The descending tracts are neural pathways that send motor signals from the brain to lower motor neurones, which then directly activate muscles. These pathways can be divided into two major functional categories: the pyramidal tracts and the extrapyramidal tracts.
The pyramidal tracts are named for the medullary pyramids of the medulla oblongata, which they travel through. This pathway is responsible for the voluntary control of the musculature of the body and face. It can be further divided into two functional sub-categories: the corticospinal tracts and the corticobulbar tracts.
The corticospinal tracts begin in the cerebral cortex, receiving input from the primary motor cortex, premotor cortex, and supplementary motor area. Additionally, they receive nerve fibres from the somatosensory area, which plays a role in regulating activity in the ascending tracts. After leaving the cortex, the neurones travel through the internal capsule, the crus cerebri of the midbrain, the pons, and into the medulla.
In the most inferior (caudal) part of the medulla, the tract divides into two: the lateral corticospinal tract, which crosses to the opposite side of the CNS before continuing to the spinal cord and terminating in the ventral horn at all segmental levels, and the anterior corticospinal tract, which remains ipsilateral and descends to the ventral horn of the cervical and upper thoracic segmental levels.
The corticobulbar tracts also arise from the primary motor cortex, but terminate on the motor nuclei of the cranial nerves. Here, they synapse with lower motor neurones, sending motor signals to the muscles of the face and neck.
Clinically, it is important to understand the organisation of corticobulbar fibres. Most of these fibres innervate motor neurones bilaterally. However, upper motor neurones for the facial nerve (CN VII) have a contralateral innervation, only affecting the muscles in the lower quadrant of the face. Similarly, upper motor neurons for the hypoglossal (CN XII) nerve only provide contralateral innervation.
The extrapyramidal tracts originate in the brainstem, carrying motor fibres to the spinal cord. They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture, and locomotion. There are four tracts in total.
The vestibulospinal and reticulospinal tracts do not cross over to the other side of the CNS, providing ipsilateral innervation. This helps to control the body's balance and postural reflexes, enabling the body to move easily as it adjusts to external conditions.
The neurons within the descending pathways do not synapse with each other. Instead, they synapse with lower motor neurones at the termination of the descending tracts. As a result, all of the neurons in the descending motor system are classed as upper motor neurones, with their cell bodies found in the cerebral cortex or the brain stem, and their axons remaining within the CNS.
The descending tracts are pathways responsible for sending motor signals from the brain to lower motor neurones, which then directly innervate muscles to produce movement. Motor tracts can be functionally divided into two major groups – pyramidal and extrapyramidal – both of which are responsible for voluntary and involuntary control of all muscle activity, including muscle tone, balance, posture, and locomotion.
Pyramidal tracts originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem, and are responsible for the voluntary control of the musculature of the body and face. Functionally, these tracts can be subdivided into two – corticospinal tracts and corticobulbar tracts – which respectively supply the musculature of the body and head and neck.
The pyramidal tracts derive their name from the medullary pyramids of the medulla oblongata, which they pass through. This makes them particularly vulnerable to the effects of cerebrovascular accidents (CVAs), as they pass through the internal capsule, a common site of damage from CVA lesions. Depending on the location of the lesion in the CNS, a unilateral corticospinal tract lesion will result in symptoms appearing on the contralateral side of the body.
The cardinal signs of an upper motor neurone lesion include hypertonia (an increased muscle tone), hyperreflexia (increased muscle reflexes), clonus (involuntary, rhythmic muscle contractions), Babinski sign (extension of the hallux in response to blunt stimulation of the sole of the foot), and muscle weakness.
Extrapyramidal tracts originate in the brain stem, carrying motor fibres to the spinal cord, and are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion. These tracts include the rubrospinal, reticulospinal, and tectospinal tracts.
The rubrospinal tract originates from the red nucleus, a midbrain structure. As the fibres emerge, they decussate (cross over to the other side of the CNS), and descend into the spinal cord, thus providing contralateral innervation. Its exact function is unclear, but it is thought to play a role in the fine control of hand movements.
The reticulospinal tracts have two pathways, both of which arise from different locations in the brain stem. The medial reticulospinal tract arises from the pons, and facilitates voluntary movements and increases muscle tone. The lateral reticulospinal tract arises from the medulla, and inhibits voluntary movements and reduces muscle tone.
There are two vestibulospinal pathways – medial and lateral – which arise from the vestibular nuclei that receive input from the organs of balance. The tracts convey this balance information to the spinal cord, where it remains ipsilateral. Fibres in this pathway control balance and posture by innervating the ‘anti-gravity’ muscles (flexors of the arm, and extensors of the leg), via lower motor neurones.
The tectospinal tract begins at the superior colliculus of the midbrain, a structure that receives input from the optic nerves. The neurones quickly decussate, and enter the spinal cord, terminating at the cervical levels of the spinal cord. The tectospinal tract coordinates movements of the head in relation to vision stimuli.
Upper motor neurone lesions, also known as supranuclear lesions, can affect either the corticospinal or the corticobulbar tracts. Damage to the corticospinal tracts can result in the cardinal signs of an upper motor neurone lesion mentioned above. Damage to the corticobulbar tracts can result in different degrees of muscle weakness, as many of its tracts are bilateral. However, there are a few exceptions – a unilateral lesion to the hypoglossal nerve will result in spastic paralysis of the contralateral genioglossus, for example. Similarly, a unilateral lesion to the facial nerve will result in spastic paralysis of the muscles in the contralateral lower quadrant of the face.
