The Effector Organs
The nervous system decides on the response, but it doesn't carry it out: that is the job of the effector organs. Some act in a fraction of a second; others take days or months. Imagine you are riding on a motoconcho and the driver suddenly hits the brakes: your body grabs on instantly, but your heart pounds hard for several minutes. Why are there two response speeds?
Two types of effector organs
You are standing on the sidewalk and a horn blares very close by. You jump immediately, and then you notice your heart racing for quite a while. In the same situation, your body responds in two different ways.
Voluntary and involuntary muscles
Some muscles you control at will: the skeletal muscles, like the biceps or the leg muscles when you play baseball. Others work without your deciding: the heart, the muscles of the stomach and intestines, or the diaphragm when you breathe. The first are voluntary; the second are involuntary.
A typical involuntary response is shivering when it's cold: the muscles contract again and again without your deciding, and that contraction produces heat that helps maintain body temperature.
Muscles that work with other systems
When you run on a basketball court, your muscles use a lot of energy. To get it in their cells they need glucose and oxygen. The circulatory system delivers them through the blood, and the respiratory system takes in the oxygen. That is why your heart and breathing speed up: without them, the muscle would run out of fuel.
Name the two types of effector organs that execute the body's responses and give a specific example of how each acts in the same dangerous situation.
Name the two types of effector organs that execute the body's responses and give a specific example of how each acts in the same dangerous situation.
Explain the difference between voluntary and involuntary muscles, and give an example of each type.
Explain the difference between voluntary and involuntary muscles, and give an example of each type.
When it is very cold, we shiver without deciding to. Explain what type of response this is, which effector organs act, and why these contractions help to maintain body…
When it is very cold, we shiver without deciding to. Explain what type of response this is, which effector organs act, and why these contractions help to maintain body temperature.
During intense exercise, the heart beats faster and breathing speeds up. Explain why the work of the muscles requires the collaboration of the circulatory and…
During intense exercise, the heart beats faster and breathing speeds up. Explain why the work of the muscles requires the collaboration of the circulatory and respiratory systems.
Hormones: messengers in the blood
During puberty, your height, your voice, or your body mass change over months, with no nerve wires connected to each cell. How does the body deliver messages that are slow but able to act on such specific areas?
| Muscle | Endocrine gland | |
|---|---|---|
| How the command arrives | Nerve impulse | Hormone through the blood |
| Speed | Milliseconds | Seconds to days |
| Duration | Short | Long |
| Example | Jumping at the sound of a horn | Adrenaline, growth |
Hormones of puberty
The changes of puberty are also directed by hormones. In males, the testes produce testosterone: the voice changes, body hair appears, and muscle mass grows. In females, the ovaries produce estrogens and progesterone: the breasts develop and the menstrual cycle begins. These are the sex hormones: they make the reproductive organs mature and make it possible to produce gametes. You will see them in detail in the reproduction sections.
Control by negative feedback
How does a gland know when to stop? Many hormones regulate themselves. A region of the brain (the hypothalamus and the pituitary gland) sends a signal that stimulates the gland. When the hormone builds up in the blood, that signal is slowed down and the gland produces less. If the hormone level drops, the signal increases again. It works like a thermostat: the final result switches off the command that produces it.
That is why, if a person injects a hormone from outside the body, their own gland receives the signal to stop and, over time, produces less and may shrink in size.
Explain what hormones are, which organs produce them, and how they reach the target cells they act upon.
Explain what hormones are, which organs produce them, and how they reach the target cells they act upon.
Explain what an endocrine gland is and name two examples, indicating the hormone each one produces and its main effect.
Explain what an endocrine gland is and name two examples, indicating the hormone each one produces and its main effect.
Compare the body's muscular response and hormonal response: which is faster, which lasts longer, how the signal arrives in each case, and give an everyday example of…
Compare the body's muscular response and hormonal response: which is faster, which lasts longer, how the signal arrives in each case, and give an everyday example of each type.
A child grows several centimeters over a few months without noticing it, whereas standing up from a chair happens instantly. Compare the action of growth hormone with…
A child grows several centimeters over a few months without noticing it, whereas standing up from a chair happens instantly. Compare the action of growth hormone with that of a skeletal muscle in terms of speed, duration and the way the order arrives.
