Session 5: Motor Disorders and Review of Patterns of Sensory Deficits Flashcards
Describe how upper motor neurones behave in healthy individuals normally including the production of muscle tone
In neurological terminology, upper motoneurones are made from cell bodies and axons of descending or motor fibres. They are located within high centres of the nervous system and all their processes remain within the confines of CNS.
The pyramidal system is made from upper motoneurones originating in the motor cerebral cortex with axons descending as either the lateral or ventral corticospinal tracts, making only 1 synaptic contact with motoneurones of distal limb muscles (i.e. the hands and fingers). The corticospinal system is special in that it is responsible for fractionation of finger movements that are so important to the dexterity of the hand.
The rest of the descending fibres are lumped together as the extra-pyramidal system. Although the extra-pyramidal system is responsible for the constant descending inhibition of lower motoneurones, this however, is titrated to allow some tone in all muscles of the body.
Muscle tone relies entirely on the operation of the muscle stretch reflex. Muscle tone rises and falls depending on the number and size of motor units recruited by their respective muscle stretch reflexes. In healthy subjects, descending inhibition from the extra-pyramidal system inhibits the operations of most stretch reflexes whilst a random few escape it at any given for short periods.
Are Pyramidal Signs UMN signs?
Pyramidal signs are called upper motor neurone signs and arise from damage to the corticospinal tract. These tracts travel from the motor cortex to the anterior horn cells of the spinal cord and are sometimes referred to as “long tracts”.
Extrapyramidal signs arise from damage to the extrapyramidal tracts (Rubrospinal, Tectospinal, Vestibulospinal and Reticulospinal tracts) and produce signs related to dysfunction of non-cortical motor systems such as the basal ganglia and cerebellum.
The presence of other neurological signs along with long tract signs can indicate the site of a lesion.
- Damage to the corticospinal tract impairs the volition of fine movements.
- Damage to the extrapyramidal tracts impairs the way movements are carried out (e.g. abnormality).
If abnormalities of UMNs develop, what happens?
If abnormalities of upper motoneurones develop, these will lead to reduction of descending inhibition to muscle stretch reflexes (see earlier section about muscle tone). Consequently, motor tone in the affected limbs will rise (=> stiffness). Typically, the most common site of damage is the internal capsule or the cerebral cortex. Resulting damage to these upper motor neurone tracts will result in reduction in descending inhibition.
Muscle tone to the antagonistic muscle pair attending to a given joint will usually be equally affected.
In extreme cases, the muscle tone may be so high that the limb remains in a state of spastic paralysis.
- In upper limbs, since flexors are usually stronger than extensors, spastic paralysis usually results in overall flexion of the limb.
- A lower limb affected with spastic paralysis assumes overall extension since extensors are relatively more powerful in this case.
Other disorders of the extrapyramidal system result in excessive movements in addition to alteration of muscle tone. Pill rolling, shuffling gait, choreoforms, tics etc are further examples of manifestations of extrapyramidal signs.
NB: Upper motor neurone lesions encompass both pyramidal and extrapyramidal lesions. However, most common lesions occur with the extrapyramidal tracts, thus will show extrapyramidal signs. It is very rare to get pure pyramidal signs.
Sensory abnormalities are often present as well in these disorders.
What is meant by Spinal Shock?
A period of spinal shock follows when the descending tracts of the spinal cord are severely damaged.
This period, which may last for weeks or months, is characterised by a flaccid paralysis and areflexia even though the ventral roots may be intact.
Eventually the limbs become spastic and show hyperactive deep reflexes, typical of upper motor neurone damage.
The reasons for the loss of reflex activity is thought to involve the loss of motor influences exerted by descending fibres from the reticular formation (in time GABA and glycine will be removed)
As these fibres degenerate, the intact connections in the reflex circuits become dominant and show themselves as upper motor neurone signs – may take up to 6 weeks
How can muscle weakness arise?
Muscular Weakness
Can arise from conditions, which affect the muscles (myopathies), or the neuro-muscular junction (e.g. myasthenia gravis).
