From intention to muscle

Science · The Human Body · ages 21-22

Name ______________________ Date ____________

Which statement describes cortical contributions to reaching?
- Cortical areas select a target; spinal circuits plan the whole reach.
- Cortical areas plan the reach and contribute commands to its muscles.
- Cortical areas plan the reach but send no commands that influence its muscles.
What is a motor unit?
- One motor neuron and every muscle fibre supplied by that neuron.
- One skeletal muscle and every nerve supplying that muscle.
- One sensory neuron and every receptor supplied by that neuron.
During a gradual increase in force demand, smaller, lower-threshold motor units are generally recruited before larger, higher-threshold units.
Circle one: True False
During a reach for a book, which pairing describes brainstem and spinal contributions?
- Brainstem: chooses the target. Spinal circuits: send postural commands.
- Brainstem: runs arm reflexes. Spinal circuits: choose the movement goal.
- Brainstem: supports posture. Spinal circuits: combine descending and local input.
During an increase in muscle force, recordings show the same motor units remain active, but their impulses occur closer together. Which mechanism explains the increase?
- Recruitment adds force by activating additional motor units.
- Faster firing lets successive twitch forces add before full relaxation.
- Recruitment raises force by replacing small active units with larger motor units.
A reach repeatedly overshoots its target, with poor timing but largely preserved strength. Which circuit dysfunction best fits this pattern?
- Cerebellar circuits.
- Basal ganglia circuits.
- Spinal motor neurons.
A lesion impairs movement regulation but spares descending motor pathways and motor neurons. Why can movement remain possible?
- Spared motor pathways still carry signals that activate muscle fibres.
- The lesion allows muscle fibres to contract without motor neuron signals.
- Planning circuits send commands straight to muscle fibres.
Movement is slow and rigid, with a tremor at rest. Dysfunction of which circuits best fits this Parkinsonian pattern?
- Cerebellar circuits.
- Spinal motor neurons.
- Basal ganglia circuits.

Answer key

For grown-ups. Fold this page away before handing over the rest.

From intention to muscle W1-mt_hrWOuRtPDl-s1

Cortical areas plan the reach and contribute commands to its muscles. · Cortical areas contribute to planning the action. Motor cortex also contributes commands influencing muscle activity.
One motor neuron and every muscle fibre supplied by that neuron. · A motor unit includes one motor neuron and every muscle fibre supplied by that neuron.
True · This is the usual recruitment order during a gradual rise in demand. Additional higher-threshold units join as more force is needed.
Brainstem: supports posture. Spinal circuits: combine descending and local input. · Brainstem pathways contribute to postural support. Spinal circuits combine descending commands with local sensory input during the reach.
Faster firing lets successive twitch forces add before full relaxation. · Closer impulses mean a higher firing rate. Successive twitch forces can add before full relaxation without changing which motor units are active.
Cerebellar circuits. · Overshoot and poor timing with largely preserved strength fit impaired cerebellar coordination. This pattern alone does not establish a diagnosis.
Spared motor pathways still carry signals that activate muscle fibres. · The spared pathways can still activate muscles despite impaired movement regulation. Damage to those output pathways could also cause weakness or paralysis.
Basal ganglia circuits. · Slowness, rigidity and rest tremor fit a Parkinsonian basal ganglia pattern. These signs alone do not establish a diagnosis.