Upper motor neurons

Definition

Upper motor neurons are a type of neuron that originate in the motor cortex of the brain and project down to lower motor neurons in the spinal cord or brainstem. These neurons are crucial for the planning, initiation, and control of voluntary movements, forming a key part of the corticospinal tract that transmits signals to execute these movements.

5 Must Know Facts For Your Next Test

1. Upper motor neurons primarily reside in the primary motor cortex but also include areas such as the premotor cortex and supplementary motor area.
2. Damage to upper motor neurons can lead to conditions like spasticity, characterized by increased muscle tone and exaggerated reflexes due to disinhibition of lower motor neurons.
3. These neurons send their axons through the corticospinal tract, which crosses over at the medulla, meaning that each hemisphere of the brain controls movements on the opposite side of the body.
4. Upper motor neuron disorders may present with weakness and impaired coordination, but unlike lower motor neuron disorders, they do not typically cause muscle atrophy.
5. Clinical tests for upper motor neuron function often involve evaluating reflexes and muscle tone, with hyperreflexia indicating potential dysfunction.

Review Questions

• How do upper motor neurons influence voluntary movement and what pathways do they utilize?
  • Upper motor neurons influence voluntary movement by sending signals from the motor cortex down to lower motor neurons via the corticospinal tract. This pathway is critical for initiating and controlling movements by transmitting electrical impulses that stimulate muscle contraction. The organization of this pathway also ensures that movements on one side of the body are controlled by the opposite side of the brain, allowing for coordinated and purposeful actions.
• What are the clinical implications of damage to upper motor neurons in terms of movement disorders?
  • Damage to upper motor neurons can lead to various movement disorders characterized by spasticity and hyperreflexia. Patients may exhibit increased muscle tone and exaggerated reflex responses due to reduced inhibition on lower motor neurons. This can result in difficulties with voluntary movements, poor coordination, and an overall decrease in functional mobility. Understanding these implications helps in diagnosing and managing conditions like stroke or multiple sclerosis.
• Evaluate the differences between upper and lower motor neuron lesions in relation to their clinical presentations.
  • Upper motor neuron lesions typically present with spasticity, hyperreflexia, and a lack of muscle atrophy due to preserved innervation from lower motor neurons. In contrast, lower motor neuron lesions present with flaccidity, hyporeflexia, and significant muscle atrophy since these neurons directly innervate muscles. This distinction is crucial for clinicians when assessing neurological conditions as it guides diagnosis and treatment approaches by highlighting where in the nervous system the injury or dysfunction has occurred.

Related terms

lower motor neurons: Lower motor neurons are neurons that connect the central nervous system to skeletal muscles, directly controlling muscle contraction and movement.

corticospinal tract:

The corticospinal tract is a major pathway that carries signals from the motor cortex down through the brainstem and spinal cord, facilitating voluntary movement.

motor cortex:

The motor cortex is a region of the brain responsible for planning, controlling, and executing voluntary movements, serving as the origin point for upper motor neurons.