Anatomy Test Part 2

Synaptic end bulbs are the correct answer because they are the clusters of axon terminals found at the ends of neuromuscular junctions. These bulbs contain synaptic vesicles that store neurotransmitters, which are released into the synaptic cleft to transmit signals from the neuron to the muscle fiber. The term "synaptic" refers to the connection between neurons, while "end bulbs" specifically describes the bulb-like structure at the end of the axon terminal.

After the fusion of myoblasts, the muscle fiber loses its ability to go through cell division. This is because myoblasts are the precursor cells that fuse together to form muscle fibers. Once fusion occurs, the muscle fiber is formed and it becomes a mature, differentiated cell that is specialized for contraction. As a mature cell, it no longer undergoes cell division like myoblasts do. Therefore, the muscle fiber cannot go through cell division after fusion.

The primary visual area is responsible for vision. This area, located in the occipital lobe at the back of the brain, receives and processes visual information from the eyes. It plays a crucial role in the interpretation and perception of visual stimuli, allowing us to see and recognize objects, colors, shapes, and movements. Damage to this area can result in visual impairments or blindness.

Tropomyosin and troponin are regulatory proteins that can be found on an actin molecule. Tropomyosin is a long, filamentous protein that wraps around the actin filament, blocking the myosin binding sites and preventing muscle contraction. Troponin is a complex of three subunits that is attached to tropomyosin. It regulates muscle contraction by binding to calcium ions and causing a conformational change in tropomyosin, exposing the myosin binding sites and allowing muscle contraction to occur.

Titin is a protein that is found in the sarcomere, which is the basic unit of muscle contraction. It spans from the M line to the Z disc within the sarcomere. The M line is located in the center of the sarcomere, while the Z disc is found at the ends of the sarcomere. Therefore, the correct answer is "from M line to Z disc".

The cauda equina is not considered a major part of the brain. It is actually a bundle of nerves that extends from the bottom of the spinal cord. The brain stem, cerebellum, diencephalon, and cerebrum are all major parts of the brain. The brain stem controls basic functions such as breathing and heart rate. The cerebellum is responsible for coordinating movement and balance. The diencephalon includes structures such as the thalamus and hypothalamus, which play important roles in sensory processing and hormone regulation. The cerebrum is the largest part of the brain and is responsible for higher cognitive functions such as thinking, memory, and perception.

Motor unit recruitment refers to the process of increasing the number of active motor units in a muscle. Motor units are composed of a motor neuron and the muscle fibers it innervates. When more motor units are recruited, more muscle fibers are activated, resulting in increased muscle force and contraction. This process is essential for generating stronger muscle contractions and is often seen during activities that require increased force production, such as lifting heavy weights or running at high speeds.

The falx cerebri is a fold of the dura mater that descends vertically in the longitudinal fissure, separating the two cerebral hemispheres. It acts as a partition, preventing excessive movement and providing support to the brain. The falx cerebri is a crucial structure in maintaining the structural integrity and function of the cerebrum.

A single sensory neuron can have only one modality. This means that it is specialized to detect and transmit information from a specific type of sensory stimulus, such as touch, temperature, or pain. Each sensory neuron is dedicated to a particular modality, allowing for efficient and accurate transmission of sensory information to the brain.

The sarcoplasmic reticulum is a specialized organelle found in muscle cells that is responsible for storing and releasing calcium ions (Ca2+). Calcium ions play a crucial role in muscle contraction, as they bind to proteins within the muscle fibers, allowing them to slide past each other and generate force. When a muscle is at rest, the sarcoplasmic reticulum actively pumps calcium ions from the cytoplasm into its storage compartments. During muscle contraction, the sarcoplasmic reticulum releases stored calcium ions, triggering the contraction process. Therefore, the correct answer is Ca2+.

Adaptation refers to the phenomenon where the frequency of nerve impulses in the first-order neuron decreases during prolonged stimulus. This means that the neuron becomes less responsive to the stimulus over time. This adaptation allows the nervous system to filter out repetitive or non-changing stimuli and focus on detecting new or changing stimuli. It helps in maintaining sensitivity to important sensory information while ignoring constant background stimuli.