Describe the effect of UMNs on alpha-motoneurones normally
In neurological terminology, the lower motoneurone is the cell body and axon of an alpha-motoneurone.
Cell bodies of all alpha-motoneurones are located within the central nervous system in either cranial nerve motor nuclei of the brainstem or the ventral horn of the spinal cord.
Axons of alpha-motoneurones leave the CNS to course in the PNS as either motor divisions of cranial nerves or spinal segmental nerves and terminate on skeletal muscles through the neuromuscular junction.
The close relationship between the alpha-motoneurone and its target skeletal muscle is key to understanding motor output.
In a healthy nervous system, alpha-motoneurones are under constant inhibition from upper motoneurones and in particular, the extra-pyramidal system. The intensity of the constant descending inhibition varies continuously. When we fall into deep sleep, descending inhibition paralyses virtually all skeletal muscles apart from those responsible for breathing or supplying extraocular muscles.
Descending inhibition is temporarily lifted in order for us to carry out voluntary movements. The minimal muscle power that allows us to maintain our posture and minimal stiffness in our muscles is known as muscle tone.
Muscle tone relies entirely on the operation of the muscle stretch reflex. When a muscle is stretched, its muscle spindle afferents detect the stretch, firing through muscle spindle afferents to inform the CNS of this.
In addition, muscle spindle afferents also make monosynaptic and oligosynaptic contacts with alpha-motoneurones. Thus, the continuous firing in muscle spindle afferents results in reflex contraction of muscles in which the muscle spindle itself resides.
This ongoing-reflex contraction of the muscle gives it tone and thereby the ability to oppose passive displacements.
What is implicated if the tone of a muscle or limb is weakened/absent?
All muscles of the body have tonus. If the subject is awake and the tone of a given muscle or limb is absent, this points towards a pathology pointing towards lower motoneurone signs. Here, the suggestion is that there is failure of communication between alpha-motoneurones and muscles of that limb.
The problem might lie with the cell bodies of alpha-motoneurones, axons of alpha-motoneurones, the neuromuscular junction of the muscle itself.
In an acute setting where there is no obvious trauma, it is often difficult to pinpoint the site of the problem.
Chronically however changes take place thereby betraying the source(s) of the motor deficit.
Explain the cardianal lower motor neurone signs
Flaccid paralysis: this damage to the lower motoneurones means that the muscle becomes effectively deinnervated. Consequently, the muscle cannot be signalled to contract.
Muscle atrophy: the deinnervation will cause the muscle to slowly atrophy (as a result of disuse)
Hyporeflexia
Atonia and areflexia: loss of innervation to the muscles means that even if the descending inhibitory neurones or the afferent neurones are working, there will still be no muscle tone or muscle reflexes.
Fasciculation: the deinnervation to the muscles means that the nAChR become hypersensitive for any neurotransmitter substance. The hypersensitivity means that any molecule vaguely similar to ACh (even those simply found in the blood) can cause excitations of the nAChR at the NMJ and cause slight muscle contractions – wriggling movements of the muscle.
With any lower motor neurone lesion, if the cell body is damaged (such as in poliomyelitis) then the axons cannot regenerate and so the damage is permanent, yet if the axon is damaged but the cell body remains intact, then some Wallerian degeneration can occur and attempt to restore function again.
Describe the effects of stroke
Sudden onset of neurological symptoms often arising because of some perfusion problem in the cerebral circulation.
They often occur from ischaemia of the internal capsule and because the ascending and descending tracts pass through this region, they can have devastating consequences.
The internal capsule however, like other regions of the nervous system has a topographical organisation so stroke may occasionally have only sensory or only motor consequences.
Given a set of symptoms it may be possible using knowledge of the structure of the internal capsule to localise the site of the lesion.
How does polio present?
The polio virus gains access by way of the GI tract and invades the motor neurones of the brainstem and spinal cord, many of which die. The disease presents with lower motoneurone paralysis of the affected segments but without any sensory loss (compared to shingles).
