Monosynaptic reflexes

The large diameter A/l-afferent fibre enters the dorsal horn of the spinal cord through the medial division of the dorsal root. It then descends through the medial region of lamina I or II, or alternatively, curves around the medial (central) edge of the dorsal horn down to the ventral horn. On reaching deeper laminae, laminae IV and V, the AjS-fibres ascend back up into laminae III and IV where they repeatedly subdivide and form a characteristic termination pattern. The densest arborisation appears to occur in lamina III. Axons originating from specialised muscle stretch receptors have collaterals that pass ventrally to make monosynaptic connections with neurons of laminae V, VI and VII. Some also extend to laminae VIII and IX of the ventral horn where they synapse directly onto motor neurons and form the basis of monosynaptic reflexes. 

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons a polysynaptic reflex has two or more synapses between these neurons. In this case, intemeurons lie between the sensory and motor neurons. The more intemeurons involved, the more complex the response is.

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

An interneuron together with a sensory afferent and motor efferent form a polysynaptic reflex (Figure 2.2) this comprises the initial stage of information input (sensory afferent), the processing/computing an appropriate response (interneurons) and the execution of a behavioural response (motor efferent). The simplest reflexes in the nervous system are monosynaptic reflexes, such as the familiar tendon (knee) jerk, these do not involve an interneuron. The sensory afferent activated by the mechano-receptor (the tap of the patellar hammer) forms a synapse with the motor efferent in the spinal cord, which then causes the skeletal muscle to contract and the crossed leg to jerk forward. With a synaptic delay of 1 millisecond (ms), the time between input and output increases with the number of synapses introduced into the circuit. As an... 

Monosynaptic reflex A reflex arc (pathway) with a single synapse. 

Direct evidence is also available from functional studies that LSD can act as a serotonin antagonist. Anderson (9) has reviewed experiments on spinal reflexes conducted in acute spinal cats in which stimulation of the dorsal roots elicits monosynaptic, polysynaptic, and dorsal root reflexes. Increases in CNS levels of serotonin increased the size of the monosynaptic reflex and decreased the size of the polysynaptic and dorsal root reflexes. The increase in magnitude of the monosynaptic reflex induced by serotonin was blocked by a variety of classic serotonin antagonist drugs in addition, it was blocked by the systemic administration of LSD. Decreases in the other two types of spinal reflexes were unaffected by LSD and the serotonin antagonists. 

Sastry, B. S. R., and Sinclair, J. G. (1976) Tonic inhibitory influence of a supraspinal mono-aminergic system on recurrent inhibition of extensor monosynaptic reflex. Brain Res., 117 69— 76. 

There is only one monosynaptic reflex known to exist in humans - the stretch reflex. For this reason, it is commonly examined and an overview of its components and their functions is given below. 

Tramposch A, Sangdee C, Franz DN, Karler R, Turkanis SA. (1981). Cannabinoid-induced enhancement and depression of cat monosynaptic reflexes. Neuropharmacology. 20(6) 617-21. Tripathi FIL, Vocci FJ, Brase DA, Dewey WL. (1987). Effects of cannabinoids on levels of acetylcholine and choline and on turnover rate of acetylcholine in various regions of the mouse brain. Alcohol Drug Res. 7(5-6) 525-32. 

Tizanidine is an a -adrenergic receptor agonist at supraspinal and spinal levels. This effect results in inhibition of spinal polysynaptic reflex activity. It presumably reduces spasticity by increasing presynaptic inhibition of motor neurons. Tizanidine has no direct effect on skeletal muscle, the neuromuscular junction or on monosynaptic reflex activity. 

Ioffe S, Jansen AH, Russell BJ, Chemick V. Sleep, wakefulness and the monosynaptic reflex in fetal and newborn lambs. Pflugers Arch Eur J Physiol 1980 388 149-57. 

The non-specific tritiation of papaverine ha been described,13 as have methods for the detection of the alkaloid in body fluids.1415 The effects of papaverine on the biochemical fate of thymidine phosphate in the thymus,16 on cyclic AMP and the uptake of calcium in isolated pig auricle,17 on the monosynaptic reflex,18 on the L-DOPA-influenced secretion of GH and PRL,19 on cerebral blood flow,20 and on longitudinal smooth muscle21 have been studied, as have the /3-receptor-stimulant... 

Das Gupta, S., Bass, K.N., and Wamick, J.E., Interaction of reversible and irreversible, cholinesterase inhibitors on the monosynaptic reflex in neonatal rats, Toxicol. Appl. Pharmacol., 99, 28, 1989. 

Sensory nerves contain no acetylcholine and the chemical mediator involved in the simplest unit of central nervous activity, the monosynaptic reflex, is not known. For this reason, some considerable attention has been paid to the pharmacological activity of extracts of sensory nerves, which have been shown to contain three substances of potential neuropharmacological significance—ATP, substance P (a polypeptide) and histamine. They have been reviewed at length elsewhere and little need be added here, since it now seems likely that neither ATP nor substance P will be shown to have neurohumoral functions. However, some psychotropic drugs have antihistamine activity and a brief statement is therefore necessary concerning the present status of histamine. 

Feedback Mechanism Visual perception Haptic perception Polysynaptic reflexes Monosynaptic reflexes... 

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