Neuroanatomy: Tracts of the Spinal Cord

Hi :) In this post we'll be discussing the spinal cord and how it functions. Our ultimate aim will be to use the information from this post and other neuroanatomy posts to describe the neuroanatomy of locomotion. This is quite a tricky section and may seem confusing at times but don't give up :) You can do it! You might want to listen to some good music to help you through this, why don't you try this playlist,which you can listen to online.

The Spinal Cord

The spinal cord can be described in two ways, anatomically or functionally. We'll start with the anatomical method and then we'll describe the spinal cord based on its various functions.

Anatomically:

Cross Section of a Cervical Vertebra. Source.
Please see this website if you'd like to use this diagram.

Afferent fibres, which are sensory, enter the spinal cord via the dorsal root ganglia. The efferent fibres, which are motor fibres, exit the spinal cord through ventral nerve roots. A spinal nerve is composed of afferent fibres and efferent fibres. Below is a good diagram which explains this nicely. 

The Formation of a Spinal Nerve. Source

The spinal cord also contains grey matter and white matter. Grey matter is mainly made up of nerve cell bodies but some nerve fibres may be present. The nerve cell bodies in the dorsal horns of the spinal cord receive axons from the dorsal root ganglia. On the other hand, nerve cell bodies in the ventral horns of the cord give off axons which form the ventral roots of the spinal nerves. 

White matter surrounds the grey matter inside the spinal cord and consists of myelinated and unmyelinated axons. White matter has four divisions, known as funiculi: the dorsal, ventral and two lateral columns. Within each funiculus is a group of functionally similar fibres which run along side each other to form tracts. Below is a useful diagram which shows how the positions of these tracts. Please note that this diagram refers to human anatomy, thus anterior = ventral and posterior = dorsal on domestic species.

Tracts of the Spinal Cord. Source.

 It's probably best if you click the diagram as this will take you to the original copy which is much larger and easier to read.

The Afferent System

The afferent pathways follow a general pattern which involves a chain of three neurons. The first neuronal body is in the dorsal root ganglion. The second neuron decussates (crosses over), however there are some exceptions. The third neuron generally has a cell body located in the thalamus which projects to the cortex (telencephalon, see this post). There are two types of afferent pathways: conscious and unconscious. 

Conscious:

These are involved with the forms of sensory perception with which we are aware. These include kinesthesia; proprioception; pain, heat and cold; and the special senses such as vision, hearing, balance, taste and olfaction. These stimuli cause nerve impulses to be sent along specific tracts, these include:

• Fasciculus gracile and fasciculus cuneatus: these form the lemniscal system and sense kinesthesia and prorioception. Primary sensory neurons enter the spinal cord and branches of their axons pass through to the dorsal funiculus. The dorsal funiculus is separated into two parts, the medial part contains fibres from the hindlimb and the caudal part of the trunk and is called the fasciculus gracile. The lateral part of the dorsal faniculus contains fibres from the forelimb, cranial part of the trunk and the neck and are called the fasciculus cuneatus. The fibres of both of these divisions end in the medulla oblongata in areas called the nuclei of the fasciculus gracile and fasciculus cuneatus. Here they synapse with the second-stage neurons which decussate and lead to the thalamus as the medial lemniscus. At the thalamus they synapse with third-stage neurons which lead to the somatosensory area of the cerebral cortex.  
• Spinothalamic: This is involved in sensing noxious stimuli, heat and cold. This is less developed in domestic species than the tracts mentioned above and may not exist as a discrete tract in all species. For example, the cat, cow and horse are said to rely more on the ascending reticular formation. Sharp pain, such as a pin prick, is transmitted quickly by myelinated fibres with accurately localised projection through the thalamus. Deep, or aching, pain is transmitted by thin unmyelinated fibres and is poorly localised because it is only partly projected through the reticular formation.
• Specific cranial nerves also exist and these are involved in senses such as sight, smell, vision, hearing, balance and taste. 

Unconscious:  

This includes pathways that we can't feel, such as:

• Spinocerebellar tracts: these are very important in hoofed animals (ungulates) and are involved in muscle proprioceptor responses by transmitting information from muscle and tendon receptors. They inform the cerebellum of the state of contraction of the skeletal muscles. They are essential for the adjustment of muscle tone and synergy between muscles to allow smooth, coordinated motion. Primary axons enter the spinal cord and end at the dorsal horn cells. The axons of the second-stage neurons can be separated into the dorsal and ventral spinocerebellar tracts. Information from muscle spindles travels along the dorsal tract which follows a direct pathway towards the brain and enters the cerebellum through the caudal peduncle without decussating. Information from tendon receptors travels along the ventral tract. This fibres in this tract decussate within the cord near their origins and then ascend to the midbrain. Once there they turn back and enter the cerebellum through the rostral peduncle. The fibres decussate again within the cerebellar cortex which is also where they end. These tracts are only concerned with information from the trunk and hindlimb. 
• The reticular activation system (RAS): This is the large polysynaptic core of the brainstem and spinal cord and contains fifty per cent of their neurons. The RAS system receives sensory information from most of the body and also has motor connections with most of the body. Its neurons are situated around the central canal and its axons form the spinal reticular tract in the lateral funiculus. It continues through all parts of the brainstem and to the thalamus. From the thalamus its projection to the cerebral cortex is very diffuse and indistinct. The RAS functions in arousing the cortex to alert it to use its more specific sensory systems. Inhibition of the RAS results in sleep or coma. It's also involved in non-specific deep or severe pain transmission.   

