Science Made Simple

Why do humans have protruding noses and how fast do signals travel down nerves?

We explore some of the curious questions that science can answer

Wednesday 11 August 2021 21:30 BST
No snout about it: our conks help us breathe easy
No snout about it: our conks help us breathe easy (Getty/iStock)

What is the evolutionary advantage to Homo sapiens in having a prominent nose, as most other primates have flat noses?

The nose serves two main purposes: to moisten the air we breathe and to warm it. Races from Equatorial regions tend to have flatter noses, as they don’t need to put extra moisture into the air they breathe, whereas people who live in desert countries tend to have longer noses as the air is dry and needs more moisture added. On the other hand, races from colder countries, such as Inuit people and Tibetans, have flatter noses because the importance of warming the air you breathe is outweighed by that of not losing heat through your extremities.

It is difficult to establish an exact evolutionary advantage, as selective pressures vary between people living in different environments. But the overall advantage compared with primates is that Homo sapiens roamed to practically every environment in the world – and adapted through selective pressures to survive in each particular environment.

How long does it take for the alcohol in a drink to reach your bloodstream?

On average, it takes 20 minutes for alcohol to reach the bloodstream after the first drink, and then approximately 1 hour per unit after this. This varies per person depending on their tolerance to alcohol, of course.

How many hairs are there in a human cochlea?

The cochlea, a part of the ear that helps us hear, has about 15,500 separate tiny hairs that wave from side to side sending signals to our brain.

What is myelin and why is it important to nerve cells?

In vertebrates, specialised cells called Schwann cells wrap themselves around the long thin bits of nerve cells in the peripheral nervous system. The Schwann cells form a thick insulating layer rich in lipids (a sort of fat) called the myelin sheath. This insulates the axon, rather like the plastic layer around a copper wire in an electrical flex. Nerve cells with myelin sheaths are called myelinated nerves.

Curiously, there is a type of mouse that has a genetic mutation that means it has no myelinated nerves. Without the insulation that myelin provides, nerve impulses passing along one nerve cell also affect nearby nerve cells, some of which connect to muscles. The affected mouse shivers and makes jerking movements as its muscles are stimulated.

People whose myelin sheaths are damaged as a result of multiple sclerosis can experience similar difficulties in controlling their muscles.

What causes hiccups?

Underneath your lungs is a large muscle called the diaphragm. When it contracts, it pulls down on the lungs, making you breathe in. Hiccups result from a nervous reflex that causes your diaphragm to contract suddenly, making you breathe in involuntarily – generating a peculiar sound.

Why do doctors on TV tap the knee of a patient to see if their leg jerks?

They do this to test whether the knee jerk reflex, which helps the body control the movement of the legs during running, is working properly. If it is, this gives the “doctor” information about how well the person’s nervous system is working, and they test to see if it is working on both sides of the body.

A reflex is a rapid, automatic response to a stimulus: pulling your hand away from a hot pan-handle is a reflex action.

A simple reflex involves communication between neurones in the peripheral nervous system and the spinal cord. The brain may be informed but does not take part in the response.

Reflexes can also help to coordinate complex muscular events, such as swallowing.

The knee-jerk is a “stretch” reflex – one of many that work together to help us maintain an upright posture. You can test it by holding your knee suspended, leg down, and tapping the leg just below the kneecap. Your lower leg should lift automatically.

Is it true that cold weather stimulates urination?

Cold weather itself would not directly stimulate urine production or excretion. But it does lead to reduced sweat production, so proportionally, more water has to be excreted via the kidneys – hence, more visits to the toilet.

How fast do signals travel down a nerve?

The speed at which a nerve impulse, or action potential, travels is known as its conduction velocity. In human nerve fibres, values range from 1 and 3 metres per second (2 to 6mph) in unmyelinated fibres (without a fatty sheath) and between 3 and 120m/s (6 to 270 mph) in myelinated fibres.

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