Ventral horn damage and lower motor neuron signs
The ventral horn is the front part of the spinal grey matter and holds the cell bodies of lower motor neurons, which directly stimulate skeletal muscle to contract.
Remember
Because the ventral horn holds lower motor neuron cell bodies, damage there weakens the muscles supplied at that level, with reduced tone and reflexes but normal sensation.
In the spinal cord, grey matter forms a central core surrounded by white matter. The grey matter has regions called horns. The dorsal horn (the posterior horn) is responsible for sensory processing, and the ventral horn (the anterior horn) sends motor signals out to the skeletal muscles.
The ventral horn contains the cell bodies of the lower motor neurons that supply skeletal muscle. These neurons leave the spinal cord and directly stimulate the muscle to contract. The ventral horns of the different spinal levels line up as a column, and each level supplies particular muscles. The level L1 to L2, for instance, supplies the iliopsoas, which flexes the thigh at the hip.
Damage to the ventral horn therefore removes the lower motor neurons for the muscles supplied at that level. Those muscles become weak, with reduced muscle tone, called flaccidity, and reduced reflexes, and in time they show muscle wasting. Sensation stays normal, as the sensory pathways are not involved.
Damage to upper motor neurons gives the opposite picture in muscle tone and reflexes. Their cell bodies lie in the motor cortex, a region of the cortex (the folded sheet of grey matter on the surface of the brain). Their axons run down in the corticospinal tract, a white matter pathway, and damage to it causes spasticity (stiffness) and brisk reflexes.
Key facts
- Where
- the anterior part of the spinal grey matter, which sends motor signals to skeletal muscles
- Holds
- the cell bodies of lower motor neurons, which stimulate skeletal muscle directly
- Level map
- each level of the ventral horn column supplies particular muscles, and L1 to L2 supplies the iliopsoas, which flexes the thigh at the hip
- Signs
- the muscles supplied at that level have reduced tone (flaccidity) and reduced reflexes, become weak, and in time show wasting
- Sensation
- stays normal, as the sensory pathways are not involved
- Upper motor neuron signs
- damage to upper motor neurons, such as to the corticospinal tract, gives spasticity (stiffness) and brisk reflexes
Exam traps
- Upper motor neuron cell bodies are not found in the ventral horn, because they lie in the motor cortex, and their loss causes stiffness and brisk reflexes instead.
- The lateral corticospinal tract is white matter made of upper motor neuron axons, so damaging it gives spasticity and brisk reflexes rather than flaccid weakness with absent reflexes.
- The lateral horn does not supply skeletal muscle, because it holds the cell bodies of autonomic motor neurons, which control involuntary functions.
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