Left Side Internal Capsule Lesions Neuro Physiology

1. A 60 year old male presented to neurological clinic with the following features.  
A.Weakness of hand grip with spasticity more on flexors than extenders of the right   upper limb. 
B. Circumduction gait 
C. Spasticity more on extensors than flexors of the right lower limb 
D. Upward movement of the left angle of the mouth when smiling E. Right side positive Babinski sign 
F. Rest of the neurological examination normal   

1.1. State the most likely site of the lesion.    (20 marks)   
1.2. Explain the physiological basis of the weakness and spasticity in this patient.         (80 marks)

Answers:-

1. 1.1 Left side internal capsule

1.2 Spasticity is caused when there is increased muscle tone in the body and this is a usual sign of upper motor neurone lesions. Muscle tone is the basal level of contraction that occurs in the muscles even when it is not voluntarily contracted. This occurs due to the stretch reflex. Skeletal muscles have 2 types of muscle fibres. 

1. Extrafusal fibres which takes part in muscle contraction innvervated by Aα motor neurones 
2. Intrafusal fibres in the muscle spindle innervated by Aγ motor neurones  

The stretch receptors present in the muscle spindle receive sensory supply from Ia and II afferent fibres. The stretch reflex in a monosynaptic reflex. The stimulus is stretch of the muscle. This stretch is sensed by the muscle spindle stretch receptors which send the impulse through the Ia afferent neurone into the spinal cord. Here the α motor neurone is stimulated and results in the contraction of the extrafusal fibres. The γ motor neurone is usually activated in parallel with the α motor neurone. This causes the contractile part of the intrafusal fibres to contract. This stimulates the stretch receptors causing the stretch reflex to be activated and the extrafusal fibres contract. So the γ neurones maintain the sensitivity of the spindle. The γ motor neurone activity is normally modified by descending pathways. The net effect is inhibitory. But in the case of a stroke, there is damage to the internal capsule which is where all the descending pathways are aggregated. But the reticulospinal pathway is still intact. This pathways activates the γ motor neurone thus indirectly causing the stretch reflex to occur and contract the muscle, increasing muscle tone. This is called the gamma loop.  All of the cortical fibres from the cerebrum that control the muscle movements pass through the internal capsule. Due to damage of these fibres there will be significant loss of muscle power. But some extrapyramidal tracts are still present which may provide some power leading to muscle weakness. The left side of the brain controls the right side of the body so in this case it is the left internal capsule that is affected.  This spasticity being more in the upper limb flexors is due to the intact rubrospinal tracts and being more in the lower limb extensors is due to the intact reticulospinal tracts. Normally these actions are inhibited but higher centres but in a stroke patient this inhibition is lost.