Upper Motor Neurons

Hello :) In this post we'll be discussing the physiology behind upper motor neurons (UMNs). We'll take a look at the major descending UMN tracts as well as what happens when these tracts are damaged.

Upper motor neurons (UMNs) have cell bodies that are in the cerebral cortex or brain. Their axons descend in the white matter of the spinal cord to synapse with interneurons or lower motor neurons in the grey matter of the ventral horn. The UMNs that have cell bodies in the cerebral cortex are involved with initiating, maintaining and planning sequences of voluntary movements. On the other hand, UMNs with cell bodies in the cerebellum are involved with the regulation of muscle tone, the control of posture and basic “navigational” movements.  Basically, upper motor neurons are synonymous with the central nervous system.

Descending Upper Motor Neuron Tracts

The UMN tracts can be divided into the pyramidal and extrapyramidal systems and this depends on whether the tracts traverse the pyramids of the medulla or not. However, this distinction is more important in primates than in domestic animals as the pyramidal system is more developed in primates. The separation is related to the capacity to perform finely skilled motor movements.

In addition, two general forms of movement exist:

• Voluntary, conscious, skilled movement (learned): mediated by flexor muscles from discrete contractions of a few groups of muscles, most of which are distant to the spinal cord. The cell bodies of neurons innervating these muscles are located more dorsolaterally on the cross section of a spinal cord. 
• Postural, anti-gravity muscle tone: mediated by extensor muscles from the contractions of large groups of muscles, most of which are located closer to the spinal cord. Cell bodies of neurons innervating these muscles are arranged ventromedially in the spinal cord.

Pyramidal System

These tracts have neurons with cell bodies that are mostly in the motor area of the cerebral cortex. These fibres cross over to the contralateral side at the pyramids of the medulla. In primates, most of the neurons form a monosynaptic pathway from the cerebrum and influence the LMNs directly. Lesions of tracts within this system in humans results in severe contralateral motor deficits. However, similar lesions in domestic animals results in minimal weakness although contralateral placing deficits may occur. 

Below is a summary of the major descending UMN tracts. Included is information on their beginning, ending and function. 

Corticospinal and Corticobulbar Tracts

Beginning: both are from the cerebral cortex.

Ending: the corticobulbar tract innervates the nuclei of cranial nerves in the brain stem (bilaterally, but mainly contralaterally).

The Corticospinal tract continues to the medullary pyramids where 75-90% of the axons decussate and continue as the lateral corticospinal tract (LCST). In the dog, 50% of LCST axons terminate in the cervical grey matter, 20% in thoracic grey matter and 30% in lumbosacral grey matter where they influence LMNs of distal musculature via interneurons.

Those axons that don’t decussate continue as the ventral corticospinal tract on the ipsilateral side. These synapse with motor neurons of the axial and proximal limb muscles.

Function: not very important in domestic animals.

Extrapyramidal System

These neurons originate in the cerebral cortex, including the motor area. These fibres travel to the brain stem directly or via subcortical nuclei and synapse with additional neurons along the way. The fibres pass through the spinal cord and don’t cross at the pyramids. These are usually multisynaptic pathways.

Rubrospinal Tract

This is the most important lateral descending pathway in animals.

Beginning: It originates in the red nucleus which receives afferent axons from an ipsilateral (same side) area of the cerebral cortex.

Ending: lateral grey matter of spinal cord, influencing LMNs of flexor muscles of the thoracic and pelvic limbs.

Function: Since it is found in the lateral part of the spinal cord it is involved with conscious, voluntary, skilled movement.

Vestibulospinal Tracts

This is involved mainly with the maintenance of posture and balance and consists of lateral and medial tracts.

Lateral Vestibulospinal Tract:

Beginning: vestibular nuclei which receives input from the vestibular apparatus and inhibitory input from cerebellum.

Ending:  descends ipsilaterally and terminate mainly on interneurons which activate LMNs of the trunk and limbs.

 

Function: This facilitates extensors and inhibits flexors and is involved in the maintenance of posture and balance.

Medial Vestibulospinal Tract

Beginning: rostral, medial and caudal vestibular nuclei

Ending: contains crossed and uncrossed fibres and terminates in the cervical and cranial thoracic spinal cord segments.

Function: adjusts head and neck position in response to change in posture.

Tectospinal Tract

Beginning: the visual tectum of midbrain (superior colliculus)

Ending:  crosses to the contralateral side and projects to the cervical and upper thoracic spinal cord.