The adult brain represents only 2% of the total body weight. This is because the brain is a highly complex and specialized organ that requires a lot of energy to function properly. Despite its relatively small size, the brain plays a crucial role in controlling all bodily functions and processes. Therefore, it is not surprising that it accounts for a small percentage of the total body weight.

The blood-brain barrier is a protective mechanism that prevents harmful substances and pathogens from entering the brain. It is composed of specialized cells lining the blood vessels in the brain, which tightly regulate the passage of molecules. This barrier helps maintain the delicate balance of the brain's internal environment, ensuring that only essential nutrients and molecules are able to cross into the brain while keeping out potentially damaging substances.

Cerebrospinal fluid carries chemicals from the blood to neurons. This is because the blood supplies the brain and spinal cord with essential nutrients, oxygen, and other chemicals necessary for proper neuronal function. The cerebrospinal fluid acts as a medium to transport these substances from the blood to the neurons, ensuring their proper functioning and maintaining the homeostasis of the central nervous system.

The reticular formation is a netlike region of white and gray matter that extends through the brain and plays a crucial role in maintaining consciousness. It is involved in regulating arousal, attention, and sleep-wake cycles. The reticular formation receives sensory information from various parts of the body and relays it to the cerebral cortex, allowing us to be aware of our surroundings and respond to stimuli. Additionally, it helps filter out irrelevant information and focus on important stimuli.

The direct pathway is responsible for conveying nerve impulses from the cerebral cortex to cause precise, voluntary movements of skeletal muscles. This pathway allows for conscious control over movements and is involved in initiating and executing planned movements. It is responsible for transmitting signals that result in the activation of specific muscles, allowing for coordinated and controlled movements.

The inferior colliculus is responsible for somatic (startle) reflexes in response to loud sounds. It is a structure located in the midbrain that receives auditory input from the ears and processes it to generate appropriate reflexive responses. When a loud sound is detected, the inferior colliculus triggers a startle reflex, causing involuntary muscle contractions and other physiological responses. This reflexive reaction helps to protect the body from potential harm or danger associated with loud noises.

During fetal development, Schwann cells begin to form myelin sheaths around axons. This process is crucial for the proper functioning of the nervous system. Myelin sheaths help to insulate and protect the axons, allowing for efficient transmission of electrical signals. This development occurs before birth and continues throughout early childhood. After birth, Schwann cells continue to myelinate axons in response to the maturation of the nervous system, but the initial formation of myelin sheaths occurs during fetal development.

When a muscle is stretched, the muscle spindle, which is a sensory receptor located within the muscle, generates nerve impulses. These impulses then travel through the sensory neuron, which is connected to the muscle spindle, and enter the spinal column. The structure through which these nerve impulses enter the spinal column is the posterior root of the spinal nerve.

When a muscle is overstretched, the muscle spindle, which is a sensory receptor within the muscle, generates a somatic spinal reflex. This reflex leads to the contraction of the agonist muscle, which is the muscle that is being stretched. Additionally, it causes the relaxation of the antagonist muscle, which is the muscle that opposes the action of the agonist muscle. Therefore, the correct answer is 1 and 2, as both the contraction of the agonist muscle and the relaxation of the antagonist muscle are responses to the muscle being overstretched.

When there is excessive tension on a tendon, the tendon organ detects this and initiates a somatic spinal reflex. This reflex leads to the contraction of the antagonist muscle (the muscle opposite to the one causing the tension) and the relaxation of the agonist muscle (the muscle causing the tension). Therefore, the correct answer is 3 and 4.

The pons is a region of the brainstem that contains various nuclei, including the pontine nuclei, apneustic area, and pneumotaxic area. Therefore, all of these choices are correct as they represent nuclei found in the pons.

