Monday, 4 June 2012

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.


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. 


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. 

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 :)


Saturday, 2 June 2012

Neuroanatomy: Primary Divisions of the Brain

Hello :) The brain consists of several divisions including the: telencephalon, diencephalon, mesencephalon, metencephalon, and myencephalon. In this post, we'll take a look at each of these divisions.

When an animal embryo is developing, the brain starts out as a simple tube beneath the skin. As the embryo grows and develops, the anterior end of this tube swells and becomes the brain. Shortly after this, the brain forms three divisions which later expand and change to form 5 divisions, we'll go through each of these divisions below.


The forebrain is composed of the telencephalon and the diencephalon.


An aside: The word 'telencephalon' is derived from the Greek word 'telos', meaning end, and the word 'encephalon' which refers to the brain. So telencephalon refers to the end of the brain. This may help you remember what part of the brain it refers to. 

The telencephalon refers to the cerebrum, this includes the left and right cerebral hemispheres.The largest parts of the telencephalon are the left and right cerebral cortices which consist of the outer cortical grey matter. Below this grey matter is white matter which partly encloses the basal ganglia. The telencephalon contains the lateral ventricles (see this post). The telencephalon can be divided up into regions which are named after the bones of the cranium which cover them, these are shown in the diagram below. Blue represents the frontal lobe, green - temporal, yellow - parietal, and red - occipital. 

Lobes of the Cerebrum (Telencephalon)
Each of these areas contain specialised functional areas which are referred to as projection and association areas. The primary projection areas of the cerebral cortex are associated with certain somatic tracts which are projected via the thalamus. The thalamus is an area of the brain between the cerebral cortex and midbrain and is involved in relaying sensory and motor signals to the cortex. The projection areas are surrounded by a region called the association area which is involved in the processing and cognition of stimuli. The primary projection and association areas include:
  • Somatic Motor Area: this is found in each of the cerebral hemispheres and functions in learnt motor skills. These are the origins of the cerebrospinal tract which is responsible for the initiation of the movements of the musculoskeletal system.
  • Somatic Sensory: this is found in the parietal lobe and functions in conscious sensations. In regards to movement, it also senses tensions, positions of limbs and the forces acting on various body parts. 
  • Visual: located in the occipital lobe and involved in vision.
  • Auditory: located in the temporal lobe and involved in hearing.
  • Olfactory: this is located in a special area of the brain known as the piriform cortex, it functions in smell.
The telencephalon also includes the basal ganglia, which are also called basal nuclei. These are masses of grey matter which lie beneath the cortex within the white matter of the cerebral hemispheres. They are a series of discrete grey matter units that have many interconnections. Their job is to support the body's voluntary movements through postural adjustments as well as to assist or initiate the performance of automatic movements.

Neocortical Dominance

The neocortex is the outer part of the cerebral cortex and is involved in higher functions such as sensory perception and the generation of motor commands. Out of all the vertebrates, humans have the greatest ability to conceptualise, communicate, remember, associate and analyse input into their central nervous systems. The large neocortex of humans allows highly sophisticated and flexible responses to environmental change. Domestic animals also have many of these abilities but the extent varies between species.

Other species however, such as retiles and amphibians, have more stereotypical responses. Their responses to environmental stimuli are mainly reflexes. The more highly evolved the species is, the more it is able to vary its responses on the basis of learned and remembered past experiences.


The telencephalon also includes an area of the brain known as the Rhinencephalon. This region is mainly related to olfaction and has olfactory and non-olfactory components. In the olfactory component, the axons of olfactory neuroepithelial cells pass through the cribiform plate and synapse with another neuron in the olfactory bulb. This neuron passes via interneurons to the cerebral cortex. The non-olfactory component includes the hippocampus which is an association area. This is believed to be involved with emotion, recent memory and autonomic function.