Function: influences LMN circuits which work to control the axial musculature of the neck. This tract is important in the reflex co-ordination of head and eye movements in response to visual stimuli.

Reticulospinal Tracts

There are two reticuolspinal tracts: the medullary (lateral) reticulospinal tract and the pontine (medial) reticulospinal tract (RST). 

Beginning: Originate in the Reticular formation which receives projections from the cerebellum, spinal cord and higher levels of the brain including extrapyramidal nuclei. 

Ending: all levels of the spinal cord.

Function: The medullary RST projects bilaterally and supresses extensor spinal reflex activity. The proximal RST projects ipsilaterally and facilitates the extensor spinal reflex activity. 

The diagram below shows the arrangement of these tracts on a cross-section of the spinal cord. 

Spinal Tracts. Source

Damage to the Descending Pathways

Damage to upper motor neurons is common and is characterised by:

• Paresis: this is weakness due to interference with the ability of the upper motor neurons to initiate gait generation. This paresis is different to that seen in LMN damage which interferes with the ability to support weight not to initiate gait. UMN disorders result in a delay or absence of protraction of the limb with attempts to walk or hop. 
• Hypertonia: The inhibition of gamma motor neurons, which innervate muscle spindles, is lost. This is most evident in antigravity extensor muscles. Thus the increased firing of gamma motor neurons, especially to chain fibres which detect length, causes them to shorten and this increases the firing of Type 1a and Type 2 sensory fibres. This leads to a reflex increases in the firing of alpha motor neurons which shortens the muscle to a new length and increases muscle tone. 
• Spasticity: This refers to an increased muscle tone when the limbs are moving. UMN damage results in the loss of gamma motor innervation to the nuclear bag fibres of the muscle spindles which causes them to shorten. At rest, there is no increase in the firing of type 1a primary fibres. It is only when the muscle is lengthened that there is greatly increased Type 1a firing and excessive contraction. This is found predominantly in the extensors of the pelvic and thoracic limbs. 
•  Hyperreflexia: Because the UMNs are damaged, the LMNs are no longer inhibited. This leads to an increased force and amplitude of spinal reflexes which is especially evident in the patellar tendon reflex. 

That's all for this post, see you next time :)

Lower Motor Neurons

Hi :) In this post we'll take a look at how lower motor neurons work. We'll discuss neuromuscular transmission, motor unit recruitment, the patellar tendon and withdrawal reflexes, as well as muscle spindles and Golgi tendon organs. We'll also take a look at some of the signs of lower motor neuron dysfunction as well as some important definitions. Enjoy! 

Neuromuscular Transmission

Several steps are involved in the transmission of an action potential across a neuromuscular junction:

1. First, an Action Potential arrives at the presynaptic terminal of the motor neuron.
2. This causes voltage-gated Calcium channels to open which leads to an influx of calcium ions into the cell.
3. The entry of calcium triggers the release of Acetylcholine via exocytosis into the synaptic cleft.
4. Acetylcholine then binds to nicotinic acetylcholine receptors (nAChRm) on the motor end plate which causes ligand-gated channels to open.
5. This causes the flow of sodium ions into the muscle cell which generates an end-plate potential (EPP).
6. The end-plate potential causes the generation of an action potential which spreads via voltage gated sodium channels and ultimately leads to muscle contraction.
7. The neuromuscular transmission is stopped by acetylcholinesterase which degrades acetylcholine to choline and acetate.
8. Choline is taken up by the sodium dependent transporters.  

Lower Motor Neurons

Lower motor neurons (LMNs) are found in the grey matter of the spinal cord and innervate skeletal muscles. LMNs provide coordination between muscle groups which is required for organised movement. The cell bodies of LMNs are located in the ventral horn of the spinal cord while their axons exit via the ventral root. They reach muscle, where their axons branch so that each muscle fibre is innervated by the axon of a single neuron, through peripheral nerves.

Interestingly, the fibres of the LMNs are arranged somatotopically in the spinal cord. The cell bodies of the neurons which innervate the trunk and proximal limbs are found arranged ventromedially in the spinal cord. The neurons which innervate the muscles of the distal limb have cell bodies which are located more dorsolaterally in the spinal cord.

Motor Unit Recruitment

A motor unit consists of a motor neuron and all the muscle-fibres it innervates. Individual muscle fibres are innervated by a single motor neuron but a motor neuron may innervate more than one muscle fibre.

