before Your Brain Even Knows It Was Tapped

Solving a maths problem, keeping your balance on a bicycle and keeping your heart beating all depend on the brain — but different parts of it. Some responses do not wait for the brain at all: the spinal cord handles them directly.

This part covers the main parts of the brain, the structure of the spinal cord, the reflex arc, and the difference between voluntary, involuntary and reflex actions.

The cerebrum controls thinking and voluntary actions, the cerebellum controls balance and coordination, the medulla controls vital involuntary actions, the thalamus relays sensory messages, the hypothalamus regulates temperature, hunger and thirst, and the pons relays messages and helps control breathing.

The brain is protected by the skull, three membranes called meninges, and cerebrospinal fluid that cushions it.

- Cerebrum — the largest part, with two folded hemispheres; thinking, memory, intelligence, speech, voluntary movement and interpreting what the senses detect
- Thalamus — below the cerebrum; relays sensory impulses such as pain, touch and temperature to the cerebrum
- Hypothalamus — below the thalamus; controls body temperature, hunger, thirst, sleep and emotions, and controls the pituitary gland
- Cerebellum — at the back, below the cerebrum; maintains balance and posture and coordinates muscle movements
- Pons — in front of the cerebellum; relays impulses between parts of the brain and helps regulate breathing
- Medulla oblongata — joins the spinal cord; controls heartbeat, breathing, blood pressure, swallowing, coughing, sneezing and vomiting

An everyday example. Riding a bicycle along a narrow village lane without wobbling is the cerebellum's work; deciding which turning to take is the cerebrum's.

The boundary case. Serious injury to the medulla can be fatal, because the centres for breathing and heartbeat are there.

The spinal cord is a long cylinder of nerve tissue running down through the backbone, with butterfly-shaped grey matter inside and white matter outside — the reverse of the brain, where grey matter forms the outer layer.

Structure:

- Runs from the medulla down a canal inside the vertebral column, which protects it
- Covered by meninges and bathed in cerebrospinal fluid
- A central canal runs down its middle
- **$31$ pairs of spinal nerves leave it; each joins by a dorsal root carrying sensory fibres and a ventral root carrying motor fibres

Grey and white matter:

- Grey matter — mainly cell bodies of neurons, without myelin
- White matter — mainly myelinated axons; myelin gives the white colour
- Spinal cord — grey matter inside, white matter outside
- Brain — grey matter outside as the cortex, white matter inside

Functions: it conducts impulses to and from the brain and controls reflex actions.

An everyday example. A spinal injury in a road accident can cause paralysis below the injury, because messages can no longer pass that point.

The link. Relay neurons in the grey matter** are where the reflexes in the next section are processed.

A reflex action is a quick, automatic response to a stimulus, carried out along a reflex arc from receptor to effector without conscious thought; natural reflexes are inborn, while acquired reflexes are learned through repetition.

The reflex arc — touching a hot object:

$$\text{receptor in skin}\to \text{sensory neuron}\to \text{relay neuron in spinal cord}\to \text{motor neuron}\to \text{effector muscle}$$

- Receptor — heat receptors in the skin detect the stimulus
- Sensory neuron — enters the spinal cord through the dorsal root
- Relay neuron — in the grey matter, links sensory to motor neuron
- Motor neuron — leaves through the ventral root
- Effector — arm muscle contracts and pulls the hand away

Natural reflexes — present from birth, need no learning: blinking, sneezing, coughing, the knee jerk and a baby suckling.

Acquired reflexes — learned by practice until they become automatic: mouth watering at the smell of a favourite dish, braking at a red light while cycling, typing without looking.

An everyday example. A doctor taps just below the kneecap and the leg kicks forward — a quick check that the reflex arc through the spinal cord is working.

The substance. You feel the pain only after your hand has moved, because the message reaches the brain after the spinal cord has already acted.

Voluntary actions are done by choice under the control of the cerebrum, involuntary actions happen continuously without our will under the medulla and autonomic system, and reflex actions are sudden automatic responses to a stimulus controlled mainly by the spinal cord.

Compared:

- Voluntary — under conscious control; cerebrum; can be started or stopped at will; examples: writing, walking, speaking
- Involuntary — not under conscious control; medulla and autonomic system; continuous; examples: heartbeat, peristalsis, secretion of digestive juices
- Reflex — automatic, rapid, triggered by a stimulus; spinal cord or brain; examples: withdrawing a hand from a flame, blinking when dust approaches

An everyday example. You can hold your breath while diving into a pool, but after a short time breathing restarts on its own.

The boundary case. Breathing is both voluntary and involuntary — you can control it for a while, but the medulla takes over to keep you alive.