In one breath
Movement is the displacement of the body or one of its parts, produced when agonist muscles contract and their antagonists relax. Movements are either automatic (reflex: fast, stereotyped, set off by a stimulus) or volitional (slow, adaptable, under voluntary control). Three feedback systems make them better, trading speed for accuracy: local (spinal only, fastest), central (cord, brainstem and cortex, precise) and special sensory (vision and hearing, most accurate). The output is built in levels: the muscle and its motor neurons, the spinal segmental circuit, brainstem centres and the cortex, with the basal ganglia, cerebellum and thalamus shaping it.
Builds on: Divisions of the nervous system · Afferent fibers, dorsal root and spinal laminae · Leads to: Segmental organization of the motor system · Cortical integration and motor cortex · Basal ganglia: circuits and functions · Cerebellar divisions, histology and connections
Two types of movement
Your book sorts all movements into automatic (reflex) and volitional (voluntary, intentional) movements.
| Feature | Automatic (reflex) | Volitional |
|---|---|---|
| Latency | Short | Long |
| Execution | Rapid | Slow |
| Pattern | Stereotyped, usually fixed | Easily modified |
| Voluntary control | No | Yes |
| Trigger | A specific sensory stimulus | Rarely a specific stimulus |
| Example | Pulling away from a painful stimulus | Painting; threading a needle |
- Only volitional movements are listed as open to attention, emotion and motivation.
What a movement is
- Movement: any shift of a body part that puts that part, or the whole body, in a new position. Muscles produce it by contracting and relaxing.
- Agonists are the muscles that bring the movement about. Your book defines them as the muscles that decrease the angle at the joint.
- Antagonists oppose the movement; your book says they increase the joint angle. The movement goes more easily when the antagonist relaxes.
- Worth knowing, though not in your pages: the joint-angle rule fits flexion only. The agonist is whichever muscle produces the movement in question, so when the elbow is straightened the triceps is the agonist, and it opens the joint.
How a movement is produced
- The signal that reaches a muscle is generated in motor neurons in the spinal cord, but what drives those neurons differs:
- Basic (reflex) movement: the cord makes the motor signal itself and sends it to the muscle along its motor neurons.
- Complex (volitional) movement: the signal starts in the forebrain (chiefly the motor cortex) and brainstem, then is relayed down to the cord.
- The cord blends the descending (supraspinal) signals with its own inputs to refine the final output.
- Feedback control is what gives a movement its precision and stability.
Feedback control systems
Feedback makes a movement better: more accurate and more stable. There are three levels.
| Level | Integrated in | Wins on |
|---|---|---|
| Local | Spinal cord only | Speed |
| Central | Cord, brainstem, cortex | Accuracy |
| Special sensory | Many CNS levels | Greatest accuracy |
Local feedback
- The lowest level, wired entirely inside the spinal cord.
- The afferent signal never meets signals from higher centres, so the loop is fast. It runs gross reflex movements.
- It is used where speed matters more than accuracy. Example: pulling a limb away from a noxious stimulus, where getting it out of the way fast matters more than doing it neatly (see Withdrawal reflex).
- The pattern of the movement still depends on how strong the afferent input is, and what kind it is.
Central feedback
- The second level, integrated in the spinal cord, brainstem and cortex.
- The afferent signal is combined with signals from the supraspinal levels. Processing at so many levels slows the movement, but the slowness buys precision.
- It is used where accuracy matters more than speed. Example: threading a needle, which needs balance and coordination.
- How the signal travels up: proprioception from muscles and joints reaches the spinocerebellum along the dorsal and ventral spinocerebellar tracts, and reaches the sensory cortex along the dorsal column pathways, which then pass it on to the motor cortex.
- How the correction comes back: the cerebellum sets the original plan against what the limb actually did and trims the movement through the extrapyramidal pathways; the motor cortex adjusts its own output down the corticospinal and corticobulbar tracts.
- Smoothing, and posture held during a movement, are handled mainly by the extrapyramidal systems that arise in the brainstem.
- The extrapyramidal tracts your book names are vestibulospinal, reticulospinal and tectospinal. Because the cerebellum and basal ganglia shape what these tracts do, both are counted as extrapyramidal structures too. Worth knowing, though not in your pages: most other textbooks count the rubrospinal tract in this group as well.
Special sensory feedback
- Uses special senses such as vision and hearing. Their inputs act on different parts of the CNS to sharpen the movement.
- It gives the most accurate movements. Example: striking a nail with a hammer goes far better when you can see it.
Match each loop to its example
The book’s examples are fixed pairs, and MCQs use them:
- Local: withdrawing from a painful stimulus (fast, rough).
- Central: threading a needle (slow, precise).
- Special sensory: hitting a nail with the eyes open (most accurate).
Draw it: local and central feedback
- Draw a muscle at the bottom and a cross-section of the spinal cord above it.
- Local loop: an afferent from the muscle enters the cord and connects, directly or through an interneuron, to a motor neuron whose axon goes back to the same muscle. Write “fast, speed first” beside it.
- Central loop: from the same afferent, draw a branch rising to the brainstem and cortex, and a descending arrow coming back down to the motor neuron. Write “slower, accuracy first”.