The disease is now rare in the UK following vaccination. It remains endemic in certain third world countries, though there are efforts to completely eradicate this scourge.
Describe the autonomic consequences of UMN/LMN lesions, using the bladder as an example
Autonomic Consequences: upper and lower motor neurone lesions can also have an effect on the autonomic nervous system and the organs they innervate. This can be highlighted by looking at the bladder innervation.
Autonomous Bladder: an autonomous bladder is caused by LMN lesions, when there is damage to above S2-S4 level. There is consequently a loss of parasympathetic and afferent neurones This results in an individual with overflow incontinence and no ability to micturate.
Automatic Reflex Bladder: any damage above the sacral region (i.e. T12 and above) can cause damage to the UMN and result in automatic reflex bladder. There is a loss of the descending inhibitory control which results in loss of bladder control and involuntary leakage of urine, producing an urge urinary incontinence.
What happens in ALS?
Amyotrophic Lateral Sclerosis (Lou Gehrig’s disease) is a progressive disease in which the corticospinal tracts and ventral horn cells degenerate, often beginning with lower limbs and later involving the upper limbs.
Degeneration results in weakness and loss of control to muscles in the hand, trunks and lower limbs.
Bladder and bowel function can become impaired due to loss of descending autonomic pathways.
Cause of the disease in is unknown.
What is Brown-Sequard Syndrome?
loss of sensation and motor function (paralysis and ataxia) that is caused by the lateral hemisection of the spinal cord.
It can be caused by a tumour, trauma, ischaemia or infectious or inflammatory conditions.
As a consequence of the damage, the individual will present with
- Spastic paralysis of the ipsilateral side
- Loss of fine touch and proprioception to the ipsilateral side due to damage to fasciculus gracilis and cuneatus.
- Loss of pain, temperature and pressure sensation to the contralateral side due to damage to the spinothalamic tract.
Treatment involves correcting the underlying condition; the classic cause is a stab wound to the back.
Describe Anterior Spinal Artery Syndrome and Syringomyelia
Anterior Spinal Artery Syndrome: caused by ischaemia by the anterior spinal artery, affecting the corticospinal tracts.
- It results in motor paralysis and impaired pain and temperature sensation.
Syringomyelia is the term used to describe the development of a cyst/cavity around central canal, which grows and spreads out over time.
- It typically disrupts spinothalamic tract as this decussates just ventral to central canal.
- The result is reduced temperature and pain sensation at level of lesion, yet fine touch, proprioception and vibration are affected.
- It can affect motor system as it extends
Describe the Hierarchy of Movement Control
3 levels
High: involved with the strategy or what is to be achieved by the movement according to recognized demands and how the movement will be carried out. Structures for this level include the frontal lobes and limbic system and basal ganglia
Middle: the tactics or the sequence of muscle movements in time and space required to achieve the goal. This includes the selection of appropriate motor programmes. Structures that carry out this level include the motor cortex and the cerebellum
Low: execution or the activation of motor neuron and interneuron pools that generate the movement. This level also adjusts posture if necessary. Structures at this level include the brain stem and spinal cord.
What is the main function of the Basal Ganglia?
The main function of the basal ganglia is to provide a feedback mechanism to the cerebral cortex for the initiation and control of the motor responses – involved in movement planning; much of the output of the basal ganglia is to reduce or dampen down the excitatory input to the cerebral cortex.
The basic feedback circuit involves both the thalamus and basal ganglia communicating to modulate the output of the cerebral cortex; they have no link to lower motoneurones.
The basal ganglia regulate the amplitude and velocity of planned movement, particularly in relation to the use of internal (e.g. proprioceptive) information.
Describe the composition of the basal ganglia
the basal ganglia consist of the neostriatum (composed of caudate nucleus and putamen), globus pallidus (external and internal), substantia nigra (composed of pars compacta and pars reticulate) and the subthalamic nucleus. These multiple subcortical nuclei regulate the amplitude and velocity of planned movements, in relation to internal formation (proprioception).
