Anatomy and Physiology 2e · The Somatic Nervous System
Motor Responses
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In 30 seconds
Motor responses are the output stage of the somatic nervous system: the chain that turns plans into movement. Every voluntary movement of skeletal muscle runs through a two-neuron pathway. The Upper motor neuron Motor neuron with cell body in cortex/brainstem, axon descending to the cord Full entry → (UMN) has its cell body in the motor cortex (or brainstem) and sends its axon down the spinal cord; the Lower motor neuron Motor neuron with cell body in anterior horn/cranial nucleus, axon to muscle Full entry → (LMN) has its cell body in the anterior horn of the spinal cord (or a cranial nerve nucleus) and sends its axon out to skeletal muscle fibers at the Neuromuscular junction Synapse between motor neuron and muscle fiber Full entry →. The LMN is the final common pathway — the only way signals reach skeletal muscle.
Two supporting systems adjust the command: the basal nuclei help initiate and smooth voluntary movement, and the cerebellum coordinates timing, balance, and error correction. The spinal cord also runs automatic motor programs — reflexes such as the stretch, withdrawal, and crossed extensor reflexes. Because the two neurons sit on opposite sides of the synapse in the cord, injuring one versus the other produces recognizably different weakness patterns — the classic "upper vs. lower motor neuron" distinction.
Why this matters
Motor examination is the everyday work of assessing stroke, spinal cord injury, neuropathy, and neuromuscular disease. The key clinical skill taught everywhere is separating UMN from LMN lesions. A stroke affecting the motor cortex produces UMN signs: weakness with increased Muscle tone Low-level resting tension maintained by reflexes Full entry → (spasticity), brisk reflexes (hyperreflexia), and, in classic teaching, an upgoing toe when the sole is stroked (Babinski sign Extensor plantar response (big toe extends, toes fan) Full entry →). Damage to anterior horn cells or their axons — as in polio (a classic teaching example), severe nerve injury, or nerve compression — produces LMN signs: flaccid weakness, reduced or absent reflexes, and muscle atrophy. Understanding motor units and recruitment also explains how strength builds gradually and underpins rehabilitation. Exams regularly test the corticospinal tract route, the decussation, the homunculus, and the reflex circuits.
The college version
Core Concepts
The two-neuron motor chain
Upper motor neurons originate in the primary motor cortex (precentral gyrus) or brainstem and descend to synapse on LMNs; their axons travel in the Corticospinal (pyramidal) tract Main descending pathway from motor cortex to spinal cord Full entry →, with fibers to cranial nerve nuclei forming the corticobulbar tract. Lower motor neurons have cell bodies in the anterior horn or cranial nerve nuclei; their axons leave through ventral roots and spinal nerves to reach skeletal muscle at the neuromuscular junction, where acetylcholine triggers contraction.
The corticospinal tract route
Corticospinal fibers run from the motor cortex through the corona radiata and internal capsule, descend through the cerebral peduncles, and reach the pyramids on the ventral medulla. There, the great majority (commonly taught as roughly 85–90%) cross at the Pyramidal decussation Crossing point of most corticospinal fibers in the medulla Full entry → and descend as the lateral corticospinal tract, which controls limb muscles. The rest continue as the anterior (ventral) corticospinal tract and cross at the spinal level they supply, mostly controlling trunk muscles. Because of this crossing, the right motor cortex controls the left body. Corticobulbar fibers reach cranial nerve nuclei, most receiving bilateral input — the classic exception is the lower facial nucleus (mostly contralateral), so a UMN facial lesion spares the forehead while an LMN lesion does not.
The motor homunculus
The primary motor cortex is somatotopically organized like the sensory cortex: the body maps onto the precentral gyrus as a distorted motor homunculus. The hands, face, and tongue occupy disproportionately large areas, reflecting the fine control they require — small cortical strokes can therefore produce very focal weakness.
Basal nuclei and cerebellum: the modulators
Neither sends fibers directly to muscle; both adjust the UMN–LMN pathway.
- Basal nuclei (caudate, putamen, globus pallidus, substantia nigra, subthalamic nucleus) help initiate voluntary movement, suppress unwanted movement, and regulate tone. Classic teaching examples: Parkinson disease (tremor at rest, rigidity, bradykinesia — too little movement) and Huntington disease (chorea — too much, uncontrolled movement).
