A football player suffers from a partially torn anterior cruciate ligament. Which joint has been injured?

Sensory information from the body is transmitted to the central nervous system through afferent nerve pathways. In many of these pathways, sensory fibers decussate (cross over) to the opposite side of the spinal cord or brainstem before reaching the cerebral cortex. This organization is important for integrating sensory input with motor control and higher brain processing. The result is a contralateral representation of the body in the brain, meaning each hemisphere processes sensory input from the opposite side of the body.

A. Brain cannot tell from which side of the body a sensory impulse originated: the brain is highly organized in its processing of sensory input and can precisely localize the origin of stimuli. Even though sensory fibers cross over, the central nervous system maintains a detailed somatotopic map of the body (e.g., sensory homunculus). This allows accurate identification of the side and location of a stimulus. Crossover does not impair localization.

B. Each cerebral hemisphere receives sensory impulses from its opposite side of the body: most major sensory pathways, such as the spinothalamic and dorsal column-medial lemniscus pathways, decussate either in the spinal cord or brainstem. As a result, the left hemisphere processes sensory input from the right side of the body, and the right hemisphere processes input from the left side. This contralateral organization is a fundamental principle of neuroanatomy and allows coordinated integration of sensory and motor function.

C. Each cerebral hemisphere receives sensory impulses from its own side of the body: This describes ipsilateral processing, which is not the typical arrangement for major sensory pathways. While some limited pathways may remain ipsilateral, most conscious sensory pathways cross to the opposite side of the CNS. This does not reflect the general organization of sensory nerve fibers.

D. Sensory impulses are blocked at the level of the spinal cord: sensory crossover does not block signal transmission. Instead, it allows signals to be redirected to the appropriate contralateral brain region for processing. The spinal cord serves as a conduit and integration center, not a barrier. Therefore, sensory impulses are actively transmitted, not blocked, during decussation.

Skeletal muscle fibers are highly specialized cells designed for rapid and coordinated contraction. To achieve this, they require an efficient system for transmitting electrical signals from the cell surface deep into the muscle fiber. Transverse (T) tubules are invaginations of the sarcolemma that penetrate into the cell interior. They work closely with the sarcoplasmic reticulum to ensure uniform and rapid activation of muscle contraction throughout the fiber.

A. To store calcium ions needed for activating tropomyosin: calcium storage in muscle cells is primarily handled by the sarcoplasmic reticulum, not the T-tubules. The sarcoplasmic reticulum releases calcium ions in response to an action potential, allowing calcium to bind troponin and shift tropomyosin away from actin binding sites. T-tubules do not store calcium; they serve as conduits for electrical signals.

B. To transmit action potentials (impulses) into the cell interior: transverse tubules are invaginations of the sarcolemma that rapidly conduct action potentials from the cell surface into the deeper regions of the muscle fiber. This ensures that the entire muscle fiber contracts simultaneously rather than in a wave-like fashion. The electrical signal traveling through T-tubules triggers calcium release from the sarcoplasmic reticulum. This coupling is essential for coordinated and efficient muscle contraction.

C. To synthesize ATP for muscle contraction: ATP production occurs primarily in mitochondria through oxidative phosphorylation and in the cytoplasm via glycolysis. T-tubules have no role in energy metabolism or ATP synthesis. Their function is electrical signal transmission, not biochemical energy production. Therefore, this option describes a mitochondrial function rather than a T-tubule function.

D. To break down acetylcholine at the neuromuscular junction: acetylcholine breakdown is performed by the enzyme acetylcholinesterase located in the synaptic cleft of the neuromuscular junction. T-tubules are located inside the muscle fiber and are not involved in synaptic transmission or neurotransmitter degradation.