Plasticity of the neurons Suppose the median nerve of the right hand is destroyed and consequently there is sensory loss of The palm of the right ha

are liable la alter slightly if the situation alters The post central gyms, according la Brodman's mapping, consists of areas 3,2.1. Plasticity of the neurons Suppose the median nerve of the right hand is destroyed and consequently there is sensory loss of The palm of the right hand. The corresponding area of the sensory homonculus. (SI), in the left hemisphere, is Thus, now, useless With passage of lime it will be noted that impulses from some neighbouring nerves (neighbours of the now destroyed median nerve], somehow, project [to some extent), on the palm aiea of the left SI, so thai this area now regaains some activity. This is an example of plasticity of the neuronal activity Further projection and body imaging From the post central gyrus, further order neurons arise to end in parietal lobe, wtiere various informations are further integrated and body image made For a short account of body image (and parietal lobe neglect) see introduction to sec XB l and chap 6 sec XD, parietal lobe) Comparison between the lemniscal and the anterolaleral systems (1) The lenimscal system appeared late in the evolution and The anterotaleral system is more primitive. (2) The anterolaleral system conveys rather crude senses, whereas Ihe lernniscal system conveys rather finer senses. (3) Conduction through the lernniscal system is faster as the journey To The thalamus is accomplished by two neurons whereas the same is accomplished in the anTerolaleral system, very often, by more than two neurons (4) The anterolaleral system gives collateral;, during its upward journey to the (a] penaqueductal gray (thus controlling the pain sensation) and (b) RAS (thus influencing the state of alertness), whereas no such thing happens wtlh the lemniscal system However, the lernniscal system gives collate rats to SGR for gate controlling in pain. (5) Localization of the sense is much better in the lerrmiscal system than that in the anterolateral system SPINO CEREBELLAR TRACTS (fig 10B1 3.3) These tracts, as staled earlier, do not reach Ihe level of consciousness There are two such well recognized tracts, (i) antenor or ventral, and (ii) posterior or dorsal spino cerebeliar tracts In addition, some fibers of the dorsal column, after reaching the medulla, enter the cerebellum Dorsal spinocerebellar tract (fig. IDA 1 3) The first order neuron arises from the muscle spindle, joints and The skin enters the spinal cord via the DRG -+ makes a short ascent in The dorsal fuinculus of The same side then terminate in the Clarke's column (also called, nucleus dorsahs) situated al Ihe base of the dorsal horn The second neuron arises from the Clarke's column proceeds up through The dorsal spinocerebellar tract of Ihe same side reaches medulla turns posteriorly to enter cerebellum through the inferior cerebellar peduncle Fig in Spino cerebeliar tracts Ventral spino cerebellar tract First order neuron anses from the muscle spindle, joint and shin enters the spinal cord via Ihe DRG relays in the dorsal horn The second order neurons anse from the Rexed lamina V to VII (fig IDA 1 6) Most of (he second order neurons cross to Ihe opposite side proceed up via the vantral spinocerebellar tract [tig 10A1.3) medulla pens mid brain In The mid brain the fibers turn posteriorly via superior c e rebellar peduncle enters the cerebellum The spinocerebellar tracts carry impulses arising from the muscles and joints (= non conscious propnocepfive senses), and [ii) impulses from the skin (non conscious cutaneous senses) Bolh these are sensory input to The cerebellum; on the basis of these (and other inputs from Ihe brain), Ihe cerebellum acts properly Nerve fibers from The neck and supenor extremity muscles enter The dorsal column (instead of The spnoc erebell at tracts) and relay in the accessory cuneale nucleus in the medulla From this nucleus, fresh relay occurs and rhese second order neurons enter the cerebellum via the external areuale fibers The accessory nucleus is, therefore, homologous to the Clarke's column (Clarke's column of nucleus is absent in the cervical regron). The two spino cerebeliar tracts, bnng information from the two inferior extremities and the trunk only. Figure I0A 1 3 shows the positions the major tracts (both ascending and the motor) m Ihe spinal cord. Tract of Goll and Burdach To give a comprehensive idea, the anatomy of the classical tract ofGoll and Burdach is descnbed below The first order neuron, carrying (i) conscious senses as well as (10 non conscious propnoceptive senses from skin, muscles and joints enters the spinal cord and then continues up via the dorsaf funiculus (Some of these fibers, from the upper extremity, however, relay m the posterior horn) All such fibers relay in the medulla, either in the DCN (majority of the fibers) or in the accessory cuneate nucleus From the DON, the second order neuron via the medial lemmscus reaches Ihe Thalamus and then cortex as shown in fig 10S1 3.1 Nonconscious propnocepTive fibers, arising from The accessory cunaate nucleus enter the cerebellum (for the superior eKtrernrties, homologous fibers of the spinocerebellar tracls therefore, ascend via the tract of toll and Burdach and ante: the cerebellum via the external arcuate fibers). FUNCTIONAL ANATOMY OF THE EYE Each eyeball, aboul 2.5 cm in diameter, is lodged within the bony socket called orbrit Between the orbital wall and the eyeball, there are several structures, viz. (i) the voluntary muscles, (hj fat, (in) connective tissue and (iv) lacnmal gland All these structures are separated from the eye ball by a fascia called Buck's fascia. Fig 1QB2.1 1 Structure of the eye (i) The voluntary muscles, called Ihe extrinsic muscles of the eye, are concerned with the movements of the eye ball, viz, medial (canver gent), lateral,