The big idea: When the cerebral hemispheres decide on a movement, motor neurons carry impulses out of the CNS: down through the spinal cord and out to skeletal muscles. The impulses stimulate the muscles to contract, and the body moves.
The points to remember
- The cerebral hemispheres decide on a voluntary movement.
- Motor neurons carry impulses away from the CNS.
- They run down the spinal cord and out in nerves.
- They end at skeletal muscles.
- The impulses stimulate the muscle to contract.
Remember it as: Motor: from the CNS, to make things move.
Real example: kicking a ball. To kick, your cerebral hemispheres send impulses along motor neurons down the spinal cord and out to the muscles of your leg. Each impulse that arrives stimulates muscle fibres to contract.
Trap: Motor neurons do not carry information to the brain: they carry impulses from the CNS to effectors, such as muscles.
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How it is asked: Expect an MCQ on which neurons signal skeletal muscle, or a short question on how the brain makes a muscle contract.
IB-style questionOutline[2 marks]
Outline how the cerebral hemispheres cause a skeletal muscle to contract.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.