The primary afferents of the basal ganglia are the caudate nucleus and the putamen.
Caudate nucleus + putamen = striatum
Putamen + globus pallidus = lenticular nucleus.
The signals received through the basal ganglia come from cortical inputs and ultimately result in a distinct response transmitted back to the motor areas of the cerebral cortex; the output of the basal ganglia act on the cerebral cortex to affects its motor output. Dysfunction of the basal ganglia leads to hypokinetic and hyperkinetic disorders, but movement coordination is normal (cerebellum).
Describe the direct and indirect pathways in the basal ganglia
The circuits set up within the basal ganglia by which signals are transmitted back to the cerebral cortex may be direct or indirect.
Direct Pathway: (overall excitatory – disinhibitory action “opens movement gate”
- Cortex (excitatory action)
- Neostriatum (inhibitory action)
- Globus pallidus interna / substantia nigra pars reticulata (inhibitory action)
- Ventrolateral nucleus of thalamus (excitatory action)
- Cortex
Indirect Pathway: (overall inhibitory – antagonises disinhibitory action)
- Cortex (excitatory)
- Neostriatum (inhibitory)
- Globus pallidus externa (inhibitory)
- Subthalamic nucleus (excitatory)
- Globus pallidus interna/ substantia nigra pars reticulata (inhibitory)
- Ventrolateral nucleus of thalamus (excitatory)
- Cortex (excitatory)
As this pathway goes via additional areas of the brain, this pathway takes longer to reach the thalamus.
What is the role of the substantia nigra? And describe the overall pathway in the basal ganglia
The Substantia Nigra is made up of a pars compacta and a pars reticulata
- The pars reticulata appears to be functionally analogous with the globus pallidus internal.
- The pars compacta feeds into the neostriatum to modulate its activity; the pars compacta can increase the activity of the direct pathway and decreases the activity of the indirect pathway, consequently acting to increase the size and speed of movements.
What happens in Parkinson’s Disease
Progressive degeneration of the dopaminergic neurons of the Substantia Nigra – decreased activity of nigro-striatal pathway. Consequently, there is a reduced direct pathway action and an increased indirect pathway action. The overall outcome is reduced so there becomes a reduction in movement amplitude
Left: Normal pathways; Right: Parkinson’s Disease
Describe the Cerebellum
The cerebellum lies beneath the occipital lobe, posterior to the brainstem, in the posterior cranial fossa.
The overall effect of the cerebellum is to allow for precise and effective execution of purposeful movements as well as the presence of appropriate posture in standing and with movement, doing so by integration and organising all the sequence of events associated with this response.
The cerebellum is attached to the brainstem and can be divided into 3 lobes (anterior, posterior and flocculonodular). It is highly folded – grey matter cortex (lots of cell bodies) and white matter core (containing deep nuclei).
3 peduncles ‘bridges’ – carry input/output fibres from and to the brainstem.
The core contains 3 pairs of deep nuclei – generate output projections to brainstem.
The vermis is located in the medial, cortico-nuclear zone of the cerebellum. Functionally, the vermis is associated with bodily posture and locomotion.
Describe the functional zones of the cerebellum
Vestibulocerebellum: the main input is from the vestibular system. It is involved in balance and ocular reflexes (maintain fixation on target). It sends back fibres to vestibular nuclei. Damage causes disturbances of balance and gait.
Spinocerebellum: it is involved in regulating body movements by allowing error correction (= match the motor prediction and sensory feedback). It contains a map of body and received proprioceptive information (complex computation). It applies a predictive correction – predicts where the movement will have go to by the time the correcting signal reaches it.
Cerebrocerebellum (2 hemispheres): it’s involved in planning movements and motor learning. It receives inputs from the cortex and sends fibres mainly to the thalamus and red nucleus. It regulates coordination of muscle activation and it’s very important in visually guided movements (parietal cortex inputs).
