Question

22. The unrestricted passage of ions between adjacent cardiomyocytes, required for the synchronous contractions that create the heart beat, is possible due to the presence of

​a. adhering junctions

​b. desmosomes

​c. gap junctions

​d. tight junctions

​e. hemidesmosomes

23. The cell bodies of sensory neurons are located

​a. in the anterior horns

​b. in the lateral horns

​c. in the posterior horns

​d. in the posterior columns

​e. outside the spinal cord

24. The scleral venous sinus is also called the

​a. canal of Schlemm

​b. hyaloid canal

​c. lacrimal canal

​d. ora serrata

​e. conjunctiva

25. The thickest layer in the artery is:

​a. tunica media

​b. tunica intima

​c. tunica externa

​d. subendothelial layer

​e. none of the above

26. The papillae that form an inverted V on the tongue posterior dorsal surface, indicating the 2/3:1/3 border are

​a. filiform papillae

​b. circumvallate papillae

​c. fungiform papillae

​d. foliate papillae

​e. gustatory papillae

27. The heart is located within the

​a. pericardium

​b. mediastinum

​c. thoracic cavity

​d. a and b

​e. all of the above

28. Melatonin is secreted by the

​a. thyroid gland

​b. anterior pituitary gland

​c. pineal gland

​d. posterior pituitary gland

​e. hypothalamus

Answer 1

22. The unrestricted passage of ions between adjacent cardiomyocytes, required for the synchronous contractions that creat the heart beart, is possible due to the presence of gap junctions(Option C). Gap junctions are the channels by which iorganic ions, intracellular signalling molecules (cAMP, Calcium ion) up to 1000 dalton size, from one cell cytoplasm to the adjacent cell cytoplasm. This passage of signalling molecules and inorganic ions couples the adjacent cells electrically and metabolically. This results in synchronous contraction of the whole cell. Heart muscle consist of this gap junction, this channel consists of a protein connexins (transmembrane protein) which allows this passage of ions through a center pore (connexon) for synchronous contraction of adjacent cell. When the connexon of two plasma membrane of two cells are aligned, its allows the passage of ions through a continuous channel. This continuous channel is named gap junction. So, by this gap junction the cardiac myocytes excites as a whole unit and produce heart beat. This synchronous electrical signal from one cardiomyocte to adjacent cell is nessesary for the continuous production of heart beat. So Option C is correct.

Option A is incorrect because, adherens junction is a type of anchoring junction which connects the actin filaments from cell to cell or cell to extracellular matrix. The junction is basically anchored two cell or a cell to extracellular matrix tightly. This junction can't form any channel by which ion can pass from one cell to adjacent cells, So by this junction no electrical current can pass for synchronous contraction. So for synchronous contraction of cardiomyocytes this adherens junction has no role.

Option B is incorrect because, Desmosomes is also a anchoring junction. It connects two adjacent cells by cadherin proteins like, desmoglein, desmocollin. These cadherin proteins binds with the cytoplasmic plaque of two adjacent cells, these cytoplasmic plaque then interact with keratin and intermediate filaments of the cells. This whole interaction holds the neighbouring cells tightly. This junction also has no role for passing electrical currents as a form of ions, so this desmosome has no role for production of continuous heart beat(synchronous contraction of cardiomyocytes).

Option D is incorrect because, Tight junction is a occluding junctions, which prevents ions or small molecules to pass from one cell to adjacent cell. This tight junction is made up of two transmembrane protein caludins, occludins. These two proteins associates with ZO proteins to occluds the passage of ion and make the closest contact between two cells. So, this tight junction prevent passage of ions and molecules from one cell to another, that's why this junctions can't be responsible for passage of ions between cardiomyocytes.

Option E is incorrect because, Hemi-desmosome is also called half-desmosome, so this is also a anchoring junction as desmosome. But this hemidesmosome connects two adjacent cells by integrin protein not by cadherin proteins. This hemidesmosome connects the basal surface of epithelial cell to the basal lamina (a type of extracellular matrix). So this junction also helps for the anchoring of the adjacent cells not for the transferring of inorganic ions. So this junction doen't have any role for the unrestricted passage of ions through the cardiomycytes for the production of synchronous contraction and heart beat.

So, Option C is correct, Because gap junction is a direct connection between two cardiomyocytes and form a continuous channels through connexins proteins. This helps for the unrestricted passage of ions between adjacent cardiomyocytes, required for the synchronous contractions that creat the heart beart