Force Generation

The generation of force in muscles is dependent on several factors. The number of motor units that are recruited plays an important effect on force as the more muscle fibres are active, the more force is generated. The force that a muscle produces is also dependent on the force generated by individual muscle fibres. Three factors affect this:

1. Frequency of stimulation: a high frequency of stimulation allows cytoplasmic calcium ion concentrations to build up and this leads to the summation of contractions.
2. Fibre Diameter: this is dependent on the number of sarcomeres in parallel.
3. Resting Fibre Length: this is the overlap of thick and thin filaments in sarcomeres.

Muscle fibre type also affects how much force is generated, three types exist:

1. Slow Muscle Fibres: these contract slowly and generate small forces.
2. Fast-Fatigable: These contract quickly and generate large forces.
3. Fast-Fatigue Resistant Fibres: these are an intermediate between the slow and fast-fatigable fibres.

The Patellar Tendon Reflex

This is the simplest type of reflex as it only involves one synapse at the spinal cord. The reflex involves several steps:

1. The tap of the hammer stretches the tendon of the extensor muscles which stretches sensory receptors.
2.  a) The sensory neuron synapses with the motor neuron in the spinal cord and excites it.
   b) The sensory neuron also synapses with an interneuron in the spinal cord which in turn synapses with, and inhibits, the motor neuron innervating the flexors of the leg.
3. a) The motor neuron conducts the action potential to the extensor muscle fibres and causes them to contract.
   b) The flexor muscles relax because their motor activity has been inhibited.
4. The leg extends.

The Withdrawal Reflex

This is a basic reflex that helps to avoid harmful stimuli. A harmful stimulus, such as a pin prick, causes electrical impulses to be transmitted along sensory nerve fibres to the spinal cord. Here they synapse with an interneuron (also known as local circuit neurons) which subsequently synapses with a motor neuron. The motor neuron innervates the muscles near the source of the stimulus and causes them to flex in order to move the body part away from the noxious stimulus. This is known as a polysynaptic reflex because more than one synapse occurred between neurons. Interestingly, spinal reflexes will continue to occur even if that spinal cord segment has been isolated from the rest of the central nervous system.

Crossed Extensor Reflex

In some situations, such as stepping on a nail, activation of nociceptors may stimulate the crossed extensor reflex. Once the nociceptors are activated, afferent neurons send impulses to the spinal cord where they synapse with interneurons. These interneurons synapse with extensors and flexors on the ipsilateral leg (the one that stood on the nail) causing the flexors to contract and the extensors to relax. This removes the harmful stimulus. Some of the interneurons synapse with flexors and extensors on the contralateral leg (the one that didn’t step on the nail) and causes them flexors to relax and the extensors to contract. This causes the leg to extend which supports the body. This reflex is important for the maintenance of posture and locomotion.  

Muscle Spindles

Muscle spindles are groups of specialised skeletal muscle fibres (known as intrafusal muscle fibres) contained in a capsule and arranged in parallel with striated (or extrafusal) muscle fibres. They are attached to the rest of the muscle by connective tissue and do not contribute the force of muscle contraction. In addition they are innervated by different neurons compared to extrafusal fibres: intrafusal fibres receive Ia and II sensory and γ motor innervation while extrafusal fibres receive α motor innervation.


Muscle spindles have three components:

• Specialised intrafusal muscle fibres whose ends are contractile while its centre is non-contractile.
• Type Ia (primary) and Type II (secondary) sensory fibres which originate from non-contractile regions of the intrafusal fibres. 
• Myelinated γ motor neurons which innervate the contractile polar regions of the intrafusal fibres.

Function:

The function of muscle spindle fibres is to provide information about the muscle length and changes in muscle length (which is velocity or the speed of contraction).  The nerves of the spindle allow it to do this. At the spinal cord, the sensory neurons of the spindle synapse with γ motor neurons that innervate the same spindle and α motor neurons that innervate the same muscle that the spindle is situated in.

The alpha motor neurons cause the extrafusal muscle fibres to contract while the gamma motor neurons cause the intrafusal polar regions to contract too. This allows the spindle fibres to always be under tension while the muscle is contracting which allows more information about the length and velocity of the contraction to be sent to the CNS.

But how do the spindles actually provide information about length and changes in length? The lengthening and shortening of skeletal muscle fibres involves two phases: the dynamic phase, when the muscle length is changing, and the static phase, when the muscle has stabilised at the new length after lengthening/shortening. Separate components of the muscle spindles signal each of these phases.