Autonomic motor neurons regulate visceral activities by both increasing and decreasing activities in effector tissues. This means that they have the ability to stimulate or inhibit the functioning of these tissues, depending on the needs of the body. This regulation allows for the control of various bodily functions, such as digestion, heart rate, and respiration.

The flocculonodular lobe of the cerebellum is responsible for maintaining equilibrium and balance. It receives input from the vestibular system, which detects changes in head position and movement. This lobe helps coordinate eye movements and adjust posture and muscle tone to maintain balance. It plays a crucial role in preventing falls and maintaining stability during movements.

A typical spinal nerve has two connections to the cord. These connections are known as the dorsal root and the ventral root. The dorsal root carries sensory information from the body to the spinal cord, while the ventral root carries motor information from the spinal cord to the muscles. Together, these two roots form the spinal nerve, allowing for communication between the body and the spinal cord.

In the sliding filament mechanism, the thin filament is being pulled towards the M line. The M line is located in the center of the sarcomere and serves as the attachment point for the thick filaments. As the thick filaments slide past the thin filaments during muscle contraction, the thin filaments are pulled towards the M line. This movement causes the sarcomere to shorten, resulting in muscle contraction.

During the relative refractory period, the membrane potential of a neuron is hyperpolarized but gradually returning to its resting state. This means that a larger than normal stimulus can still generate a second action potential, although it would require a stronger stimulus compared to the initial action potential. Therefore, during the relative refractory period, a second action potential can be initiated by a larger than normal stimulus.

The parasympathetic division of the autonomic nervous system is responsible for promoting rest and relaxation in the body. It is characterized by long preganglionic neurons, which extend from the brainstem or sacral spinal cord to ganglia near or within the target organs. The vagus nerve, which is part of the parasympathetic division, carries parasympathetic output to various organs in the body. However, the parasympathetic division does not synapse with smooth muscle in blood vessel walls. Instead, the sympathetic division is responsible for regulating smooth muscle in blood vessels.

Diffusion, enzymatic degradation, and uptake by cells are all mechanisms by which a neurotransmitter can be removed from the synaptic cleft. Diffusion refers to the movement of the neurotransmitter molecules from an area of high concentration to an area of low concentration. Enzymatic degradation involves the breakdown of the neurotransmitter molecules by specific enzymes. Uptake by cells refers to the process where neurotransmitters are taken up by neighboring cells, such as glial cells or presynaptic terminals, through specific transporters. These processes collectively contribute to the removal of neurotransmitters, thus terminating their signaling effects in the synaptic cleft.

A fibers have the largest diameter among the given options. This is because A fibers are myelinated and have a larger diameter compared to B and C fibers. Myelination increases the conduction speed of nerve impulses, and larger diameter axons allow for faster transmission of electrical signals. Therefore, A fibers are responsible for transmitting information quickly and efficiently in the nervous system.

A postsynaptic neuron may respond to inhibitory and excitatory effects in either EPSPs or IPSPs. EPSPs (excitatory postsynaptic potentials) are depolarizations of the postsynaptic membrane, which increase the likelihood of an action potential. IPSPs (inhibitory postsynaptic potentials) are hyperpolarizations of the postsynaptic membrane, which decrease the likelihood of an action potential. Therefore, the postsynaptic neuron can respond to both excitatory and inhibitory effects by either depolarizing or hyperpolarizing its membrane potential, depending on the type of synaptic input it receives.

When a light touch is applied, it triggers a lower frequency of impulses sent to the sensory centers in comparison to a touch with more pressure. This difference in frequency is what causes the perception of a different sensation. The sensory centers receive and process these impulses, and the brain interprets the frequency as a different type or intensity of touch. Therefore, changing the frequency of impulses sent to sensory centers is the phenomenon that explains why a light touch feels different than a touch applied with more pressure.

A motor unit consists of a somatic motor neuron and all the skeletal muscle fibers it stimulates. When the motor neuron is activated, it sends an electrical signal to the muscle fibers, causing them to contract. This coordinated contraction of multiple muscle fibers allows for precise control and movement. Therefore, the motor unit is the correct answer in this context.

