Sympathetic nerve fibres ___________the heart rate while parasympathetic nerve fibres _____________the heart rate.

Answer

To answer the question, we need to fill in the blanks regarding the effects of sympathetic and parasympathetic nerve fibers on heart rate.

1. **Understanding Sympathetic Nerve Fibers**:
- Sympathetic nerve fibers are part of the autonomic nervous system that prepare the body for 'fight or flight' responses.
- They release a neurotransmitter called epinephrine (also known as adrenaline).
- The release of epinephrine leads to an increase in heart rate, as it stimulates the heart to pump faster.

2. **Understanding Parasympathetic Nerve Fibers**:
- Parasympathetic nerve fibers are also part of the autonomic nervous system but are responsible for 'rest and digest' functions.
- They release a neurotransmitter called acetylcholine.
- The release of acetylcholine results in a decrease in heart rate, as it promotes relaxation and a slower heart rhythm.

3. **Filling in the Blanks**:
- Therefore, we can conclude:
- Sympathetic nerve fibers **increase** the heart rate.
- Parasympathetic nerve fibers **decrease** the heart rate.

**Final Answer**: Sympathetic nerve fibres **increase** the heart rate while parasympathetic nerve fibres **decrease** the heart rate.

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