Mammals_Concept_6

Subject Facts

Types of muscle tissue

There are three types of muscle in the mammalian body:

Smooth muscles are what give the internal organs, skin and blood vessels their shape and tone. They are normally very elastic though in old age they become more brittle and begin to lose their elasticity.

Cardiac muscles are very special muscles that are only found in the heart. They seem to have a lattice of fibres, rather than bunches of parallel strands. Scientists are still trying to discover exactly how they work.

Skeletal muscles are the ones that can be flexed to cause movement, because they are easier to investigate than the other types, they are the focus of this section.

Muscle structure

Skeletal muscle tissue consists of long, fibrous muscle cells held together in bundles. Each muscle fibre is divided into many smaller filaments that fit together in a similar way to the fingers of two hands (see Figure 10). When relaxed, these filaments are quite well separated; but when stimulated, they slide closer together and bunch up, which shortens the effective length of the fibre. This causes the whole muscle to contract, resulting in a movement.

There are two forms of this type of muscle fibre: 'slow^’ and 'fast-twitch' fibres. Slow fibres are lighter in colour and have greater endurance; fast-twitch fibres are darker and react faster to stimuli, giving more immediate power Most skeletal muscles are made of a mixture of both types, but some will have more of one type than the other. Cheetahs have a lot of fast-twitch fibres in their muscles: they can sprint very fast, but not for long. Antelopes have more slow fibres: they can run quite fast for long periods of time. Olympic sprinters and tong-distance runners probably have different

Energy release

When skeletal muscles are used, they require significant amounts of energy. The blood system delivers glucose and oxygen to the muscles. Under normal, relatively restful conditions, enough oxygen is delivered to combine with the glucose to release sufficient quantities of energy for the muscles to work (aerobic respiration). In times of high energy requirements, such as running more energy is needed — so breathing becomes deeper and faster, and the heart rate increases to deliver more oxygen to the muscles. If more energy is needed than there is oxygen to convert, anaerobic respiration takes place. This is less efficient, and leads to the build-up of a waste product called lactic acid, which causes muscle fatigue, Fitness is often judged by recovery time: how long it takes to replace the *oxygen debt' (see Concept 3,).

Some of the energy released in skeletal muscles is not converted into movement, but released as heat. This is why running warms us up. It is also why we shiver when we are cold: the rapid muscle movements generate heat.

Making movements

All of these contracting and relaxing skeletal muscles are attached to bones across joints, resulting in a fairly effective levering system. Almost all muscles act in antagonistic pairs, so that once a bone is moved, it can be moved back (see Concept 2).

Why you need to know these facts

When children become aware that muscles are where most energy use in the body takes place, they can begin to understand the links between diet, fitness and exercise. In this way, understanding the role of the muscles leads to a deeper understanding of both respiration and digestion.

Vocabulary

Muscle — fibrous tissue capable of contracting and combining glucose with oxygen to release energy.

Antagonistic pair — two muscles attached to bones such that they can cause a movement and its reverse movement

Amazing facts

The human body has over 650 muscles, some of which we never use and have allowed to become weak (such as the ear-waggling muscles that everyone has).

Almost all of the meat that we eat is muscle tissue,

The biggest muscles in your body are in your buttocks: the gluteus maximus muscles.

Common Misconceptions

Stronger people have more muscles.

No, not more muscles: just more highly developed muscles. Exercising a particular muscle makes the body devote more tissue-building activity to that area, and so makes the muscle grow.

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Teaching ideas

Antagonistic muscle pairs (observing, modelling)

Ask the children to place a hand, palm up, under a desk and push up until they can feel pressure. They should note how the muscle at the front of their upper arm (the biceps) is stiff and the muscle at the back (the triceps) is flaccid Now ask them to place one hand on top of the desk and push down — the biceps will now be flaccid and the triceps rigid. The biceps flex to push up, the triceps to push down.

Feeling muscle movement (observing)

Ask the children to place their fingers against their head, just above the ears, then clench their teeth. They will be able to feel their jaw muscles become tense: these muscles are attached high up on the skull to allow for maximum flexibility of movement.