Which of the following is not a function of the central nervous system (CNS)?

A. Spectrin: This is a specialized cytoskeletal protein that provides the cell with the structural integrity and flexibility needed to deform through narrow capillaries. While it is essential for maintaining the biconcave shape and mechanical endurance of red blood cells, it is a molecular component rather than a diagnostic measurement for blood volume.

B. Hematocrit: Also known as the packed cell volume (PCV), this test measures the proportion of total blood volume that is composed of erythrocytes following centrifugation. It is a critical clinical parameter used to assess a patient's oxygen-carrying capacity and to diagnose conditions such as anemia (low hematocrit) or polycythemia (elevated hematocrit).

C. Hematopoiesis: This term refers to the biological process of blood cell formation and development that occurs primarily within the red bone marrow of adults. It involves the differentiation of pluripotent hematopoietic stem cells into various myeloid and lymphoid lineages, including erythrocytes, leukocytes, and thrombocytes.

D. Hemocytometer: This is a precision-etched glass slide used primarily for performing manual cell counts under a microscope by viewing a specific, calibrated volume of fluid. While it can be used to determine the absolute concentration of red blood cells (cells per microliter), it does not directly measure the volume percentage or "packed" fraction of the blood sample.

A. Skeletal muscles: These tissues are under the exclusive control of the somatic nervous system, which utilizes myelinated motor neurons to release acetylcholine directly onto the motor endplate of the muscle fiber. This system is characterized by voluntary control and always results in excitatory responses leading to contraction.

B. Smooth muscles: These are a primary effector organ for the autonomic nervous system, found within the walls of hollow organs such as the stomach, intestines, and blood vessels. The autonomic system regulates the tone and rhythmic contractions of these muscles through a two-neuron relay (preganglionic and postganglionic).

C. Glands: The secretory activity of both exocrine and endocrine glands, including sweat glands, salivary glands, and the adrenal medulla, is fundamentally regulated by autonomic input. For instance, parasympathetic stimulation typically increases the secretion of digestive enzymes and saliva, while sympathetic stimulation can trigger the release of sweat or catecholamines.

D. Cardiac muscle: The heart is an intrinsic, autorhythmic organ whose rate and force of contraction are modulated extensively by the autonomic nervous system. The sympathetic division acts through the cardiac accelerator nerves to increase heart rate and contractility, whereas the parasympathetic division acts via the vagus nerve to decrease the heart rate.