An upper motor neuron lesion is a lesion

Lesion

A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...

 of the neural pathway above the anterior horn cell or motor nuclei of the cranial nerves

Cranial nerves

Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...

. This is in contrast to a lower motor neuron lesion, which affects nerve fibers traveling from the anterior horn of the spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 to the relevant muscle(s).

Upper motor neuron lesions occur in conditions affecting motor neurons in the brain or spinal cord such as stroke

Stroke

A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, traumatic brain injury

Traumatic brain injury

Traumatic brain injury , also known as intracranial injury, occurs when an external force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...

 and cerebral palsy

Cerebral palsy

Cerebral palsy is an umbrella term encompassing a group of non-progressive, non-contagious motor conditions that cause physical disability in human development, chiefly in the various areas of body movement....

.

Symptoms

Changes in muscle performance can be broadly described as the Upper Motor Neuron Syndrome. These changes vary depending on the site and the extent of the lesion, and may include:

• Muscle weakness. A pattern of weakness in the flexors (lower limbs) or extensors (upper limbs), is known as 'pyramidal weakness'
• Decreased control of active movement, particularly slowness
• Spasticity
  Spasticity
  Spasticity is a feature of altered skeletal muscle performance in muscle tone involving hypertonia, which is also referred to as an unusual "tightness" of muscles...
  
  , a velocity-dependent change in muscle tone
• Clasp-knife response
  Clasp-knife response
  Clasp-knife response refers to a stretch reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination. It is one of the characteristic responses of a upper motor neuron lesion...
  
   where initial higher resistance to movement is followed by a lesser resistance
• Babinski sign is present, where the big toe is raised (extended) rather than curled downwards (flexed) upon appropriate stimulation of the sole of the foot. The presence of the Babinski sign is an abnormal response in adulthood. Normally, during the plantar reflex, it causes plantar flexion and the adduction of the toes. In Babinski's sign, there is dorsiflexion of the big toe and abduction of the other toes. Physiologically, it is normally present in infants and during deep sleep.
• increase deep tendon reflex (DTR)
• Pronator drift
  Pronator drift
  In medicine, pronator drift refers to a pathologic sign seen during a neurological examination. Jean Alexandre Barré is credited with having first described it thus it is sometimes known as the Barré test. A positive result indicates spasticity...
  
  

Corticospinal/pyramidal tract

These are the neural tracts which descend in the spinal cord, in the lateral columns, carrying signals for voluntary movement of skeletal muscle. These nerve fibers usually originate in the primary motor cortex

Primary motor cortex

The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

, pass via the corona radiata

Corona radiata

In neuroanatomy, the corona radiata is a white matter sheet that continues caudally as the internal capsule and rostrally as the centrum semiovale. This sheet of axons contains both descending and ascending axons that carry nearly all of the neural traffic from and to the cerebral cortex...

 to gather in the internal capsule

Internal capsule

The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....

 before crossing over to the opposite side (decussation

Decussation

Decussation is used in biological contexts to describe a crossing.Examples include:* In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin...

) in the medullary pyramids and proceeding down the spinal cord to meet lower motor neuron

Lower motor neuron

Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...

s in the anterior horn

Anterior horn

The term anterior horn may refer to either of two separate anatomical structures within the central nervous system:...

.

External links

• http://www.med.yale.edu/caim/cnerves/cn7/cn7_13.html
• http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch3/s8ch3_34.htm