General sensory systems



PATHWAYS FOR PROPRIOCEPTION, FINE TOUCH AND VIBRATIONS
	Receptors for discriminative / fine touch are Meissner’s corpuscles. Proprioceptors consist of neuromuscular spindles, Golgi tendon organs, pacinian corpuscles and un-encapsulated endings in joint capsules and ligaments. Dendrites of primary neurons are contained in the spinal nerves. Cell bodies are contained in the dorsal root ganglia. The axons entering dorsal column and divide into three One terminates in the gray mater at the same level of the spinal cord A second one descends two or three lower segments and constiute fasciculus inter-fascicularis, and fasciculus septomarginalis. The third one ascends in the dorsal columns towards the brain. The long ascending branches arising from mid-thoracic and lower spinal roots ascend in the fasciculus gracilis. Those arising from upper thoracic and higher spinal dorsal roots ascend in the fasciculus cuneatus, which is therefore only seen above T6. The fasciculus cuneatus and gracilis form the dorsal column - lemniscal system. The fibres in these fasciculi ascend on the ipsilateral side of the spinal cord, and synapse in the gracile and cuneate nuclei of the medulla oblongata respectively. Axons of second order neurons cross in the midline as the great sensory decussation and ascend as the medial leminscus. The medial lemniscus synapses in the ventral posterolateral nucleus, pars caudalis (VPLc) of the thalamus, Third order neurons, traverse the internal capsule, corona radiata to end in the sensory cortex.

Cranial Sensations:
	Discriminative touch Axons of large primary neurons in the semilunar ganglion synapse in the chief nucleus of the trigeminal nerve. Axons of second order neurons constitute the crossed trigeminothalamic tract. Third order neurons from the ventral posterolateral nucleus terminate in the sensory cortex Proprioception Primary sensory neurons for proprioception have their cell bodies in the mesencephalic nucleus of the trigeminal nerve rather than semilunar ganglion. They mainly arise from the temporo-mandibular joint and the muscles of mastication. The central processes make contacts with cells in the reticular formation. Axons of the latter join the trigeminothalamic tract. Some are however, involved in the reflex circuits of mastication.