The semicircular canals are responsible for sensing changes in rotational acceleration of the head. These canals are fluid-filled and contain hair cells that are sensitive to movement of the fluid. When the head rotates, the fluid in the canals also moves, causing the hair cells to bend. This bending of hair cells sends signals to the brain, which helps in maintaining dynamic equilibrium and balance. The other structures mentioned, such as the cochlea, maculae of the vestibule, organ of Corti, and vestibulocochlear nerve, are not specifically involved in sensing changes in rotational acceleration.

The subarachnoid space is the correct answer because it is the area between the arachnoid mater and the pia mater in the meninges. This space is filled with cerebrospinal fluid, which acts as a cushion and provides nutrients to the brain and spinal cord. The other options listed do not contain cerebrospinal fluid.

Slow oxidative fiber is the least powerful type of muscle fiber because it has a slower contraction time and lower force production compared to other types of muscle fibers. These fibers are primarily used for endurance activities and are highly resistant to fatigue. They rely on oxidative metabolism to produce energy, which allows them to sustain contractions for longer periods of time. However, due to their slower contraction speed and lower force production, they are not as powerful as fast oxidative or fast glycolytic fibers.

Intercalated discs are found in cardiac muscle tissue but not in skeletal muscle tissue. These specialized structures are responsible for connecting individual cardiac muscle cells, allowing them to work together as a coordinated unit. Intercalated discs contain gap junctions, which enable electrical impulses to pass quickly from one cell to another, facilitating synchronized contractions of the heart. In contrast, skeletal muscle tissue does not have intercalated discs, as skeletal muscle cells are not required to contract in a coordinated manner.

The epineurium is the outermost layer that surrounds the entire spinal nerve. It provides protection and support to the nerve fibers within the spinal nerve. The dura mater, pia mater, endoneurium, and perineurium are also layers associated with the spinal nerve, but they do not surround the entire nerve like the epineurium does.

The given answer states that all of the options listed (sensory function, integrative function, and motor function) are functions of the nervous system. This means that each of these functions is performed by the nervous system. The nervous system is responsible for receiving sensory information from the environment, integrating and processing this information, and then sending out motor signals to initiate appropriate actions. Therefore, all of the options listed are indeed functions of the nervous system.

Intercostal nerves are the nerves that run between the ribs. They do not enter into a plexus, which is a network of nerves, and instead directly connect to the structures they supply. This means that each intercostal nerve supplies a specific area of the body without branching or merging with other nerves. This direct connection allows for precise control and sensation in the muscles and skin of the chest and abdomen.

Muscle spindles are sensory receptors found within muscles that are responsible for detecting changes in muscle length. They provide important feedback to the central nervous system about the position and movement of our muscles. When a muscle is stretched or contracted, the muscle spindles within it are activated and send signals to the brain, allowing us to perceive and adjust to these changes. This feedback loop helps to maintain proper muscle tone, coordination, and balance.

A first order neuron is responsible for conducting impulses from the somatic receptors into the brain stem or spinal cord. This neuron is the initial step in the sensory pathway, receiving signals from sensory receptors located in the body and transmitting them to higher levels of the nervous system for processing. It serves as the primary conduit for sensory information to reach the central nervous system, where further processing and interpretation of the signals occur.

Upper motor neurons are axons that extend from the brain to the lower motor neurons. They are responsible for transmitting signals from the brain to the lower motor neurons, which in turn transmit these signals to the muscles, allowing for voluntary movement. Upper motor neurons play a crucial role in the coordination and control of motor functions.

Olfactory hairs are responsible for the sense of smell. They are located in the olfactory epithelium, which lines the nasal cavity. Olfactory hairs contain receptors that detect different odors in the air. When these receptors are stimulated by odor molecules, they send signals to the brain, allowing us to perceive and identify smells. Therefore, olfactory hairs play a crucial role in our sense of smell.