Now, there are two types of intrafusal spindle fibres and a typical spindle will contain: 2-3 large nuclear bag fibres and about 5 nuclear chain fibres. The nuclear bag fibres may be dynamic or static. The primary (Type Ia) sensory neurons innervate the centres of all the fibres. The secondary (Type II) sensory fibres innervate the chain and static bag fibres. The dynamic γ motor neurons innervate the contractile regions of the dynamic bag fibres while the static γ motor neurons innervate the contractile region of the chain and static bag fibres.

The sensory neurons respond through stretch-sensitive ion channels. Primary afferent fibres respond to muscle length but more strongly to changes in length (velocity) while secondary afferents provide information mainly about the steady-state length of muscle. Dynamic γ motor neurons are able to control the sensitivity to velocity by increasing the dynamic sensitivity of the endings of the primary sensory neurons. The sensitivity to length can be controlled by static γ neurons which may increase the tonic level of activity in primary and secondary endings and decrease the dynamic sensitivity of primary endings.

This allows the CNS to control the sensitivity to velocity or length to suit the activity of the muscles. Static γ motor neurons are active when the muscle length changes slowly and predictably (such as when standing or walking) while dynamic gamma neurons are active when length changes rapidly and unpredictably (such as when shaking a paw or balancing).

This video explains it all really well:

Golgi Tendon Organs

Golgi Tendon Organs (GTOs) provide information about the change in muscle tension. They are afferent nerve endings covered by a capsule and located at the junction between the muscle and tendon. Importantly, they are arranged in series with extrafusal fibres and this allows them to sense tension. GTOs are innervated by Type Ib sensory neurons and at the spinal cord they inhibit α motor neurons which innervate the muscle that the GTO is situated in. This prevents the muscle from generating too much tension.

So, to sum this all up, spindles provide sensory information about the length and change in length (velocity) of skeletal muscle while Golgi tendon organs provide information about muscle tension. In addition, the spindles provide a feedback system which monitors and maintains the length and tone of muscles. The Golgi organ system provides a feedback system which monitors and maintains muscle force. Both of these systems receive information from multiple sources including sensory receptors in the skin and descending spinal motor pathways from the brain. Both of these systems also send information to the brain through ascending spinal pathways which are part of the proprioception system.     

LMN Dysfunction

This section is quite important as it often comes up in exams! The signs that one would see in an animal that has a lower motor neuron problem include:

• Weakness/paralysis: This is because motor neurons to the muscles aren’t working and the muscles are unable to contract.
• Hyporeflexia: this refers to reduced or absent flexor reflexes and occurs because the nerves in the reflex circuit are not working.
• Neurogenic Atrophy: muscles waste away because the alpha motor neurons no longer communicate with them.
• Muscle fasciculation: muscles contract slowly in a disordered fashion.
• Flaccidity/hypotonia: this is because spindle fibre activity is inhibited.

Important Definitions

The following definitions all regard motor neurons.

• Paresis (-paretic): a partial deficit in motor function
• Paraparesis: when both pelvic limbs are affected
• Tetra- or quadriparesis: when all four limbs are affected
• Hemiparesis: when the thoracic and pelvic limb on the same side of the body are affected.
• Monoparesis: when only one limb is affected.
• Paralysis: the complete loss of voluntary movements
• The same prefixes apply as for paresis.

That’s it for this post, if you have any questions please let me know :)

Nasal Cavity and Paranasal Sinuses

Hello, in this post we’ll take a look at the external and internal features of the nasal cavity as well as the species differences in the arrangement of the paranasal sinuses.

The Nasal Cavity 

The nose includes the external nares and associated cartilages, the nasal cavity and paranasal sinuses. It consists of the nasal bones dorsally; the maxillae laterally; the palatine, incisive and maxillae bones ventrally; while the cribriform plate of the ethmoid bone forms the caudal limit. A rostral continuation of the crista galli of the ethmoid bone creates the median septum.  

The nasal cavity consists of the two external nares which lead to two nasal vestibules that are separated by the median nasal septum. These vestibules merge caudally and form a large nasal cavity which is lined with ciliated nasal mucosa. These cilia waft mucus and debris to the choanae (the paired openings between the nasal cavity and the nasopharynx) to be swallowed. Within the nasal cavity are nasal conchae which project medially from the walls of the cavity. They increase the respiratory surface area and are more complex in animals with highly developed olfaction. These conchae also have an extensive network of anastomosing vascular plexuses.

Externally, the nares are surrounded by hairless skin, except in the horse and humans, and form a structure which differs between species. In carnivores and small ruminants, a nasal plate exists and this is divided by a median groove called the philtrum. In large ruminants a nasolabial plate exists while in pigs a rostral plate is present and this is used to dig for food. The nares are supported by nasal cartilages which form the moveable part of the muzzle. In horses, which are obligate nasal breathers, the nares are held open by wing-like extensions known as the alar cartilages. The deeper parts of the nasal cavity are formed from the nasal and incisive bones.  