The descending motor tracts of the central nervous system (CNS) allow for the organization of motor signals originating from the cerebral cortex, brainstem, and internal capsule. These tracts are comprised of the pyramidal tracts, the extrapyramidal tracts, the corticospinal tracts, and the corticobulbar tracts, each of which serve a specific role within the CNS. Comprehension of the upper motor neurone lesions is essential for defining the outcome of masses in the CNS. Additionally, it is important to understand the same structural organization of the cervical, lumbar, and sacral segments of the cord and the motor nuclei of the cranial nerves in order to control muscle tone, balance, posture, and locomotion.
The pyramidal tracts, also known as the corticospinal tracts, are a set of descending tracts originating from the cerebral cortex. The primary motor cortex, premotor cortex, and supplementary motor area have a range of inputs that develop into the neurones that will converge and descend through the internal capsule. This connectivity is clinically important, as the internal capsule is particularly vulnerable to compression caused by haemorrhagic bleeds, or a “capsular stroke”, which can lead to lesions of the descending tracts. From the internal capsule, the neurones pass through the crus cerebri of the midbrain, the pons, and eventually the medulla.
The tracts divide into two parts once they enter the lowest, or caudal part of the medulla. The fibres of the lateral corticospinal tract cross over to the other side of the CNS, referred to as a decussation, and descend into the spinal cord. Here, they terminate in the ventral horn at all segmental levels, from where the lower motor neurones transmit signals to the muscles of the body. The anterior corticospinal tract does not cross over but rather remains ipsilateral, descending into the spinal cord and terminating in the ventral horn of the cervical and upper thoracic segmental levels.
The corticobulbar tracts originate from the lateral aspect of the primary motor cortex, receiving the same inputs as the corticospinal tracts. They converge and pass through the internal capsule to the brainstem, where they terminate on the motor nuclei of the cranial nerves. Here, the lower motor neurones become responsible for sending the motor signals to muscles in the face and neck.
The extrapyramidal tracts are responsible for conveying motor fibres from the brainstem to the spinal cord. This is an important role as they are responsible for the involuntary and automatic control of musculature, such as muscle tone, balance, posture, and locomotion. The tracts consist of four parts; the vestibulospinal and reticulospinal tracts remain ipsilateral, while the rubrospinal and tectospinal tracts decussate, providing contralateral innervation.
The vestibulospinal tracts comprise of two pathways; medial and lateral. These tracts originate from the vestibular nuclei, which receive input from the organs of balance. From there, information is sent to the spinal cord where the pathways remain ipsilateral. The fibres innervate anti-gravity muscles (flexors of the arm and extensors of the leg) to control balance and posture in lower motor neurones.
The reticulospinal tracts, the two pathways of which have different functions, originate from either the pons or the medulla. The medial reticulospinal tract from the pons facilitates voluntary movements and increases muscle tone, while the lateral reticulospinal tract from the medulla inhibits voluntary movements and reduces muscle tone.
The rubrospinal tract originates from the red nucleus, situated in the midbrain. The fibres emerge from the nucleus and cross over to the other side of the CNS, before descending into the spinal cord. This pathway has contralateral innervation, and its function is still unclear, though it is thought to have a role in fine-tuning hand movements.
The tectospinal tract begins at the superior colliculus of the midbrain. This structure receives input from the optic nerves, which the neurones quickly cross over (decussate) before entering the spinal cord. They terminate at the cervical levels of the spinal cord and are responsible for coordinating the movements of the head in relation to visual stimuli.
An understanding of the descending motor tracts is essential for clinicians to properly diagnose diseases, and to be able to accurately assess the outcome of motor lesions in the CNS. The pyramidal tracts, extrapyramidal tracts, corticospinal tracts, and the corticobulbar tracts have distinct functions that must be taken into consideration when assessing motor signals.
The brain stem, specifically the medulla oblongata, and lower motor neurons are responsible for controlling the muscles of the body, face, and neck. Depending on the electrical signals sent from the brain to the muscles, a range of muscle contraction can be achieved. This motor control is essential for our everyday movements, from walking to facial expressions.
However, dysfunction in this process can be caused by several events, such as haemorrhagic bleeds, capsular strokes, and decussation lesions. A haemorrhagic bleed is when a vessel ruptures, causing blood loss, while a capsular stroke involves the area of brain tissue that controls movement and sensation. A decussation lesion refers to an injury in the area in which nerve fibres cross from one side to the other.
Haemorrhagic bleeds, capsular strokes, and decussation lesions can all result in a disruption of the signals travelling between the brain and the muscles. This disruption can lead to difficulties in controlling the muscles of the body, face, and neck, and thereby interpret into various motor control problems. Symptoms range from paralysis to general muscle weakness.
In severe cases, where the disruption of signals between the brain and the muscles is very significant, surgery may be required to correct the problem. Surgery for this type of condition can involve reconnecting the nerves, repairing the damage, or implanting electrical stimulators to help recuperate the motor control. Additionally, if the cause of the problem is a stroke, medications might be used as well.
Once the underlying cause of the motor control issues is identified and treated, physical therapy is often beneficial in helping a patient regain normal motor control. Physical therapy can include exercises to improve strength, balance, coordination, endurance, and flexibility. Additionally, improving posture, body mechanics, and movement strategies can help a patient get back to the level of physical activity and mobility they had prior to the motor control issues.
In conclusion, motor control is an essential part of our everyday lives. Problems with motor control can be caused by haemorrhagic bleeds, capsular strokes, and decussation lesions. Treatment methods such as surgery, medications, and physical therapy can help to address these issues and help restore motor control. It is important to get the appropriate medical attention for any motor control related issues in order to prevent long-term damage and ensure that a person is able to continue with their daily activities.
