Cutaneomuscular refle

When cutaneous and/or muscle afferents are activated, a reflex in ongoing EMG activity can be observed (Caccia et al. 1973; Jenner and Stephens 1982). This reflex is composed of three parts. An initial short-latency excitatory phase (E l) occurs approximately 30-40 ms following peripheral stimulation; after that is a longer-latency inhibitory phase (II) occurring at 40-50 ms; finally there is a secondary excitatory phase (E2) occurring at 50- 70 ms. Cutaneous and muscle afferents can be activated naturally by stretch or by electrical stimulation of digital nerves o f the hand (activates cutaneous afferents) or nerves at the wrist (activates muscle and cutaneous afferents).

T H A L A M U S ( N u c l e u s ^ v e n t r a l i s p o s t e r o l a t e r a l i s ) M e d i a ! l e m n i s c u s A N u c l e u s c u n e a t u s DORSAL C O L UM N NUCLEI N u c l e u s g r a c i l i s CERVICAL CORD LUMBAR CORD M e d i a l ^ l e m n i s c u s C e r e b e l l u m N u c l e u s c u n e a t u s D o r s a l s p i n o c e r e b e l l a r t r a c t N u c l e u s Z F a s c i c u l u s c u n e a t u s F a s c i c u l u s g r a c i l i s C l a r k e ' s c o l u m n

c

Figure 1.4: Main anatomical features of the dorsal column-medial lemniscal pathway and spinothalamic pathway. (A) The main route in the dorsal column-medial

lem niscal pathway taken by fibres projecting from cu tan eou s receptors. (B) Afferent fibres from receptors in joints and m u scles a scen d this pathway by a different route and also travel in (C) the spinothalamic pathway. (Adapted from Rothwell 1994, p219, 221).

Cerebral cortex C ereb ella r cortex V isu al F rontoi C e r e b e lla r n u clei

m

Figure 1.5: C onnections betw een subcortical brain regions and the motor cortex. Afferent fibres that arrive at the thalam us m ake direct con n ection s with the motor cortex (Ml ) or indirect con n ection s via the so m a to sen so ry cortex (SI ). The thalam us also receives input from cerebellar nuclei, via which additional afferent fibres projecting to the cerebellum can eventually m odulate motor cortex activity. In addition to the corticospinal tract (0), cortico-reticulo-spinal projections d escen d from the sensorim otor a rea s of the cortex (A). There are also d escen d in g pathways that m odulate sen sory input from the periphery (B). Note the d e n s e projections from the basal ganglia and inferior olive to the motor cortex. (Adapted from Porter and Lemon 1993, p26).

Jenner and Stephens (1982) suggested that at least part o f the cutaneomuscular reflex involved transcortical pathways and studies of patients with mirror movements has provided strong evidence to support this claim (Farmer et al. 1990; Mayston et al. 1997). When these patients make voluntary movements o f the fingers of one hand, there is an involuntary mirroring of the same movements in the fingers o f the other hand. Mayston et al. (1997) showed that this was probably due to an abnormal bilateral fast conducting corticospinal pathway. This was indicated by the fact that TMS evoked MEPs of similar latency were observed in contralateral and ipsilateral (to the stimulated motor cortex) hand muscles (Mayston et al. 1997). Digital nerve stimulation on one side, resulted in E2 responses in the IDI muscles on both sides of the patient. Taken together, TMS evoked responses and cutaneomuscular reflexes in mirror movement patients, provide evidence that the reflex pathway giving rise to the E2 phase involves the corticospinal tract. Sensory evoked potentials recorded over the sensory cortex in mirror movement patients were similar to control subjects suggesting that the afferent pathways in these patients were normal (Mayston et al. 1997).

Despite the fact that the early inhibitory (II) phase has been considered to be of a spinal origin (Jenner and Stephens 1982; Farmer et al. 1990), bilateral inhibitory responses have also been observed in some patients with mirror movements, suggesting a transcortical pathway may also be involved in this phase of the cutaneomuscular reflex (Mayston et al.

1997).

It therefore seems likely that activation o f cutaneous and muscle afferents can have effects on the excitability of the motor cortex. This prompted work investigating the effects of peripheral input on TMS evoked MEP responses. The latency o f effects on TMS evoked responses was then compared to the timing o f the reflex response in ongoing EMG activity. In this way, any inhibitory or excitatory effects o f peripheral input at the cortex, could then support claims about the origins o f the transcortical reflex.