- For special sensory feedback, add an eye with an arrow into the brain.
Organization of the motor system
The book’s components, from the bottom up:
| Component | Main job |
|---|---|
| Muscle and its motor neurons | Tone, force, final output |
| Spinal segmental circuit | Rapid reflexes; basic limb patterns |
| Brainstem centres | Posture |
| Cortex | Direct and indirect control of the cord |
| Basal ganglia | Initiating and smoothing movement |
| Cerebellum | Every stage, from planning to stopping |
| Thalamus | Sensory relay; sensorimotor link |
Muscle and its motor neurons
- The muscles and their efferent supply (the motor neurons) are the foundation of the whole system.
- A muscle has tone only while its nerve supply is intact, and without tone it cannot build enough force to move anything.
- Destroy the motor neurons and tone and function go together, so the muscle is completely paralysed (see Upper vs lower motor neuron lesions).
- The working unit at this level is the motor unit: one α motor neuron and all the muscle fibres it supplies. Your book defines it in the next chapter; see Segmental organization of the motor system.
Spinal segmental circuit
- A muscle’s own afferents bring its sensory signals into the cord. There they act, directly or indirectly, on the motor neurons supplying that same muscle.
- This loop is the local (segmental) spinal circuit, and every fast reflex movement depends on it.
- It also sets up the basic firing patterns that limb movements are built from, and keeps them coordinated.
Brainstem controlling centres
- Descending input from the brainstem motor nuclei strongly shapes the spinal motor neurons and interneurons.
- These brainstem-to-cord paths are mainly extrapyramidal (see Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts):
- the reticulospinal tract, from the reticular nuclei;
- the vestibulospinal tract, from the vestibular nuclei.
- Both act mainly on the motor neurons that control posture.
Cortex
- Direct route: motor cortex → corticospinal tracts → spinal motor neurons (Corticospinal (pyramidal) tract).
- Indirect route: motor cortex → corticobulbar fibres → brainstem nuclei → extrapyramidal tracts → cord.
- Some corticospinal fibres come from the sensory cortex, which also feeds the motor cortex (Somatosensory cortex).
- That somatosensory input is feedback: it lets the descending commands be corrected and improved. This is why the cortex counts as one of the levels where sensation and movement are coordinated.
Basal ganglia
- Subcortical nuclei with a strong effect on movement (Basal ganglia: circuits and functions).
- Nothing somatosensory reaches them directly from the cord. Their output runs through the thalamus to the motor cortex, so they shape the commands the cortex sends down.
- Roles: initiating, smoothing and coordinating movement.
Cerebellum
- It lies behind the brainstem and cord (the motor neuraxis).
- Its inputs come from nearly all the senses; its outputs go in large numbers to the brainstem motor nuclei and the motor cortex. That makes it crucial for posture and movement.
- It takes part in nearly every stage of a movement: planning, programming, initiation, smoothing, coordination and termination (Cerebellar divisions, histology and connections).
Thalamus
- The major sensory relay station: body sensation stops here on its way to the cortex (Thalamus: nuclei, functions and thalamic syndrome).
- The cerebellum and the basal ganglia both feed into it as well.
- That puts it at a meeting point of the sensory and motor systems: a site of sensorimotor coordination.
Basal ganglia and cerebellum: different inputs
The cerebellum is fed by almost every sensory modality. The basal ganglia get no direct somatosensory input from the cord. Both send output to the thalamus.
Applied: where the lesion shows
- Motor neurons: tone and function are lost, so paralysis is complete.
- Basal ganglia, as in Parkinsonism: control of posture and movement is badly impaired (Parkinson’s and Huntington’s disease).
- Cerebellum: every aspect of movement becomes abnormal (Cerebellar functions, lesions and tests).
Draw it: general organization of the motor system
- Draw a column of boxes from the top: cerebral cortex (motor and sensory), brainstem centres, spinal cord (interneurons and motor neurons), muscle.
- Put the basal ganglia and the cerebellum as side boxes, each with an arrow to the thalamus, and draw an arrow from the thalamus up to the motor cortex. Add an arrow from the cerebellum to the brainstem.
- Descending arrows: cortex → cord (corticospinal); cortex → brainstem (corticobulbar); brainstem → cord (reticulospinal, vestibulospinal).
- Close the bottom loop: cord → muscle (motor neurons) and muscle → cord (muscle afferents).
- Sensory arrows: cord → thalamus → sensory cortex → motor cortex, and a sensory arrow into the cerebellum. Draw no direct arrow from the cord to the basal ganglia.
Exam-answer skeleton: "Feedback control systems for movement" (short note)
- Purpose: accuracy and stability of movement; three levels.
- Local feedback: spinal cord only; no supraspinal input, so fast; speed over accuracy; withdrawal from a noxious stimulus.
- Central feedback: cord, brainstem and cortex; slower but precise; accuracy over speed; threading a needle.
- Special sensory feedback: vision and hearing acting on many CNS levels; most accurate; hammering a nail with the eyes open.
- A comparison table: level, speed, accuracy, example.
- A diagram of the local and central loops.
Asked in exams
- Motor unit: Sep/Oct 2019, 2 marks