Limbic System

This includes the limbic lobe as well as the subcortical nuclei. The limbic lobe consists of the deep medial cortical grey matter while the subcortical nuclei includes structures such as the amygdala, hypothalamus, and hipocampus. This system receives and associates visceral, oral, sexual and basic sensory (olfaction, optic and auditory) impulses and then projects them to the hypothalamus. The system is also invloved with emotional and behavioural patterns via the hypothalamus through the autonomic nervous system.  


Diancephalon refers to the most rostral part of the brain stem. 

Another aside: The word 'diencephalon' is derived from the Greek word 'dia', meaning 'through' and, 'encephalon' which refers to the brain. So one way to remember where the diencephalon is located is to imagine looking through the brain :)
The Diencephalon (red). Source.
Please see this website if you'd like to use the animation.
 The diencephalon lies beneath the cerebral hemispheres, as shown in the animation above. In 'lower' vertebrates the diencephalon is the main association centre of the brain. However, in 'higher' vertebrates the cerebral hemispheres have taken over this role. The diencephalic nuclei still remain and are woven into the pathways that lead to and from the cerebrum. The largest part of the diencephalon is the thalamus, which I wrote a little about above. 

The thalamus is a complex intercommunicating network which lies along the full length of the diencephalon on either side of the third ventricle. The two thalami are connected by an interthalamic adhesion. The thalamus is mostly made up of grey matter in the form of many closely packed thalmic nuclei which relate to specific pathways. The thalamus functions as:
  • a relay centre for all sensory input to the cerebral cortex. This includes stimuli such as proprioception, pain and temperature, stimuli from the cerebellum as well as some stimuli from the basal ganglia.
  • a recognition centre for some specific sensations such as tactile, thermal and pain sensations. In addition, there is a basic thalmic awareness of pain, touch, heat and vibration but there is poor localisation. Ie. the thalamus can tell something is going on but it can't tell where it is occurring. 
The diencephalon also contains the hypothalamus which functions in regulating several hormones as well as other bodily functions such as eating, drinking and sleeping. The hypothalamus forms the floor of the brain and responds to neural input (this includes cortical input as well as ascending tracts from the brain stem and spinal cord). It also responds to factors within the circulating blood such as temperature, osmotic pressure and hormone levels.

The hypothalamus exerts is effects through the autonomic nervous system as well as the endocrine system. The anterior hypothalamus is the origin of the parasympathetic nervous system while the posterior is the origin of the sympathetic system.  


The Mesencephalon

The mesencephalon refers to the midbrain.
Etymology: I find that knowing what the root of the word that we need to know is helps me to remember what it means. The word 'mesencephalon' is derived from the Greek word 'mesos', which means 'middle', as well as the word 'encephalon' which refers to the brain. So mesencephalon = 'middle brain'. Its easy to remember where the mesencephalon is if you know the origin of the word :)
 The main role of the mesencephalon is as a channel for fibres to pass through. It also contains structures such as the red nuclei which are the origins of the rubrospinal tract. The mesencephalon has a stratified structure and is composed of several layers. The most dorsal of these layers is the tectum whose major features are four rounded surface swellings. The paired caudal swellings are known as the caudal colliculi and are the integration centres of auditory pathways. It receives auditory input from the inner ear and some of these impulses are transferred to the relevant part of the cerebral cortex for interpretation. Some impulses are also transferred to the rostral calliculus and ultimately to the tectospinal system in the reflex turning of the head towards the source of a sudden loud noise.

The rostral colliculi receive some input directly from the optic tracts. It also mediates reflex impulses such as blinking and pupillary adjustments. 



The main part of the metencephalon is the cerebellum but it also contains the pons. The main function of the cerebellum is to maintain motor synergy throughout the body by the coordination of motor activity, the maintenance of muscle tone and equilibrium. If an animal wants to move, the somatic motor area of the cerebral cortex will communicate its intentions to the cerebellum through corticopontine tracts which will ensure that these intentions are followed out. The cerebellum does this through pre-control, that is, the cerebellum compares the current state to the intended state and modifies the intended movement accordingly. The cerebellum also exerts feedback control to ensure that the movements occur smoothly. In addition, the cerebellum can remember information related to motor events, this is how people can learn to walk. 

