2.2.2 Feedback oscillators
Feedback oscillations occur in a loop between the muscle and the central nervous system, for example in the loop between the muscle and spinal cord or in the loop between muscle and cerebellum, and they occur because of reflexes. A reflex is defined as an involuntary response to a stimulus. An example of a stretch reflex is the patellar reflex, where a physician taps the tendon just below the patella resulting in stretch of the muscle which produces a reflex in which the muscle contracts.
When the stretch reflex is regarded as an under damped negative feedback loop, oscillations with a period of double the loop time may occur. Such oscillations may result in synchronized activity of EMG and tremor at this frequency. Taking the travelling time of the signals and the muscle activation time into account, the loop time for a stretch reflex in the finger is about 50ms (Marsden, 1978) and would thus tend to result in a tremor frequency of 10Hz (McAuley & Marsden, 2000).
Mechanical loading results in a change in reflex oscillation frequencies (Berthoz and Metral, 1970). A higher limb inertia results in lower accelerations and thus in slower movements, so the movement is detected later by the muscle spindles which results in an increased delay before the reflex is induced (McAuley & Marsden, 2000). Mechanical loading will thus directly influence the power spectrum of the EMG when reflexes are active.
2.2.3 Central oscillators
There are also tremors which are not affected by changes in stiffness and mass, and do not depend on the length of the loop. An example is such a tremor is orthostatic tremor (see section 2.3.***). Therefore it is believed that these tremors are produced by a central oscillator, which is an oscillating neural network within the central nervous system (Grimaldi & Manto, Tremor: From Pathogenesis to Treatment, 2008). Oscillators in the central nervous which are also present in healthy subjects are those involved in motions like walking or cycling.
2.2.4 Central nervous system
The central nervous system (CNS), which consists of the brain and the spinal cord, receives information, integrates that information and controls the human body. Figure 2 shows the parts of the CNS involved in tremor genesis and the pathways between them.
Figure 2 Pathways that are involved in tremor genesis
Peripheral nervous sytem: MNα/MNγ (α and γ motor neuron), Ia (Ia and II*** afferent fiber from muscle spindles), Golgi (Ib afferent fiber from Golgi tendon organs)
Brainstem: RN (red nuclei), mf (mossy fibers), cf (climbing fibers), IO (inferior olivary nucleus)
Cerebellum: CC (cerebellar cortex), CN (cerebellar nuclei)
(Adapted from (Grimaldi & Manto, Tremor: From Pathogenesis to Treatment, 2008))
The purpose of showing this figure is not so much to describe all aspects of human motor control nor to identify the role of each part of the CNS, but to give an indication of all the possible loops and pathways involved in tremor genesis. In some subsections of section 2.3***, parts of the CNS are mentioned which are affected in certain disorders. ***verwijzen naar studie materiaal over motor control?***
The simple stretch reflex as described in section 2.2.2*** involves only the muscle spindles (Ia afferent fiber), the spinal cord and the alpha motor neuron.
2.3 Different forms of pathological tremor
For each of the disorders discussed in this section the following information will be presented: the symptoms of the tremor, the frequency range of oscillation of the tremor, the prevalence, the etiology, the pathology and the possible treatments. In most cases there are different frequencies reported for a disorder. These different frequencies indicate that there is no consensus in literature on a specific frequency range for each disorder (Accardo, Chiap, Marino, Lanzafame, & Bramanti, 2010). Prevalence numbers for tremor are largely unavailable, however an attempt is made to sort the pathological tremors by prevalence.
2.3.1 Essential tremor
Symptoms
Essential tremor (ET) is a kinetic tremor which mainly affects hands and forearms and may include a postural component in more severe cases (Louis, 2005). ET may also involve the head, neck and voice and less occasionally the trunk, legs and facial structures (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007) (Louis, 2005). The tremor which affects the head and neck appears as a yes-yes or no-no head movement (Smaga, 2003). ET is slowly progressive and can become very disabling (Louis, 2005) (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007). The amplitude of ET increases with stress, fatigue, and certain drugs (Smaga, 2003).
Frequency range of oscillation
The frequency of oscillation of ET has been reported to be between 4 Hz and 12 Hz (Smaga, 2003) (Harish, Venkateswara Rao, Borgohain, Sairam, & Abhilash, 2009) (Kuks & Snoek, 2007) (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007) and between 4 Hz and 8 Hz (Eidelberg & Pourfar, 2007).
Prevalence
ET is the most common movement disorder (Ahmed & Sweeney, 2002) (Smaga, 2003). Prevalence ranges from 410 to 3920 cases per 100,000 persons, to as high as 5050 per 100,000 in persons older than 60 years (Smaga, 2003). Since up to 50 percent of persons suffering from ET are unaware of it, the true prevalence is probably higher (Smaga, 2003) (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007).
Etiology
The etiology of ET is unknown. Often the patient has a positive family history of tremor (Ahmed & Sweeney, 2002). In 60% of the cases ET is an inherited disorder (Smaga, 2003). Age is another risk factor, where the age of onset is usually after 50 years. It also appears that ethnicity is a risk factor. Several studies showed that Caucasians show higher prevalence than African-Americans (Louis, 2005).
Pathology
The pathology of ET is largely unknown, but it is often associated with the olivocerebellar system (Kuks & Snoek, 2007) because of presence of lesions in the cerebellum, pons and thalamus (Rubchinsky, Kuznetsov, Wheelock, & Sigvardt, 2007).
Treatment
Options for treatment of ET are rest, beta blockers, and primidone (Mysoline). Alcohol ingestion appears to decrease the tremor. When the disability caused by ET is small, no treatment is performed since the side effects of the medication can outweigh the benefits (Ahmed & Sweeney, 2002).
