Proprioception & Postural Reactions

The Proprioception Pathways

Cranial spinocerebellar tract

Sensory input from general proprioceptive receptors (found in muscles, joints, and tendons) travels up the peripheral nerve to the nerve cell body in the spinal ganglion.  The axon enters the spinal cord through the dorsal nerve root and synapses on a neuron in the centrobasilar nucleus (CBN), within the dorsal grey horn.  The axon from this neuron travels cranially within the cranial spinocerebellar tract (SCT) through the caudal cerebellar peduncle and into the cerebellar cortex.  This pathway mostly originates in the neck (C1-T1).

Cuneocerebellar tract

Another pathway involves the axon from the spinal ganglion nerve cell traveling cranially in the fasciculus cuneatus (FC).  These axons synapse on neurons within the lateral cuneate nucleus, located in the medulla oblongata.  Axons from these neurons travel through the caudal cerebellar peduncle to synapse in the cerebellar cortex. This pathway mostly originates in the thoracic limbs (C1-T8).

Sensory input from general proprioceptive receptors (found in muscles, joints, and tendons) travels up the peripheral nerve to the nerve cell body in the spinal ganglion.  The axon enters the spinal cord through the dorsal nerve root.  There are then two paths the impulse may take:

Dorsal spinocerebellar tract

The axon from the spinal ganglion cell synapses in the nucleus of the spinocerebellar tract (NSCT).  Axons from these neurons travel cranially in the dorsal spinocerebellar tract, through the caudal cerebellar peduncle and into the cerebellar cortex.  The axons are ipsilateral to the sensory afferent.

Ventral spinocerebellar tract

The axons from the spinal ganglion synapses on neurons within the dorsal grey column.  Most of these axons cross midline in the ventral white commissure and travel cranially in the ventral spinocerebellar tract through the rostral cerebellar peduncle into the cerebellar cortex where they decussate again to end up, ultimately, ipsilateral to the sensory afferent.

Conscious Proprioception

Sensory input from peripheral receptors travels up the peripheral nerve to the nerve cell body in the spinal ganglion.  The axon enters the spinal cord through the dorsal nerve root and then travels cranially within either the fasiculus cuneatus or the fasiculus gracilis, depending upon where the sensory input occurred.  Sensory input originating cranial to T6 will travel in the fasciculus cuneatus and axons will synapse on neurons in the medial cuneate nucleus (dorsal part of caudal medulla).  For any sensory input caudal to T6 impulses travel in the fasciculus gracilis and axons synapse on neurons in nucleus gracilis (dorsal part of caudal medulla).  Axons from these nuclei then cross midline, traveling rostral in the medial lemniscus system.  Axons then synapse with neurons in the thalamus (ventral caudal lateral nucleus) whose axons travel through the internal capsule to the somesthetic cortex.

Evaluation of Proprioception - The Postural Reactions

The primary aim of postural reaction testing is to detect subtle deficits that were not obvious on gait evaluation.  In a patient that is recumbent with tetraplegia or paraplegia, evaluation of the postural reactions in the affected limbs is redundant.  However, evaluation of the thoracic limbs postural reactions in a paraplegic patient will be important to detect abnormality that could suggest a focal cranial thoracic lesion or a multifocal disorder.  The postural reactions test the animal’s awareness of the precise position and movements of parts of its body and especially the limbs as well as the animal’s ability to generate movement in the part tested.  

How to perform - The postural reactions commonly tested are:

• The paw replacement reaction:  evaluated by placing the paw in an abnormal position (turned over so that the dorsal surface is in contact with the ground) and determining how quickly the animal correct the paw position.  It is fundamental to support the majority of the animal’s weight in order to improve test sensitivity. Paw replacement reaction can be very difficult to assess in cats that resent having their feet handled during this test.  Other postural reaction testing such as the hopping response, weelbarrowing and tactile placing are preferred in this species and should be considered in animals in which the paw replacement reaction is equivocal or difficult to interpret.  

• Hopping response:  tested by holding the patient so that the majority of its weight is placed on one limb while the animal is moved laterally.  Normal animals hop on the tested limb to accommodate a new body position as their center of gravity is displaced laterally. 

• Wheelbarrowing: the animal’s pelvic limbs are lifted off the ground by supporting the animal under the abdomen and forcing it to walk forward.  

• Tactile placing response:  the animal is lifted and the distal part of the thoracic limb is brought in contact with the edge of a table.  When the dorsal surface of the paw makes contact with the edge of the surface, the animal should immediately place its foot on the surface

How to interpret - All components of the peripheral and central nervous system that affect the limb tested are needed to be able to perform postural reactions.  These responses are complex in their pathways but generally involve an afferent arc 1) joint proprioceptor, 2) peripheral sensory nerve, 3) spinal cord and brainstem ascending pathways, 4) contralateral forebrain – and an efferent arc 1) contralateral forebrain, 2) descending motor pathways within the brainstem and spinal cord, 3) peripheral motor nerve and skeletal muscle.   Lesions affecting any of these components could potentially result in abnormal postural reactions.  Although these reactions are a sensitive test to detect neurological dysfunction, they do not provide specific information for lesion localization.  Their importance in localizing the lesion is dependent on the results of the rest of the neurologic examination.  In general, postural reactions remain normal in neuromuscular diseases as long as the animal has the strength to support its weight.