In terms of structure, the metencephalon lies above the fourth ventricle in the caudal cerebral fossa. It is connected to the brainstem by three pairs of peduncles, which are thick bundles of fibres. These are the superior, middle and inferior cerebellar peduncles. The cerebellum also has hemispheres which are connected to each other by the 'median vermis'. The cerebellum contains several important subcortical nuclei, these are:
  • The dentate nucleus: this is the largest and most important and receives information from the cerebellar hemispheres. Its main role is limb coordination, posture and muscle tone. 
  • The fastigial nucleus: this receives information mainly from the vestibular apparatus and is mainly involved with equilibrium, muscle tone and axial posture. 
The Myelencephalon

The myelencephalon refers to the medulla oblongata (or medulla for short). The medulla is located directly beneath the cerebellum and is physiologically important because of its many control centres and ascending and descending pathways. The control centres include those involved with respiration, cardiac and vasomotor activity as well as those regulating vomiting, swallowing, coughing, gastric secretion, urinating and defecating. In addition, it has the origin of the sixth to twelfth cranial nerves. The medulla is the most primitive region of the brain and functions mainly at a reflex level.

That's it for this post, if you have any questions please feel free to ask :)

Friday, 1 June 2012

Neuromuscular Anatomy: Protection of the CNS

Hi :) This post will discuss the ways in which the central nervous system (CNS) is protected, that is, by bone, meninges and cerebrospinal fluid. I'll discuss the bones and their relevant landmarks of the skull and vertebral column as well as the meninges of the CNS and the ventricles of the brain.


The Skull

The brain is encased by the skull which provides protection. This website has some good diagrams of what the skull looks like. I'm not going to go into too much detail here, the best way to learn these bones is to actually look at a real skull. I'd suggest going to an anatomy museum or lab, if you have access to one, to handle a skull. This is probably one of the best ways to learn the bones. You may also find it helpful to try and sketch and label the bones or to even quiz yourself with a game such as this. Another good way to remember the major bones of the skull is to use a mnemonic. One of the best I've heard (and probably the strangest) is: "I Never Masticate Lean Protein From Zebra Babies, Please Tell Our Moms".
  • I (incisive),
  • Never (Nasal),
  • Masticate (Maxilla), 
  • Lean (Lacrimal), 
  • Protein (Palatine), 
  • From (Frontal), 
  • Zebra (Zygomatic), 
  • Babies (Basisphenoid wing), 
  • Please (Parietal), 
  • Tell (Temporal), 
  • Our (Occipital), 
  • Moms (Mandible)
The stranger the better because you're more likely to remember it!

 The Vertebral Column

In dogs, the vertebral column is divided into 5 main regions: cervical (which includes 7 bones), thoracic (13), lumbar (7), sacral (3) and caudal (20). The general structure of a vertebra is shown below, although there are differences in the vertebra of different regions in the spinal cord. C1 and C2, the atlas and axis are heavily modified.
Source. Please see this website if you'd like to use the diagram.


Meninges are three continuous membranes which surround and help protect the brain and spinal cord. 

Dura Mater

This is the tough outermost membrane and in the skull is fused with the inner periosteum.

Spinal Dura

The spinal cord only has a single layer of dura which is separated from the walls of the vertebrae by an epidural space to form a tube (called the dural tube). The epidural space is filled with fat and the internal vertebral venous plexus, which are veins inside the vertebral canal. The fat and veins cushion the spinal cord and allow it to adjust to the movements of the back and neck. The dural tube is attached to the upper surface of the caudal vertebrae. Distally, it tapers with the pia mater and anchors to the periosteum of the proximal coccygeal vertebrae to form the filum terminale. The role of this is to prevent the spinal cord from sliding cranially and caudally when the animal bends and moves around.  

