The Nervous System
You're at recess, you step on a sharp rock in your worn-out sneakers, and you lift your foot before you even realize it hurt. Something in your body decided to "pull the foot away" faster than you could think. That something is the nervous system, and on this page you'll see how it does it.
The Neuron, the Cell That Transmits Information
You touch a hot pot in the kitchen and pull your hand back in just a few hundredths of a second. What cells make up those "living wires" that can carry messages so fast?
The Myelin Sheath
The myelin sheath is a fatty covering that wraps the axon in segments, like the plastic that insulates a wire. It works as an insulator: the nerve impulse "jumps" from one uncovered segment to the next, and that is why it travels much faster than along an axon without myelin.
If the myelin is destroyed, as happens in multiple sclerosis, the impulse slows down, gets lost, or is diverted, like a current leaking out of a bare wire. Then the brain's commands reach the muscles poorly (movement problems) and information from the skin and the senses reaches the brain poorly (sensation problems).
Look at the image of the neurone. State which number points to the dendrites and which points to the axon, and explain the difference between the function of the former…
Look at the image of the neurone. State which number points to the dendrites and which points to the axon, and explain the difference between the function of the former and the latter in the transmission of the nerve impulse.
In the image of the neurone, a sheath covering sections of the axon is shown. Name it, explain its function, and reason why a disease that destroys it, such as multiple…
In the image of the neurone, a sheath covering sections of the axon is shown. Name it, explain its function, and reason why a disease that destroys it, such as multiple sclerosis, causes movement and sensory problems.
When the Synapse Fails or Is Altered
At the synapse, the neurotransmitter is released, binds to receptors on the next cell (another neuron or, for example, a muscle fiber), and is then removed: an enzyme breaks it down or it is taken back up. If something blocks the receptors, the signal does not get through. If something prevents the neurotransmitter from being removed, the signal repeats nonstop.
Analyze how two different toxins act on the neuromuscular synapse (the junction between the motor neuron and muscle fibers): Substance X (curare): Binds to the…
Analyze how two different toxins act on the neuromuscular synapse (the junction between the motor neuron and muscle fibers):
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Substance X (curare): Binds to the neurotransmitter receptors on the muscle cell and blocks them, preventing the neurotransmitter from interacting with them.
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Substance Y (nerve gas): Destroys the enzyme that breaks down the neurotransmitter after its release, causing it to remain indefinitely in the synaptic cleft.
- a) Predict what will happen to the muscles of a person exposed to substance X and to those of a person exposed to substance Y (will the muscles remain flaccid and paralyzed, or in permanent contraction and spasm?). Justify your answer based on how chemical synapses work.
- b) Although they act through completely opposite mechanisms on the muscle, explain why both substances cause death by asphyxiation if they affect the respiratory muscles.
How the Nervous System Is Organized
To keep so much information in order, the body organizes its cells in levels: neurons, nervous tissue, organs, and system. How is the work divided among those levels?
The Spinal Cord
The spinal cord runs inside the vertebral column. The nerves leave it at different levels: those from the upper part reach the arms and those from the lower part reach the legs. That is why an injury to the spinal cord leaves everything below the damaged point without sensation or movement, because messages no longer pass between that section and the brain.
Name the two main divisions of the nervous system and list at least two organs that form part of each of them.
Name the two main divisions of the nervous system and list at least two organs that form part of each of them.
Order the following structures of the nervous system from highest to lowest level of organisation, and explain your criterion: neurone, brain, nervous system, nervous…
Order the following structures of the nervous system from highest to lowest level of organisation, and explain your criterion: neurone, brain, nervous system, nervous tissue.
A motorcyclist has an accident and loses sensation and movement in his legs, but keeps those in his arms intact. Based on your knowledge of the spinal cord, reason in…
A motorcyclist has an accident and loses sensation and movement in his legs, but keeps those in his arms intact. Based on your knowledge of the spinal cord, reason in which part of the spinal column the injury probably occurred and why only the lower limbs were affected.
Sensory Nerves and Motor Nerves
When the dentist numbs your gum, you stop feeling the instrument but you can still move your mouth. How can you lose sensation without losing movement?
Describe the pathway of information in a simple voluntary movement, such as writing your name on paper: which nervous center gives the order, where the impulse travels,…
Describe the pathway of information in a simple voluntary movement, such as writing your name on paper: which nervous center gives the order, where the impulse travels, and which types of neurons are involved.
Voluntary Actions and Reflex Actions
To take a free kick on the field, you aim calmly and decide where to shoot, but if a ball is thrown at your face, you blink without having planned it. Why do some responses require a decision while others happen instantly?
Explain what a reflex action is and how it differs from a voluntary action. Give an example of each and state which part of the nervous system mainly coordinates each…
Explain what a reflex action is and how it differs from a voluntary action. Give an example of each and state which part of the nervous system mainly coordinates each type of response.
In a neurology consultation, the patellar reflex (tap on the tendon below the kneecap) and tactile sensitivity are examined in three patients, obtaining these results:…
In a neurology consultation, the patellar reflex (tap on the tendon below the kneecap) and tactile sensitivity are examined in three patients, obtaining these results:
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Patient A: Upon receiving the tap on the tendon, extends their leg automatically as normal, but states that they did not feel the touch or the tap.
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Patient B: Feels the tap completely clearly, but the leg does not move reflexively (although they can extend it if the doctor voluntarily asks them to do so).
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Patient C: Feels nothing and the leg does not move at any point either.
- a) Based on the elements that make up a reflex arc (receptor, sensory neuron, spinal cord, motor neuron, and effector) and its connection to the brain, reasonably deduce at what point in the nervous circuit the alteration may be located for each of the three patients.
- b) Why does a healthy person pull their foot back from a prick before becoming conscious of the pain?
Hormones: Messengers
The nervous system does not work alone. Between ages 11 and 14 your body changes (you grow, your voice changes, menstruation begins) and nobody "decides" for that to happen. Who gives those orders?
The endocrine system is made up of glands that produce hormones: chemical messengers that are released into the blood and travel to the organs that can detect them. Together, the nervous system and the endocrine system coordinate the body's functions.
| Nervous response | Hormonal response | |
|---|---|---|
| Messenger | Nerve impulse and neurotransmitters | Hormones |
| Pathway | Neurons and nerves | Blood |
| Speed | Very fast (fractions of a second) | Slower |
| Duration of the effect | Short | Long (minutes, days, or years) |
| Example | Pulling your hand away from a hot pot | The changes of puberty |
Sex hormones play a central role in sexual maturation: at puberty, the ovaries produce estrogens and the testes produce testosterone. These hormones cause the secondary sex characteristics and make the reproductive systems mature so they can fulfill their function in reproduction. Insulin, which regulates sugar in the blood, is another example of a hormone.
Addictive Substances and the Nervous System
What happens when a substance alters communication between neurons?
Later, in the section on habits, you will see their consequences for health, the economy, and society.
Name the two main divisions of the nervous system and list at least two organs that form part of each of them.
Name the two main divisions of the nervous system and list at least two organs that form part of each of them.
Go back to the beginning: when you stepped on the rock, the receptor in the foot sent the impulse along a sensory nerve to the spinal cord, and the spinal cord sent the command along a motor nerve to the muscle. It all happened before the brain registered the pain.