Control and Coordination · Lesson 2 of 11
Animals – Nervous System
“Neurons pass messages quickly, but thankfully without endless notifications.”
- Identify receptors and explain how they detect environmental information. - Describe the structure and function of a neuron. - Trace an impulse from a dendrite to an axon ending. - Explain how a synapse changes an electrical signal into chemical communication and back again. - Relate neurons, muscles and glands through neuromuscular and neuroglandular junctions. - Investigate how smell contributes to the perception of flavour.
When a fingertip meets a hot surface, the skin does not send heat itself towards the brain. Instead, specialised cells detect the change and convert it into information the nervous system can carry. That information travels through nerve cells in a definite direction and eventually reaches another neuron, a muscle or a gland. The entire process depends on the structure of the neuron.
Receptors And Detection Of Stimuli
The specialised tips of certain nerve cells detect information from the environment. These receptive endings are commonly associated with sense organs. Receptors in the skin respond to touch, pressure, temperature or pain; receptors in the eye respond to light; receptors in the inner ear contribute to hearing and balance. Different receptors are best suited to different forms of energy or chemicals.
Gustatory receptors in the tongue detect substances responsible for basic taste sensations. Olfactory receptors in the nose detect airborne chemicals responsible for smell. What we commonly call flavour is a combined experience: signals from taste receptors and smell receptors are interpreted together. This is why food may seem less flavourful when the nose is blocked during a cold.
A specialised cell or nerve ending that detects a particular kind of stimulus and initiates information about it.
Structure Of A Neuron
A neuron is the structural and functional unit of nervous tissue. Its branched dendrites provide a large surface for receiving information. The cell body contains the nucleus and maintains the cell. A long fibre called the axon carries the electrical impulse away from the cell body. Near its far end, the axon divides into branches ending in small terminals.
A specialised nerve cell that receives, conducts and transmits information through electrical and chemical signals.
Transmission Of A Nervous Impulse
When an appropriate stimulus acts at a dendritic ending, it produces chemical changes in the membrane. These changes create an electrical impulse. The impulse travels from the dendrite to the cell body and then along the axon. The direction matters: dendrites mainly receive information, while the axon conducts it towards its terminals.
An electrical impulse cannot simply jump through open space. At an axon ending, the impulse triggers the release of chemical messenger molecules. These molecules cross a microscopic gap and bind to receptors on the next cell. If the signal is sufficient, the next neuron develops a new electrical impulse. Thus the signal is electrical within a neuron, chemical across the gap and electrical again in the next neuron.
The functional junction between two neurons, or between a neuron and an effector cell, across which information is transmitted, usually by chemical messengers.
A typical pathway is dendrite → cell body → axon → axon ending → synapse → next cell. The one-way release of messenger molecules at an axon ending helps maintain this direction.
Neuromuscular Junction
The final cell in a pathway need not be another neuron. At a neuromuscular junction, chemicals released from a motor neuron's terminal act on a muscle fibre. The signal causes changes inside the muscle cell that lead to contraction. A similar principle allows neurons to influence glands, which then release secretions. Nervous tissue therefore collects information and carries instructions, while muscles and glands perform the resulting action.
Problem
At which parts of a neuron is information received, conducted and passed onward?
- 1.Dendritic endings receive information.
- 2.The impulse passes through the cell body and along the axon.
- 3.Axon endings convert the electrical event into a chemical signal for the next cell.
Problem
A smell receptor is stimulated. Explain how the information can reach another neuron.
- 1.Chemicals in air stimulate an olfactory receptor.
- 2.The receptor initiates an electrical impulse in a connected neuron.
- 3.The impulse travels along the neuron to its axon ending.
- 4.Messenger chemicals cross a synapse and initiate an impulse in the next neuron.
Problem
A chemical prevents messenger release at a motor neuron's axon ending. Predict the effect on the attached muscle.
- 1.The electrical impulse may still reach the axon ending.
- 2.Without messenger release, the signal cannot cross the neuromuscular junction effectively.
- 3.The muscle fibre does not receive the instruction to contract.
- 4.Sensation or earlier neural activity may remain, but the intended muscular response is weakened or absent.
Activity
Purpose: To observe how smell contributes to the experience of flavour. Use ordinary food only, and do not perform the activity if nasal blockage causes discomfort.
- Place a small amount of sugar on the tongue and notice the taste.
- Gently close the nostrils with clean fingers and taste another small amount.
- During a normal meal, briefly compare flavour with the nose open and gently closed.
- Record whether basic sweetness remains and whether the fuller flavour changes.
Observation and explanation: Sweetness can still be detected by gustatory receptors, but the complete flavour may seem weaker when smell receptors receive less information. During a cold, swelling and mucus can reduce the movement of odour molecules to olfactory receptors, producing a similar experience.
A synapse is not the dendrite or axon itself. It is the junction across which one cell communicates with the next. Also, the electrical impulse does not normally cross the gap unchanged; chemicals carry the signal across it.
Quiz
Which part of a neuron usually carries an impulse away from the cell body?
What do olfactory receptors detect?
What normally crosses the synaptic gap?
Why may food seem less flavourful when the nose is blocked?
What would most directly prevent a muscle from receiving a motor instruction?
Practice Problems
- Trace the direction of information through one neuron. Model answer: Dendritic ending → dendrite → cell body → axon → axon ending.
- Explain what happens at a synapse. Model answer: An arriving electrical impulse releases messenger chemicals; they diffuse across the gap, bind to the next cell and may initiate a new electrical impulse.
- Differentiate gustatory and olfactory receptors. Model answer: Gustatory receptors detect dissolved taste substances, while olfactory receptors detect airborne chemicals associated with smell.
- Predict the effect of damage to an axon while dendrites remain functional. Model answer: The neuron may receive information, but transmission away from the cell body would be interrupted, so the next cell would not receive the signal normally.
- Explain why a neuron-to-muscle junction is essential for movement. Model answer: Nervous tissue carries the instruction, but the muscle is the effector. Chemical transmission at the junction transfers the instruction and initiates contraction.
Key Takeaways
• Receptors convert particular stimuli into information the nervous system can transmit. • Dendrites receive information, the cell body integrates it locally and the axon carries the impulse away. • An impulse is electrical within a neuron but is usually transmitted chemically across a synapse. • The next cell develops a fresh electrical response after detecting the chemical messenger. • Neuromuscular junctions connect motor neurons to muscle fibres. • Taste and smell signals combine to produce much of the experience of flavour.