Cranial Dura

Because the dura mater is fused with the inner periosteum of the skull, no epidural space exists here. In the skull, where is it known as cranial dura, it is firmly attached to several sites within the cranial vault.  This dura has two layers: the outer endosteal and inner meningeal. The cranial dura has a ventrally directed fold which lies longitudinally between the cerebral hemispheres named the falx cerebri. The dorsal sagittal venous sinus is present on the dorsal surface of the falx cerebri between its endosteal and meningeal layers. The caudal region of the falx cerebri attaches to the tentorium cerebelli.

The tentorium cerebelli is a part of the cranial dura which separates the cerebellum from the caudal end of the cerebral hemispheres. The core part of the tentorium is made up of bone and is surrounded by meninges.

Additionally, the cranial dura extends along the special sensory nerves. The dura runs extra cranially along the optic nerve until it inserts onto the sclera of the eye. The dura also follows the olfactory tract until the olfactory nerves are through the cribiform plate. The vestibulocochlear nerve is also surrounded by dura until it reaches the inner ear.

Arachnoid and Pia Mater:

The arachnoid and pia mater membranes are relatively delicate compared to the dura mater. The arachnoid lines the deep surface of the dura mater while the pia closely attached to and supports the brain and spinal cord, it also contains numerous small blood vessels. The arachnoid and pia mater are separated by the subarachnoid space which contains cerebrospinal fluid (CSF). The role of the subarachnoid space is to provide hydraulic protection to the brain and spinal cord, to act as a temporary reservoir for CSF, it also acts as a mechanism to alter cranial capacity and provides a route for blood vessels.

The inner surface of the arachnoid is joined to the pia by many trabeculae and filaments (this looks kind of like a spider's web, which is where the name subarachnoid comes from). The subarachnoid space contains cerebrospinal fluid. 

Cerebrospinl Fluid

The cerebrospinal fluid (CSF) acts as a hydraulic skeleton within and surrounding the brain and works to buoy up and protect the brain and spinal cord. In addition, it protects the brain through chemical buffering and transports nutrients and removes wastes to and from the CNS. CSF is produced by the ependymal cells of the ventricles and central canal, the choroid plexuses and through selective leakage of vessels in the pia mater. Choroid plexuses are leaky tufts of arterioles, pia and ependymal epithelium within the brain's ventricles. Most of the CSF is produced by the choroid plexuses through dialysis from the arterioles and secretion from the ependymal cells.

CSF flows from the lateral ventricles through the interventricular foramina to the third ventricle then through the mesencephalic aqueduct to the fourth ventricle. Most of the CSF exits from each side of the lateral foramina into the subarachnoid space of the cerebellomedullary cistern. From there it flows to the surrounding brain and spinal cord.

Since CSF is derived from blood it must return to the blood stream after draining from the central nervous system. Within the central nervous system, the major drainage route for CSF are the arachnoid villi (aka. arachnoid granules). These villi are projections of the arachnoid membrane into the venous sinuses of the dura mater (such as the superior sagittal sinus). At each villus, CSF is separated from blood by flattened fibroblasts and endothelial cells. The villi act as valves which regulate the flow of CSF into the venous sinuses. When the pressure of the CSF is greater than venous pressure, the villi expand and the spaces between the cells increase, allowing more fluid to flow into the venous sinus. When the venous pressure is greater than the pressure of the CSF the villi collapse and this blocks the flow of blood into the subarachnoid space.

In addition, CSF can be absorbed directly by veins in the pia and the subarachnoid space due to an osmotic pressure effect.  


These are an interconnecting series of cavities filled with CSF within the brain. There are four ventricles: the paired lateral ventricles as well as the third and fourth ventricles. There is one lateral ventricle within each cerebral hemisphere. The third ventricle is a narrow vertical space between the two lateral ventricles. An interventricular foramen allows each lateral ventricle to communicate with the third ventricle which is linked to the fourth ventricle by the mesencephalic aqueduct. The fourth ventricle lies beneath the cerebellum and above the pons and medulla. On the lateral sides of the fourth ventricle are lateral apertures which connects the ventricle to the subarachnoid space.

It's difficult to find diagrams showing the canine ventricular system but it should look something like this :)

The Ventricular System

That's it for this post, please feel free to leave any questions in the comments section below :)