the descending pathways carry motor signals down the spinal cord and are generally divided into pyramidal or extrapyramidal tracts the pyramidal fibers travel through the medullary pyramids of the medulla oblongata which is why they are termed pyramidal these fibers originate in the cerebral cortex and are responsible for the voluntary control of the muscles of the body and of the face there are two different tracts that make up the pyramidal tracts these are the corticospinal and cortical bulbatracts the corticospinal tracts are responsible for the control of the body the cell bodies are found within the cerebral cortex with axons converging and passing through the internal capsule followed by the crust cerebri in the midbrain the pons and subsequently arriving into the medulla oblongata at this level around 75 percent of the fibers will decorate to the other side of the spinal cord and continue down to synapse with the lower motor neuron in the ventral horns at each level these fibers are known as the lateral corticospinal tract the other 25 of the fibers that remain on the ipsilateral side will continue as the anterior corticospinal tract and they will continue down ipsilaterally until the cervical and higher thoracic levels where they will decorate and synapse with their motor neurons as for the cortical bulbar tract this is the tract responsible for the voluntary control of the head the face and the neck similarly to the corticospinal tract they originate from the cerebral cortex and pass through the internal capsule however rather than descend into the spinal cord these neurons have their axon synapsing to the lower motor neurons in the brain stem specifically onto the cranial nerve nuclei a crucial difference that is present in the cortical bulbar tract is that these neurons innervate bilaterally in most instances therefore neurons from one side provide motor function for both sides of the face exceptions include cranial nerve seven involving the motor function of the lower third of the face which is why in strokes involving this tract the patient may only end up displaying drooping of the lower one-third of the face rather than the entire side of the face as occurs in bell's palsy this is also true for the hypoglossal nerve which is why in upper motor neuron lesions the tongue will deviate to the opposite side of the lesion now let's look at the extra pyramidal tracts they are different to the pyramidal tracts in that they do not pass through the pyramids and they are responsible for involuntary control and modulation of the muscles meaning they are involved in things like muscle tone and balance another difference is that they originate in the brain stem rather than the cerebral cortex the first is the rubra spinal tract which originates from the red nucleus in the midbrain these fibers are thought to be responsible for fine motor control and as these fibers terminate in the cervical and thoracic sections of the spine they are thought to primarily affect the upper limbs these have a contralateral innervation next we have the reticular spinal tracts of which there are two the medial or pontine reticulospinal tract originates from the pontine reticular formation and facilitates voluntary muscle movement it excites the antigravity extensor muscles and increases muscle tone in contrast the lateral or medullary reticular spinal tract comes from the medullary reticular formation which is shown here in dark blue and this inhibits voluntary muscle movement and decreases muscle tone it also plays a role in automatic breathing third we have the vestibular spinal tract arising from the vestibular nuclei which receive inputs from the cerebellum there are medial and lateral vestibular spinal tracts however the dominant one is the lateral tract the function is involved in maintaining balance and posture and the innervation is ipsilateral then we have the tectospinal tract known also as the colliculus spinal tract which originates from the superior colliculus of the midbrain this tract is involved in the coordination of head and eye movements and the superior colliculus receives inputs from the eyes which explains this function this track decorates and therefore provides contralateral innervation remember that differently to the ascending pathways the descending neurons do not have a second neuron within the cns they instead have a cell body within either the cerebral cortex or the brain stem and their axons travel down the entirety of the spinal cord or the brain stem to a single synapse onto the lower motor neuron which then carries the signal to the muscle itself lesions in the pyramidal tracts can result in various deficits in different locations they can be damaged at any point along the spinal cord and the internal capsule itself we said that both corticospinal and cortical bulbatracts pass through the internal capsule meaning an injury here can affect motor function of the body and of the face it is particularly susceptible to injury during a hemorrhagic event as this region can become compressed and it is also a common site for stroke as the pyramidal tracts primarily decorate lesions on one side typically lead to deficits on the contralateral side of the body the typical manifestations of an upper motor neuron lesion include hypertonicity so an increased muscle tone hyperreflexia meaning a larger muscle reflex being seen on stimulation which is due to a loss of the inhibitory signals coming from the upper motor neurons typically there will also be weakness of the affected region and clonus can be seen in some instances which is an involuntary contracture of the muscles we then have signs such as babinski's sign which is considered a positive when a sharp stimulus on the lateral palm aspect of the foot generates dorsiflexion or an upward movement of the big toe rather than the normal downward motion the reason this test is used is because normally stimulation of the s1 dermatome that is the lateral plantar aspect of the foot causes stimulation of the s1 myotome which contracts the plantar flexors causing a downward movement however if the corticospinal tract has a lesion then there is a spread of this sensory input from the s1 dermatome to the l4 and l5 horn cells which then fire and cause contraction of the extensors which causes an upward movement of the big toe hoffmann sine is a similar sign used for the upper limbs where the fingernail of the middle finger is flicked down and we're looking for the flexion and adduction of the thumb hoffmann's however is a deep tendon reflex that is normally fully inhibited by the corticospinal tract damage to the extra pyramidal tracts can often result in parkinsonism featuring rigidity bradykinesia tremors and postural deficits others include korea athetosis and dystonia you