- Cerebellum compares motor plans with sensory feedback and corrects errors in timing, trajectory, and balance. Damage produces Ataxia Uncoordinated, clumsy movement Full entry → — clumsy, uncoordinated movement — rather than paralysis.
Reflexes: automatic motor programs
Reflexes are rapid, stereotyped motor responses organized in the spinal cord or brainstem:
- Stretch reflex Monosynaptic reflex contracting a stretched muscle Full entry → (monosynaptic): a muscle spindle detects stretch and its Ia afferent synapses directly on the alpha motor neuron of the same muscle, causing contraction — the knee jerk (patellar reflex, L3–L4 in classic teaching) is the classic example. Reciprocal inhibition via an interneuron relaxes the antagonist.
- Golgi tendon reflex: Golgi tendon organs sense excessive tension; their Ib afferents activate inhibitory interneurons that relax the muscle, protecting it from overload (disynaptic).
- Withdrawal (flexor) reflex: painful stimulation flexes the limb away via interneurons — polysynaptic, and fast enough to beat conscious perception.
- Crossed extensor reflex: during withdrawal, interneurons also excite extensors on the opposite limb so it can support the body.
Muscle tone — low-level resting contraction — is maintained largely by the stretch reflex loop and is increased in UMN lesions, decreased in LMN lesions.
Motor units and recruitment
A Motor unit One LMN plus all the muscle fibers it innervates Full entry → is one lower motor neuron plus all the muscle fibers it innervates. Units vary in size: small units give fine control (eye muscles, fingers); large units generate power (thigh muscles). By the commonly taught size principle, units are recruited from small to large as force demand rises, keeping gentle movements smooth and precise. Each muscle fiber contracts fully or not at all; force is graded by how many units fire and how fast.
UMN versus LMN lesions: the classic comparison
- UMN lesion (cortex, internal capsule, corticospinal tract, or cord above the anterior horn): weakness, increased tone (spasticity), hyperreflexia, and an extensor plantar response (Babinski sign — big toe extends and toes fan; normal in infants but abnormal after about one year in classic teaching). Little atrophy occurs.
- LMN lesion (anterior horn cell, ventral root, or peripheral nerve): flaccid weakness, decreased or absent reflexes, muscle atrophy, and sometimes visible twitching (fasciculations).
These are commonly taught clinical correlations; precise signs vary and should be verified against current clinical texts.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Upper motor neuron | Lower motor neuron | UMN cell body in cortex/brainstem; LMN in cord/cranial nuclei. Signs differ: spastic/hyperreflexia vs flaccid/hyporeflexia |
| Lateral corticospinal tract | Anterior corticospinal tract | Lateral crosses in the medulla (limbs); anterior crosses at spinal levels (trunk) |
| Motor crossing | Sensory crossing | Motor fibers cross mostly at the pyramidal decussation; pain/temperature fibers cross in the cord; dorsal column fibers cross in the medulla |
| Basal nuclei | Cerebellum | Basal nuclei initiate/smooth movement; cerebellum coordinates timing and error correction |
| Stretch reflex | Golgi tendon reflex | Stretch excites the muscle (monosynaptic); Golgi inhibits it (disynaptic, protective) |
| Spasticity | Flaccidity | Increased tone (UMN) vs decreased tone (LMN) |
| Babinski sign in infants | Babinski sign in adults | Extensor plantar response is normal in infants but a classic UMN sign in adults |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of moving your arm like a delivery route. The boss in the control room (your brain) writes the order, and a messenger (the upper motor neuron) runs it down a long hallway to the delivery driver (the lower motor neuron), who is parked right at the loading dock next to the muscle. The driver is the only one who can actually load the truck, so if the driver is hurt, nothing gets delivered at all — the muscle is completely paralyzed and shrinks. If the messenger is hurt instead, the driver is still there but confused and jumpy: the muscle is stiff and twitchy rather than limp.
