……………........ nerve fibres increase the rate of heart beat.

Explanation

**Step-by-Step Solution:**

1. **Identify the Nerve Fibers**: The question asks which nerve fibers increase the rate of heartbeat. The answer is sympathetic nerve fibers.

2. **Function of Sympathetic Nerve Fibers**: These nerve fibers are responsible for the 'fight or flight' response in the body, which includes increasing the heart rate.

3. **Origin of Sympathetic Nerve Fibers**: Sympathetic nerve fibers originate from the ganglia, which are clusters of nerve cell bodies.

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