The Efferent System 

This system is composed of all the centres and tracts that have a significant influence on the motor cortex or on lower motor neurons and thus all the motor pathways. It is phylogenically primitive and so is well developed in most vertebrates. It is responsible for voluntary, involuntary and automatic movements that maintain posture and daily activities. In terms of structure, the efferent system is composed of command centres, feedback circuits and spinal pathways. 

Somatic motor activity is regulated within the central nervous system by two types of cells: upper and lower motor neurons. Lower motor neurons are located in the ventral column of the grey matter and in the somatic motor nuclei of the cranial nerves which have somatic efferent components. The axons of the lower motor neurons (LMNs) travel within the spinal and relevant cranial nerves to the skeletal muscles where each ends on a group of muscle fibres. Lower motor neurons take part in some reflexes but are mostly directed by upper motor neurons. 

Upper motor neurons (UMNs) are involved in more complicated reflexes than LMNs and also initiate voluntary movements. They are mainly located in the motor area of the neocortex (also called the neopallium) which is an area of the cerebral cortex. UMNs can also be found in the reticular formation and red nucleus. UMNs don't connect directly with muscle fibres but they exert control through the LMNs. 

Command Centres

In the forebrain the cerebral cortex and basal ganglia act as command centres. In the midbrain the reticular formation, tectum and red nucleus do this. In the hindbrain the reticular formation and vestibular nuclei behave as command centres. The peripheral pathways of the efferent system are influenced by these command centres which can be either facilitatory or inhibitory to the spinal pathway.

 Feedback circuits:

The efferent system also has feedback circuits, these include the cerebellum, olivary nucleus, thalamus and basal ganlia.

The Corticospinal and Corticobulbar Systems:

These are evolutionarily recent and so are not present in birds, reptiles, fish and amphibians but are well developed in primates and carnivores. They work together with other tracts and are responsible for skilled voluntary movements as well as involuntary postural movements. Together, the corticospinal and corticobulbar systems are known as the Pyramidal System. The pyramidal system begins with the neurons from the various regions of the neocortex, especially the primary motor area. The axons of these neurons converge as they exit the telencephalon and form part of an area of the brain known as the internal capsule. They then carry on to the medulla oblongata where they form the pyramids of the medulla oblongata. Here, the corticospinal fibres continue through to the spinal cord while the corticobulbar fibres move towards the nuclei of various cranial nerves. 

Some of the fibres of the corticospinal tract decussate at the medulla to become the lateral corticospinal tract which lies in the lateral funiculus. The other corticospinal fibres continue along the spinal cord and do not decussate at the medulla. These form the ventral corticospinal tract and are found in the ventral funiculus and decussate just before their destination. This tract is minor in primates and negligible in domestic species. 

The corticobulbar fibres control voluntary movements of the eye, jaw, facial muscles, tongue, pharynx and larynx.   

The Rubrospinal System

This tract begins in the red nucleus, decussates and then passes through the medulla oblongata. The rubrospinal tract borders the lateral corticospinal tract in the lateral funiculus of the spinal cord. Whilst travelling towards the end of the spinal cord, the tract projects on LMNs via interneurons. These LMNs are linked to flexor muscles. The rubrospinal tract is important in carnivores and is the most developed motor pathway in ungulates. Its function is to control posture and is essential for locomotion. 

 The Reticulospinal Tract

This tract originates in the reticular formation and breaks up into two efferent tracts. One is the pontine reticular formation and is more autonomous and stimulates extensor muscle tone. The other is the medullary reticular formation which is reliant on higher input and mainly suppresses extensor muscle tone. 

Reflex/Feedback motor systems:

The vestibulospinal tract is concerned with the reflex postural tone rather than voluntary movement. This tract originates in the vestibular nucleus and facilitates ipsilateral extensors but is normally dampened by cerebellar inhibition. 

The tectospinal tract, which is initiated from the tectum is a minor motor pathway for neck muscles and is only involved in unconsciuos reflex activity. 

Feedback on intended movements is sent from the brainstem to the cerebellum through the pontine nuclei, which come from the primary somatic motor complex. Feedback is also transmitted through the olivary nucleus which comes from the red nuclei and reticular formation.  

And that's what we need to know for this section. If you have any questions please let me know :)