Adaptation refers to the process by which our senses become less responsive to a constant stimulus over time. In the case of odorants, adaptation occurs rapidly, meaning that our sensitivity to a particular smell decreases quickly when we are continuously exposed to it. This allows our olfactory system to focus on detecting new or changing smells in our environment. Therefore, the correct answer is that adaptation occurs rapidly.

Taste buds are found in all of these choices, including the epiglottis, the pharynx, and the soft palate. Taste buds are sensory organs that detect different tastes such as sweet, sour, salty, and bitter. They are located on the tongue, as well as in other parts of the mouth and throat.

The conjunctiva is a thin layer that covers the anterior surface of the eyeball. It acts as a protective barrier, preventing foreign objects from entering the eye and helping to keep the eye moist. The conjunctiva also produces mucus and tears, which help to lubricate the eye and keep it comfortable.

The correct order in the flow of tears is from the lacrimal gland, which produces tears, to the lacrimal duct, which carries the tears to the superior or inferior lacrimal canal. From there, the tears flow into the lacrimal sac, which acts as a reservoir. The tears then continue through the nasolacrimal duct, which connects the lacrimal sac to the nasal cavity, where they are ultimately drained.

The lens of the eye is made up of layers of proteins called crystallins. These crystallins are responsible for maintaining the transparency and shape of the lens, allowing it to focus light onto the retina. They are highly organized and tightly packed, which helps to prevent scattering of light and maintain the clarity of vision. Crystallins also play a role in protecting the lens from damage caused by aging and oxidative stress.

The sclera is the white, tough, fibrous outer layer of the eyeball that provides protection and structural support to the inner parts of the eye. It covers most of the eyeball and helps maintain the shape of the eye. Additionally, it serves as an attachment point for muscles that control eye movements. The other options mentioned, such as the pupil, iris, cornea, and retina, have different functions and do not provide the same level of protection to the inner parts of the eyeball as the sclera does.

The tympanic membrane, also known as the eardrum, acts to convert sound waves to vibrations. When sound waves enter the ear, they cause the tympanic membrane to vibrate. These vibrations are then transmitted to the middle ear, where they are further amplified and transmitted to the inner ear. The conversion of sound waves to vibrations by the tympanic membrane is the first step in the process of hearing.

Denticulate ligaments are thickenings of the pia mater. These ligaments are delicate, tooth-like projections that extend from the pia mater and attach to the dura mater, which is the outermost layer of the meninges. The denticulate ligaments help to stabilize and anchor the spinal cord within the vertebral canal, preventing excessive movement and providing support.

The Meissner corpuscle is considered a rapidly adapting receptor. Rapidly adapting receptors are sensory receptors that quickly adapt to a constant stimulus and are responsible for detecting changes in stimuli such as touch or pressure. The Meissner corpuscle is specifically responsible for detecting light touch and low-frequency vibrations. It is found in the skin, particularly in areas such as the fingertips, palms, and soles of the feet.

The output of the ANS does not control skeletal muscle. The autonomic nervous system (ANS) is responsible for regulating involuntary bodily functions such as heart rate, digestion, and breathing. It consists of two branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). While the ANS controls smooth muscle, cardiac muscle, exocrine glands, and endocrine glands, it does not have direct control over skeletal muscle. Skeletal muscle is under voluntary control and is regulated by the somatic nervous system.

The vitreous chamber is the correct answer because it is the space between the lens and the retina in the eye. It is filled with a gel-like substance called the vitreous humor, which helps maintain the shape of the eyeball and provides support to the retina. The vitreous chamber plays a crucial role in transmitting and focusing light onto the retina for clear vision.

The brain stem is the correct answer because it consists of the medulla oblongata, pons, and midbrain. The medulla oblongata controls vital functions such as breathing and heart rate. The pons helps regulate sleep and relays information between different parts of the brain. The midbrain is involved in sensory and motor functions. Together, these structures make up the brain stem, which is responsible for many essential functions of the nervous system.

Myofibrils are the contractile organelles of the muscle fiber. They are responsible for the contraction and relaxation of muscles. Made up of thick and thin filaments, myofibrils are organized into repeating units called sarcomeres, which give muscles their striped appearance. These sarcomeres contain the proteins actin and myosin, which interact to generate the force required for muscle contraction. Therefore, myofibrils are the correct answer as they play a crucial role in muscle contraction.

