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Central Nervous System

9 out of 14

Arousal response in mediated by

A Dorsal column

B Reticulo activating system

C Spinothalamic tract

D Vestibule cerebellar tract

Ans. B Reticulo activating system

[A] RAS (Reticulo-Activating system): ­

1. RAS fibers convey impulses upwards to the thalamus and cerebral cortex.

2. RAS fibers are multisynaptic path beginning at the medulla and pass upward via pons and mid brain to reach the thalamus.

3. Function of RAS ~ when the RAS is stimulated, there is wakefulness and alertness of the subject and the subject becomes fully conscious.

[B] Spino thalamic tract or Anterolateral tract or ventrolateral tract:­

1. Consists of two part

a. Anterior (ventral) spino thalamic tract ~carries touch (crude touch) sensation.

b. Lateralspino thalamic tract ~ carries pain and temperature (cold + warmth) sensations

2. Courses: -1st order neurons passing via Dorsal root ganglia and terminate into the dorsal horn of spinal cord ~ from there 2nd order neurons arises ~ cross at the same level in the anterior commissure of cord to the opposite anterior and lateral white columns ~ Anterior spino thalamic tract cross just anterior (close) to the central canal ~ then ascend to the brain ~ ends in the thalamus ~ 3rd order neurons arises from thalamus and ends in the cerebral cortex

[C] Dorsal columns - Medial Lemniscal system: ­

1. Carries sensation of

a. Fine touch b. Two point discrimination c. Pressure vibration d. Joint position

2. Courses I st order neurons passes through posterior column do not cross at the spinal cord level ascends, and terminate into the Dorsal column nuclei (nucleus cuneatus and nucleus gracilis) from there 2nd order neuron arises and, cross to the opposite side (in the medulla) then ascends through medial lemnisciends in the thalamus from here 3rd order neurons arise and ends in the cerebral cortex.

[D] Clinical points:

1. Dissociative anaesthesia: ~in syringomyelia ~ loss of pain and temperature while touch, vibrations joint and position senses are normal.

2. Brown-sequard syndrome (Ref. Guyton 9th ed./617) If only one half of the spinal cord is transected in a single side (hemisection), the so-called Brown-sequard occurs :

a. All motor junctions are blocked on the side of the transection in all segments below the level of the transection i.e. due to pyramidal tract injury ~ same side ~since it crosses in the medulla. '

b. Sensation of the pain and temperature (cold and Heat) are lost on the opposite side of the body in all dermatomes two to six segments below the level of transection. Since pain and temp sensations are carried by spino-thalamic tract, which is cross to opposite side at the same level of the cord.

c. Kinesthetic and position sensation, vibration sensation discrete localization, and two poin discrimination -f are lost on the same side of the transection in all dermatomes below the level of the transection since all these sensations are carried by dorsal and dorsolateral columns, which do not cross in the spinal cord, instead they cross to opposite side in medulla

[E] Vestibulocerebellar tract Concerned with equilibrium and learning induced changes in VOR.

Central Nervous System Flashcard List

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