Bones, joints, and muscles: the locomotor system
A rigid bar snaps if you try to bend it, but your leg flexes with strength and smoothness when you walk or run the bases. How do hard pieces work together with soft tissues to allow movement without breaking?
The arm as a lever
When you bend your arm, the bone works as a lever: the fulcrum is the elbow joint, the effort is the force of the biceps, and the load is the weight you hold in your hand, for example a bottle of water or a gallon jug. Without a joint, the bone couldn't pivot even if the muscle pulled just as hard.
Muscles are attached to bones by tendons; at the joints, bones are held together by ligaments. Examples of joints: the elbow (flexion and extension, like a hinge) and the shoulder (movement in almost every direction).
State whether these structures are muscles, bones, or endocrine glands, and what effector function each performs: biceps, femur, thyroid, diaphragm, adrenal glands.
State whether these structures are muscles, bones, or endocrine glands, and what effector function each performs: biceps, femur, thyroid, diaphragm, adrenal glands.
Explain what a joint is and cite two examples of joints in the human body, indicating which movements each one allows.
Explain what a joint is and cite two examples of joints in the human body, indicating which movements each one allows.
Name the three parts of the musculoskeletal system and state the general function of each one in movement.
Name the three parts of the musculoskeletal system and state the general function of each one in movement.
Describe how bones, joints, and muscles work together when you bend your arm to lift a bottle of water, indicating what each element contributes to the movement.
Describe how bones, joints, and muscles work together when you bend your arm to lift a bottle of water, indicating what each element contributes to the movement.
Diagrams A and B represent the same arm with the two muscles that move the forearm, marked with ?: in each case, one appears thickened (contracted) and the other…
Diagrams A and B represent the same arm with the two muscles that move the forearm, marked with ?: in each case, one appears thickened (contracted) and the other stretched (relaxed). State which muscle (biceps or triceps) is contracted in each diagram and, taking into account that skeletal muscles can only pull and not push, explain why bending and straightening the forearm requires the coordinated action of both and what this type of muscle pair is called.
Taking care of muscles and bones
To reach old age with strong bones, it helps to exercise regularly (baseball, volleyball, walking), eat a diet with calcium and vitamin D (milk, cheese, sardines, moderate sun), and avoid cigarettes and alcohol.
State whether these statements are true or false and justify your answer: a) skeletal muscles move bones by pushing them; b) the skeleton also protects delicate organs;…
State whether these statements are true or false and justify your answer: a) skeletal muscles move bones by pushing them; b) the skeleton also protects delicate organs; c) hormones reach the organs they act on through the blood.
In an ankle sprain, the joint swells and hurts. Explain which structures of the joint can be damaged when the foot twists and why inflammation appears in the area.
In an ankle sprain, the joint swells and hurts. Explain which structures of the joint can be damaged when the foot twists and why inflammation appears in the area.
Children's bones bend more before breaking than those of older people. Relate this difference to the composition of bone at each age, and explain which habits in youth…
Children's bones bend more before breaking than those of older people. Relate this difference to the composition of bone at each age, and explain which habits in youth help to have strong bones in old age.
To study how the nervous system and the musculoskeletal system interact, three patients with different conditions in one leg are analysed after one month of progression:…
To study how the nervous system and the musculoskeletal system interact, three patients with different conditions in one leg are analysed after one month of progression:
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Patient A: has a leg immobilised by a cast following an ankle fracture (the motor nerves and muscles are intact, but the joint does not move).
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Patient B: has suffered a complete severing of the sciatic nerve that innervates the leg muscles (the muscles are disconnected from the spinal cord).
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Patient C: has the same nerve injury as Patient B, but receives daily sessions of external electrical stimulation applied directly to the skin over the muscle, causing it to contract.
- a) Predict in which of the three patients the most severe and rapid muscle atrophy will occur, and justify why Patient C better preserves muscle mass than Patient B despite having the same neurological injury.
- b) A student suggests: "To prevent Patient B's atrophy, it would be enough to stimulate the motor area of their brain with electrodes so that it sends stronger commands." Refute this statement by justifying the anatomical pathway of the motor response from the nerve center to the muscle effector.
Back to the beginning: on the motoconcho, your muscles react instantly to the nerve command, while the adrenaline from the adrenal glands travels through the blood and keeps your heart racing for much longer.