Both myelinated and unmyelinated axons contain endoneurium. Endoneurium is a connective tissue that surrounds individual axons within a nerve. Myelinated axons have a myelin sheath surrounding them, which is further enveloped by the endoneurium. Unmyelinated axons, on the other hand, lack a myelin sheath but are still surrounded by the endoneurium. Therefore, both types of axons contain endoneurium.

The indirect pathway is responsible for regulating automatic movements and coordinating them with visual stimuli. This pathway helps in controlling movements that are not consciously initiated, such as reflexes and involuntary movements. Unlike the direct pathway, which is involved in voluntary movements, the indirect pathway helps to modulate and fine-tune motor commands based on sensory inputs.

The postcentral gyri of the parietal lobes of the cerebral cortex are responsible for receiving body sensory information. This means that they play a crucial role in processing and interpreting sensations such as touch, temperature, pain, and proprioception (awareness of body position). The postcentral gyri are specifically involved in receiving sensory input from different parts of the body and integrating it to create our perception of touch and other somatosensory experiences.

The medial lemniscus can be found in all three structures mentioned: the medulla oblongata, the pons, and the midbrain.

The correct answer is "ALL of the choices." This is because a polarized cell refers to any cell that has a charge imbalance across its membrane, which is true for all cells of the body. Additionally, a polarized cell exhibits a membrane potential, which is also true for all cells. Therefore, all of the given choices accurately describe a polarized cell.

The stretch and tendon spinal reflexes do not provide protection to spinal nerves. These reflexes are responsible for maintaining muscle tone, preventing damage to muscles and tendons, and increasing awareness of muscle tension in the body. However, they do not directly protect the spinal nerves.

This answer is correct because an ipsilateral and intersegmental spinal somatic reflex involves the activation of multiple flexor and extensor muscles on the same side of the body as the sensor. This reflex pathway allows for coordinated movement and balance by activating both flexor and extensor muscles on the same side of the body in response to a sensory stimulus.

The correct answer is intercostals nerves. The intercostal nerves are a set of nerves that originate from the spinal cord in the thoracic region and run along the spaces between the ribs. They provide sensory innervation to the muscles and skin of the chest and abdomen. The other options, such as brachial nerves, lumbar nerves, sacral nerves, and cervical nerves, refer to nerves in other regions of the body and are not synonymous with the thoracic nerves.

The mesencephalon is the correct answer because it gives rise to the midbrain and the aqueduct of the midbrain. The mesencephalon is part of the brainstem and is responsible for relaying sensory and motor information between the forebrain and hindbrain. It plays a crucial role in controlling eye movement, auditory and visual processing, and motor coordination.

The correct answer is "Forms gap junctions with postganglionic neurons in autonomic ganglia." This answer is not correct because preganglionic neurons do not form gap junctions with postganglionic neurons in autonomic ganglia. Instead, they release neurotransmitters into the synapse to communicate with postganglionic neurons. Gap junctions are found between cells and allow for direct electrical and chemical communication, but this does not occur between preganglionic and postganglionic neurons in autonomic ganglia.

The given correct answer states that a postganglionic neuron releases neurotransmitter that binds to the effector cell. This means that after receiving information from the preganglionic neuron, the postganglionic neuron transmits the signal to the effector cell, which can be a muscle or gland, by releasing neurotransmitters. These neurotransmitters bind to receptors on the effector cell, causing it to respond and carry out the necessary action. This process is a crucial part of the autonomic nervous system, which controls involuntary bodily functions.

The correct answer is "inhibitory postsynaptic potential." Inhibitory postsynaptic potential refers to a temporary hyperpolarization of a postsynaptic neuron, which reduces the likelihood of an action potential being generated. This occurs when inhibitory neurotransmitters bind to receptors on the postsynaptic membrane, causing ion channels to open and allow negatively charged ions to enter the cell or positively charged ions to exit. As a result, the membrane potential becomes more negative, making it less likely for the neuron to reach the threshold for firing an action potential.

