Control and Coordination · Lesson 6 of 11
How does the Nervous Tissue cause Action?
“A nerve sends the instruction, but the muscle still has to do the heavy lifting.”
- Trace information from a receptor to a responding muscle. - Explain how a nerve impulse activates a muscle fibre. - Describe how changes in muscle proteins shorten a muscle cell. - Distinguish nervous transmission from muscular contraction. - Compare voluntary and involuntary muscle action. - Identify muscles and glands as effectors.
A decision in the nervous system cannot move a book, close an eyelid or bend an arm by itself. The message must reach a tissue capable of producing a physical change. Muscles are important effectors because they convert nervous information into force and movement.
From Information To Action
A coordinated response can be viewed as an information pathway followed by an action pathway. Receptors collect information. Neurons transmit it. Coordinating centres process it and select a response. Motor neurons then carry instructions to effectors. A muscle responds by contracting, while a gland responds by releasing a secretion.
A muscle, gland or other structure that carries out a response after receiving an instruction from a coordinating system.
Muscle Movement
When an impulse reaches the terminal of a motor neuron, chemical messengers are released at the neuromuscular junction. The muscle fibre detects these chemicals and starts a series of internal changes. Special proteins in the muscle cell change their arrangement relative to one another. This makes the fibre shorter and produces contraction.
A muscle generally produces movement by pulling, not pushing. Body movements therefore often depend on groups of muscles acting in a coordinated way. One group contracts while another relaxes, allowing a joint to move in a controlled direction. When the pattern reverses, the body part can return towards its earlier position.
| Stage | Main event | Nature of change |
|---|---|---|
| Neuron | Impulse travels along the axon | Electrical |
| Neuromuscular junction | Messenger crosses the gap | Chemical |
| Muscle fibre | Contractile proteins change arrangement | Mechanical shortening |
Voluntary And Involuntary Muscles
Voluntary muscles can usually be activated through conscious decisions. Moving a chair, writing and raising an arm use such control. Involuntary muscles act without direct conscious command. Muscles in the digestive tract and around many small arteries continue to adjust their activity automatically. The distinction concerns how activity is controlled; both kinds still require organised cellular mechanisms to contract.
Problem
Trace the events from deciding to raise a hand to movement of the arm.
- 1.The brain forms a voluntary motor instruction.
- 2.Motor impulses travel through nerves to arm muscles.
- 3.Chemical transmission occurs at neuromuscular junctions.
- 4.Muscle proteins change arrangement and selected fibres shorten.
- 5.Coordinated contraction pulls the bones and raises the hand.
Problem
Why is an electrical impulse in a neuron not the same as a muscle contraction?
- 1.The impulse is a travelling change that carries information along the neuron's membrane.
- 2.At the muscle, the signal triggers chemical and molecular events.
- 3.The muscle's proteins rearrange, shortening the cell and producing force.
- 4.One process communicates; the other performs mechanical work.
Problem
A motor neuron is healthy but messenger receptors on a muscle fibre are blocked. Predict the outcome.
- 1.The motor impulse reaches the neuron ending normally.
- 2.Messenger molecules may be released into the junction.
- 3.Blocked receptors prevent the muscle from detecting the message effectively.
- 4.Protein rearrangement and contraction are reduced or absent, so the intended movement fails.
Muscles do not receive a conscious thought directly. They receive signals through motor neurons. Also, the neuron's electrical impulse carries information; the muscle fibre's shortening produces the physical movement.
Quiz
Which tissue performs the final mechanical action in most body movements?
What happens at a neuromuscular junction?
How does a muscle fibre shorten?
Which is an involuntary activity?
Which sequence correctly links communication to action?
Practice Problems
- Define an effector and give two examples. Model answer: An effector carries out a response; muscles contract and glands release secretions.
- Explain the role of the neuromuscular junction. Model answer: It transfers information chemically from a motor neuron ending to a muscle fibre, allowing the nervous instruction to initiate contraction.
- Describe movement at the cellular level in a muscle. Model answer: Special proteins alter their arrangement, causing the muscle cell to shorten and generate force.
- Compare voluntary and involuntary muscle action. Model answer: Voluntary action is initiated through conscious decision, whereas involuntary action is regulated automatically; both depend on coordinated contraction mechanisms.
- A person senses touch normally but cannot contract a particular muscle. Suggest two possible sites of disruption. Model answer: The motor pathway to the muscle or the neuromuscular junction and muscle fibre may be damaged; normal sensation suggests the sensory pathway is functioning.
Key Takeaways
• Nervous tissue collects, transmits and processes information; effectors perform the response. • A motor neuron communicates with a muscle at a neuromuscular junction. • Muscle contraction results from changes in the arrangement of special proteins. • Electrical transmission, chemical junctional signalling and mechanical contraction are distinct stages. • Coordinated groups of muscles produce controlled movement. • Voluntary and involuntary muscles differ mainly in how their activity is controlled.