Paranasal Sinuses

The paranasal sinuses are cavities in the skull bone that are lined with mucous membranes and communicate with the nasal cavity. The size of the sinuses increase with age and are well developed in the horse and ox. The functions of the paranasal sinuses include:

• Mechanical and thermal protection of the orbit and cranial cavities 
• Increased skill dimensions for muscle attachment without increasing the weight of the skull 
• Resonance for vocalisation.

Several paranasal sinuses exist, including: maxillary, frontal, palatine, sphenoid, lacrimal (in pigs and ruminants), dorsal conchal sinus, ventral conchal sinus (in the pig, ruminants and horse), and the ethmoid cellules (in pigs and ruminants). The characteristics of these sinuses may differ between species.

Canine

The sinuses in dogs are relatively poorly developed and will vary according to the size and shape of the skull as well as with age. The frontal sinus has three compartments: the rostral, medial and lateral divisions, that are connected via ethmoidal meatuses. The lateral division is the largest. Dogs also have a maxillary sinus which is very large and lies above the roots of the molars.

Feline

The arrangement of the sinuses in cats is similar to that in dogs except that the frontal sinus is fused and has no divisions.

Porcine

The maxillary, frontal, lachrymal, shenoidal and conchal sinuses are quite prominent features of the skull and are there to protect the brain. In addition, pigs have very well developed nasal conchae, extensive olfactory mucosa and an excellent sense of smell.

Equine

The horse sinuses are extensive and are vulnerable to infection and offer potential surgical access to the roots of the molars. The frontal sinus communicates with the nasal cavity via the caudal maxillary sinus. The sinuses in the horse grow and their positions change with the jaw and teeth as the animal ages.

Bovine

The bovine sinuses are extensive and may continue into the base of the horn by the corneal process. Just like in the horse, the size and position of the sinuses changes with age.

That’s all for this post, see you next time :)

Evolution and Development of the Lungs and Respiration

Hi, in this post we’ll discuss the evolutionary origins of the lungs. We’ll take a look at the differences between the buccal and aspiration pumps, how they work and which animals use them. I’ll also describe the embryological development of the larynx, trachea and lungs.

Two types of ventilation exist in animals: unidirectional (this is seen in fish which have gills) and bidirectional (seen in mammals). It is thought that vertebrate lungs evolved from ventral swim bladders which were originally used for the maintenance of buoyancy and that gas exchange was a later adaptation.

Buccal and Aspiration Pumps

 The buccal and aspiration pumps are two forms of muscular respiratory pumps. The buccal pump involves either a two-stroke or four-stroke mechanism. With the two-stroke mechanism, the buccal cavity expands to bring air from the lungs and compresses to force air into the lungs. With the four-stroke mechanism, the first stroke is an expansion of the buccal cavity which brings air from the lungs into the cavity. The cavity then compresses in the second stroke to force air out into the external environment. In the third stroke the buccal cavity expands and this draws air into the mouth. The fourth stroke involves the compression of the buccal cavity which forces air into the lungs. This cycle is then repeated during breathing. This type of pump is found in frogs and air-breathing fish.

With the aspiration pump, the volume of the buccal cavity remains constant while the volume of the thoracic cavity changes during respiration. During exhalation, the diaphragm moves cranially while the walls of the thoracic cavity move inwards to decrease the volume of the cavity, this changes the air pressure in the lungs and forces air into the external environment. During inhalation, the diaphragm moves caudally while the thoracic walls move outward to decrease the air pressure in the lungs and draw air inwards. Aspiration pumps are found in reptiles and mammals.

Embryological Development

  The lungs have five stages of embryological development:

1. The trachea-bronchial tube grows caudally. This forms from a groove which arises from the ventral foregut. 
2.  This tube divides into two lung buds. 
3.  These buds divide into three bronchi on the right and two on the left. This defines the major lobes and bronchi of the adult lungs. 
4.  The respiratory portion of the lung develops 
5.  The alveoli develop.

In mammals, the coelom (body cavity) is divided by a thin, non-muscular oblique septum called the pulmonary fold. This fold grows from the midline to suspend the lungs and liver from the body wall. Another transverse fold from the dorsal body wall separates the liver and lungs and fuses with the pulmonary fold. This is the diaphragm. Once these developmental stages have finished, each lung is separated into its own plural cavity.

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