Central Nervous System Pathways
Introduction[edit | edit source]
The central nervous system uses ascending and descending pathways to communicate with the external environment.
Ascending Sensory Pathway[edit | edit source]
The spinal cord, basically a highway for nerves, streamlines sensory and motor signals to the brain and the body. Information detected by sensory receptors in the periphery is transmitted along ascending neural tracts in the spinal cord. Located in the white matter of the spinal cord, the ascending sensory tracts arise from either the the cells of the spinal ganglia or the intrinsic neurons within the grey matter that receive primary sensory input. There are many sensory tracts and pathways carrying different types of sensory information from the periphery to the cerebral cortex. In humans the major sensory pathways include:
- The spinothalamic tracts: The spinothalamic tract, one of the most important pathways of the nervous system, lies anteriolaterally to the ventral horn of the spinal grey matter. This pathway comprises of three neutron sets and forms part of the somatosensory system. The lateral spinothalamic tract carries information about pain and temperature, and the anterior spinothalamic tract carries information about crude touch.
- Dorsal Column Medial Lemniscal Pathway: The dorsal column pathway is one of the ascending tracts i.e. the neural pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex. In the spinal cord, this pathway travels in the dorsal column, and in the brainstem, it is transmitted through the medial lemniscus hence the name dorsal column-medial lemniscus pathway. Conveys proprioception, light touch and vibration.
- Spinocerebellar Tract: Carry unconscious proprioceptive information gleaned from muscle spindles, Golgi tendon organs, and joint capsules to the cerebellum. The cell bodies of the primary sensory neurons that bring this information from such receptors to the spinal cord are located in the dorsal root ganglia. There are three spinocerebellar tracts, viz. anterior and posterior spinocerebellar and cuneocerebellar tracts
Descending Motor Pathways[edit | edit source]
There are many motor tracts in the spinal cord. Some of these are under conscious control and others under unconscious, reflexive or responsive control. These motor tracts can be grouped functionally into Extrapyramidal and Pyramidal tracts
These functional groups contain several anatomical tracts, one for each side of the body:
- Pyramidal: conscious control of muscles from the cerebral cortex to the muscles of the body and face; and
- Extrapyramidal: unconscious, reflexive or responsive control of muscles from various brainstem structures to postural or anti-gravity muscles
Pyramidal Tracts[edit | edit source]
The pyramidal tracts are named as such due to their course through the pyramids of the medulla oblongata. The pyramidal tracts are responsible for the conscious, voluntary control of the body and face muscles. They can be divided into two tracts.
Carries information from the cerebral cortex to the spinal cord. Also called the pyramidal tract as it is the only point where all the fibres are collected together without contamination by other fibre tracts in the medullary pyramids of the brain stem. Primary motor cortex is the main source of input to this tract, but the premotor and supplementary motor cortex also contribute fibres. Its projections are primarily contralateral and have a strong influence on the activity of groups of spinal motoneurones, which innervate distal muscles of the hands and feet. Most of the fibers, approximately 85%, cross the midline in the decussation of the pyramids in the brain stem and then descend through the spinal cord in the lateral corticospinal tract, while the other 15% cross within the spinal cord at the level they terminate and are carried within the medial corticospinal tract. According to Harvey (2008), the cervical upper motor neurons are centrally located within the corticospinal tract while the lumbar and sacral neurons are peripherally located, which explains neurological patterns of loss seen with certain types of incomplete spinal cord injuries where the peripheral rim of the spinal cord is undamaged.
The corticobulbar tract is composed of the upper motor neurons of the cranial nerves. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neurons within brainstem motor nuclei. Contrast this to the corticospinal tract were the cerebral cortex connects to spinal motor neurons, and controls movement of the torso, upper and lower limbs.
Extrapyramidal Tracts[edit | edit source]
- The extrapyramidal tracts all originate in the brainstem and do not pass through the pyramids.
- These tracts all carry motor fibres to the spinal cord that allow for unconscious, reflexive or responsive movement of muscles to control balance, locomotion, posture and tone.
There are four tracts.
Reticulospinal tract is a descending tract present in the white matter of the spinal cord, originating in the reticular formation (the archaic core of those pathways connecting the spinal cord and the brain). It consists of bundles of axons that carry information or orders from the reticular formation in the brainstem to the peripheral body parts.
- The Reticulospinal tract is responsible primarily for locomotion and postural control. The Reticulospinal tract is comprised of the medial (pontine) tract and the lateral (medullary) tract.
- It is part of the Extrapyramidal system.
Originates with the axons of the vestibular nuclei (within the brainstem) and terminates by synapsing with the interneurons present in the anterior gray column of the spinal cord. The vestibulospinal tracts consist of a medial vestibulospinal tract and a lateral vestibulospinal tract. They are essential for a number of reflex actions performed by the body.
- Each tract is responsible for increasing antigravity muscle tone in response to the head being tilted to one side. Antigravity muscles are extensor muscles in the limbs
- Part of the Extrapyramidal system.
- It is part of the extrapyramidal system and is important for regulating the activity of the motor neurons.
- It plays an important role in a number of body reflexes.
Autonomic Pathways[edit | edit source]
Autonomic nerves are also carried by the spinal cord. Sympathetic nerves exit the vertebral canal via thoraco-lumbar spinal nerves, and parasympathetic nerves exit via sacral spinal nerves. As a result individuals with a cervical lesion lose supraspinal control of the entire sympathetic nervous system and of the sacral part of the parasympathetic nervous system. Individuals with thoracic, lumbar or sacral lesions lose varying amounts of supraspinal control of the sympathetic and parasympathetic nervous system as determined by the level of the lesion. Some parasympathetic fibres are carried within cranial nerves and are unaffected by spinal cord injury.
References[edit | edit source]
- Daroff RB, Aminoff MJ. Encyclopedia of the neurological sciences. Academic press; 2014 Apr 29. Available:https://www.sciencedirect.com/science/article/pii/B9780123851574011763 (accessed 29.4.2022)
- Geeky medics The Descending Tracts of the Central Nervous System Available: https://geekymedics.com/the-descending-tracts-of-the-central-nervous-system/ (accessed 29.4.2022)
- Harvey L. Management of Spinal Cord Injuries: A Guide for Physiotherapists. Elsevier Health Sciences; 2008 Jan 10
- Handwritten tutorials. Spinal Pathways 1 - Spinal Cord Anatomy and Organisation. Available from: http://www.youtube.com/watch?v=5B87zsAKmWc [last accessed 29/08/16]