Workbook questions 4 - Motor organisation Flashcards
What is the strict definition of a reflex?
It is an automatic, unlearned, repeatable response to a specific stimulus that does not require the brain to be intact
Reflex pathways are constituted from 5 generic anatomical components. Please list these
1)Receptor, 2) Afferent Neurone, 3) Integration Centre (Synapse), 4) Efferent Neurone, 5) Effector Organ
In the case of the muscle stretch reflex, please draw a fully labelled diagram of this circuit, making sure you identify the anatomical constituents of the reflex mentioned immediately above
Please note that there are only two neurones connected via a single synapse.
What do you understand by the term, “reflex movement”?
Unlearned, automatic displacement of a limb in response to a specific stimulus applied to some part of the body
When examining movements in a patient, why is it important to distinguish between voluntary movements and reflex movements?
In the process of examining a patient, a doctor can ask a patient to make certain movements voluntarily or by testing limb tendon reflexes. The central nervous system uses different and separate neural circuits to produce these two classes of movements. As such, a neurological examination of the motor system will seek to distinguish the integrity each of these neural circuits, independently. Thus, when a patient makes a movement, a doctor will need to be clear in his/her mind as to what class of movement that was. Patients who are brain dead but whose circulation and breathing are assisted artificially may still be reflexive if their spinal cords are intact and perfused normally with blood. Thus, reflex movements do not tell us much about the integrity of the brain movement bathways. In contrast, voluntary movements can only be evoked in patients with a functioning brain ( though the quality of this may be debatable).
Give an example of a common class of limb motor reflexes routinely examined as part of a medical examination
Deep Tendon Reflexes
What is the relationship (or distinction) between a monosynaptic stretch reflex and a muscle stretch reflex (also known as the MSR)?
The monosynaptic stretch reflex is the simplest reflex circuit, having just one synapse between the afferent and efferent neurone. The monosynaptic stretch reflex can be made into a disynaptic stretch reflex by adding an interneurone between the afferent and efferent neurones. A di-synaptic stretch reflex can be made into a tri-synaptic stretch reflex by adding yet another interneurone and so on. That the term muscle stretch reflex (MSR) is an umbrella term that covers the various versions of the reflex circuits built upon the monosynaptic stretch reflex. In a healthy individual, only the monosynaptic stretch reflex can easily be elicited by tapping tendons. Multi-synaptic reflexes tend to be seen in diseased central nervous systems
The term “muscle stretch reflex” is an umbrella term for a series of motor reflexes that can be evoked following stretch of a muscle. Identify the various sub-types of stretch reflexes.
The monosynaptic stretch reflex
The disynaptic stretch reflex
The oligosynaptic or multisynaptic stretch reflex
When testing for tendon jerk reflexes in a healthy individual exhibiting normal responses, which of the sub-types above (Q 4-6) of the muscle stretch reflex is most likely elicited?
The Monosynpatic stretch reflex is the most likely to be elicited because this mode of excitation of the reflex recruits only a subset of muscle spindle afferents. The di- and multi-synaptic limbs of the stretch reflex are unlikely to be elicited because their spindle afferents would not have been recruited by deep tendon reflex testing whilst at the same time, the spinal cord motor nucleus will be under tonic descending inhibition. (This inhibition will be more pronounced on the di- and multi-synaptic pathways as they are doubly or multiply inhibited whilst the monosynaptic pathways is relatively mildly affected at its single central synapse).
Apart from the muscle stretch reflex sub-type just identified in Q4-6 above, what is the explanation then for the failure to evoke the other sub-types of muscle stretch reflexes when testing for limb reflexes in a relaxed healthy individual exhibiting normal responses?
Descending inhibition of the spinal cord by upper motoneurones will tend to disable the multi-synaptic pathways more effectively as there are many synapses upon which inhibition can be exerted. Although the monosynpatic stretch reflex pathways are also susceptible to descending inhibition by upper motoneurones, this tends to be mild as it is exerted only once on that pathway.
Additionally, the severity of the descending inhibition is variable with time and when it is at its lightest, the monosynaptic reflex pathway may be less depressed, hence easily evoked by tapping tendons.
It is to be expected that when testing muscle stretch reflexes in some relaxed healthy individuals, they may be found to be areflexic. What is the explanation for this otherwise normal feature in such individuals?
Although descending inhibition is tonic/constant, its severity is variable, meaning that the nervous system can be heavily inhibited at some points in time whilst less so in others. When testing to evoke limb reflexes in patients, this can sometimes be difficult owing to the constant and heavy descending inhibition.
In other cases, a patient who has a depressed central nervous system as is the case during sleep, when taking certain formulations of medication or when intoxicated with alcohol may be found to be a reflexic.