Worked example
Walkthrough — picking up a coffee cup. You decide to reach for a cup: the premotor cortex plans the sequence and the primary motor cortex activates UMNs for the shoulder, elbow, wrist, and fingers. The UMN axons descend through the corona radiata and internal capsule, cross at the pyramidal decussation, and synapse on LMNs in the cervical anterior horn; the LMNs fire through the ventral roots and brachial plexus, where acetylcholine triggers contraction. As you move, the cerebellum compares intent with sensory feedback and makes micro-corrections while the basal nuclei suppress competing movements. If the cup is unexpectedly heavy, Golgi tendon reflexes limit excess forearm tension; if your hand touches something hot, the withdrawal reflex yanks it back ahead of conscious perception.
Scenario — localizing a lesion. A patient suddenly cannot move the right arm and leg; reflexes are brisk, tone increased, and the right big toe extends when the sole is stroked. That UMN pattern puts the lesion on the left side of the brain, because the corticospinal tract crosses in the medulla. A different patient with a flaccid, atrophied right arm, absent reflexes, and no Babinski sign points to an LMN problem at the anterior horn cells or peripheral nerves on the right. Distinguishing these two patterns at the bedside is a foundational clinical reasoning skill.
Key takeaways
- Two-neuron chain: UMN (cortex/brainstem → spinal cord) → LMN (anterior horn/cranial nuclei → muscle at the NMJ). The LMN is the final common pathway.
- Corticospinal tract: ~85–90% of fibers cross at the pyramidal decussation in the medulla (lateral tract → limbs); the anterior tract crosses at spinal levels (trunk).
- Right motor cortex → left body.
- Motor homunculus: precentral gyrus; hands, face, tongue over-represented.
- Basal nuclei initiate/smooth movement (Parkinson, Huntington are classic teaching examples); cerebellum coordinates and corrects (ataxia if damaged).
- Stretch reflex = monosynaptic; Golgi tendon reflex = inhibitory; withdrawal and crossed extensor reflexes are polysynaptic.
- Size principle: small motor units recruited before large ones.
- UMN lesion: spasticity, hyperreflexia, Babinski sign; LMN lesion: flaccidity, hyporeflexia, atrophy, fasciculations.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Trace a command from the left motor cortex to the right hand, naming the tract and crossing point.
Show answer
UMN from the left motor cortex descends in the corticospinal tract, crosses at the pyramidal decussation in the medulla, travels in the right lateral corticospinal tract, synapses on an LMN in the right anterior horn, which exits via the ventral root to the hand muscles.
A patient has a flaccid, atrophied limb with absent reflexes. Is this an UMN or LMN lesion? Where is the damage?
Show answer
LMN lesion — the anterior horn cell, ventral root, or peripheral nerve is damaged. UMN lesions produce spasticity and hyperreflexia instead.
Why does a UMN lesion affecting facial movement spare the forehead while an LMN facial lesion does not?
Show answer
Most cranial nerve motor nuclei receive bilateral corticobulbar input, but the lower facial nucleus receives mostly contralateral input. A UMN lesion leaves the forehead working; an LMN (facial nerve) lesion paralyzes the whole side of the face.
What is the functional difference between the stretch reflex and the Golgi tendon reflex?
Show answer
The stretch reflex detects muscle stretch and contracts the muscle (excitatory, monosynaptic); the Golgi tendon reflex detects excess tension and relaxes the muscle (inhibitory, protective).
A person with normal strength cannot perform smooth, coordinated movements and is clumsy. Which structure is most likely affected, and what is this sign called?
Show answer
The cerebellum; the sign is ataxia (loss of coordination with preserved strength).
Explain the size principle and why it makes gentle movements smooth.
Show answer
Recruited smallest to largest as force demand rises, so gentle efforts use only small, precise units and power builds smoothly.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Upper motor neuron
- Motor neuron with cell body in cortex/brainstem, axon descending to the cord
- Lower motor neuron
- Motor neuron with cell body in anterior horn/cranial nucleus, axon to muscle
- Corticospinal (pyramidal) tract
- Main descending pathway from motor cortex to spinal cord
- Pyramidal decussation
- Crossing point of most corticospinal fibers in the medulla
- Motor unit
- One LMN plus all the muscle fibers it innervates
- Neuromuscular junction
- Synapse between motor neuron and muscle fiber
- Muscle tone
- Low-level resting tension maintained by reflexes
- Stretch reflex
- Monosynaptic reflex contracting a stretched muscle
- Babinski sign
- Extensor plantar response (big toe extends, toes fan)
- Ataxia
- Uncoordinated, clumsy movement
Sources & references
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