The ciliary ganglion is not considered a part of the prevertebral ganglia. The prevertebral ganglia are a group of ganglia located anterior to the vertebral column, which innervate the abdominal and pelvic organs. The ciliary ganglion, on the other hand, is a parasympathetic ganglion located in the orbit of the eye. It is responsible for controlling the pupillary sphincter muscle and the ciliary muscle of the eye. Therefore, it is not considered a part of the prevertebral ganglia.

The correct answer is "All of the above are parasympathetic ganglia." This means that all of the options listed (Ciliary ganglion, Pterygopalatine ganglion, Submandibular ganglion, Otic ganglion) are examples of parasympathetic terminal ganglia.

The celiac plexus is the largest autonomic plexus in the body. It is located in the abdomen, behind the stomach, and is responsible for innervating the abdominal organs such as the liver, spleen, stomach, and intestines. It plays a crucial role in regulating the digestive system and controlling the blood flow to these organs. The other options, such as the superior mesenteric plexus, renal plexus, cardiac plexus, and hypogastric plexus, are also autonomic plexuses but are not as large or as centrally located as the celiac plexus.

White rami communicantes are structures that connect the anterior ramus of the spinal nerve with the ganglia of the sympathetic trunk. These structures contain sympathetic preganglionic axons, which are responsible for transmitting signals from the spinal cord to the ganglia. The white rami communicantes are part of the sympathetic nervous system and play a crucial role in the transmission of sympathetic signals throughout the body.

Acetylcholine is released by parasympathetic postganglionic neurons and is removed faster than Norepinephrine. This means that in the parasympathetic nervous system, acetylcholine is released from the postganglionic neurons and quickly broken down or taken up by the surrounding cells. In contrast, in the sympathetic nervous system, norepinephrine is the primary neurotransmitter released by postganglionic neurons and it is removed at a slower rate compared to acetylcholine in the parasympathetic system.

The signal to excite a muscle cell must cross the synaptic cleft. The synaptic cleft is the small gap between the axon terminal of the motor neuron and the muscle cell. Acetylcholine, a neurotransmitter, is released from the axon terminal and diffuses across the synaptic cleft to bind to receptors on the muscle cell. This binding triggers a series of events that ultimately lead to muscle contraction. Therefore, the synaptic cleft is the correct answer as it is the site where acetylcholine diffuses to transmit the signal to the muscle cell.

A diverging circuit is a neural circuit where a single presynaptic neuron synapses with multiple postsynaptic neurons. This allows for the spread of information from one neuron to multiple pathways, increasing the potential for different responses or actions. It is called a diverging circuit because the information diverges or spreads out to multiple destinations.

Creatine phosphate and ATP are both energy sources that provide the necessary fuel for muscle contractions. Creatine phosphate is used as a rapid source of energy, while ATP is the primary energy currency of cells. Together, they work to replenish and sustain the energy needed for muscle contractions. However, the limited amount of creatine phosphate and ATP stored in muscles can only support contractions for a short period of time. Therefore, the correct answer is 15 seconds, as this is the approximate duration that the available energy can last.

Plasticity refers to the brain's ability to change and adapt based on experience. This means that the brain can modify its structure and function in response to new information, learning, and environmental factors. It allows for the formation of new neural connections and the strengthening or weakening of existing ones. Plasticity is crucial for learning, memory, and recovery from brain injuries. It enables the brain to constantly reorganize itself and optimize its performance in different situations.

A twitch contraction refers to the brief contraction of all muscle fibers in a motor unit in response to a single action potential. This contraction is characterized by a quick and involuntary muscle movement. It is different from isotonic contraction, which involves muscle fibers shortening and producing movement, and isometric contraction, which involves muscle fibers generating force without changing length. Tetany refers to a sustained contraction, while the refractory period is a period of time during which a muscle fiber cannot be stimulated again.