The musculoskeletal system plays a vital role in supporting the body and facilitating movement. It is essential to comprehend both its structure and the typical process of muscle contraction. This guide aims to provide you with the necessary information for the MCAT.
Understanding the Musculoskeletal System for the MCAT
In the biology section of the MCAT, you’ll encounter questions related to the musculoskeletal system. These questions may come in different formats, either as passages or standalone inquiries.
Predicting the exact number of musculoskeletal system questions on the MCAT can be challenging. On average, you can anticipate facing approximately 2-4 questions about this system.
In the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem), introductory biology makes up 65% of the content. Additionally, it constitutes 5% of the material in the Chemical and Physical Foundations of Biological Systems (Chem/Phys) and another 5% in the Psychological, Social, and Biological Foundations of Behavior (Psych/Soc).
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Musculoskeletal System: Key Subtopics
An integral aspect of grasping the musculoskeletal system is recognizing its interaction with other organ systems, such as the nervous and endocrine systems. Frequently, these external systems provide the stimuli necessary to coordinate all functions and physiological control within the musculoskeletal system.
1. Functions of the Musculoskeletal System
The primary roles of the musculoskeletal system involve coordinating the body’s movement and providing support and protection for internal tissues and organs. Through a synergistic effort, muscles and bones collaborate to facilitate movement; muscles attach to bones, contract, and in turn, cause the bones and the body to move.
2. Types of Muscle: Skeletal, Cardiac, & Smooth
Before delving into the types of muscle, recall that muscles are organs composed of many different types of tissues and cells, with muscle tissue being the obvious main component.
With this, there are 3 main types of muscle tissue (and these muscles) distributed through the body: skeletal, cardiac, and smooth muscle.
Though they all perform muscle contract, they differ in a couple of factors including nuclei number, striation patterns, type of nervous system regulation, etc. The specific characteristics attributed to each muscle type serves a purpose. For example, the multinucleation of skeletal muscle is due to energy demands required by skeletal muscles for day to day functions!
3. Hierarchical Structure of Muscles
In understanding muscles, it’s crucial to grasp the hierarchical organization of their components.
The hierarchical organization of muscles is akin to the structure of the NBA, organized from larger entities to smaller ones. Just as the NBA is divided into two conferences, within each conference, there are 16 teams. Going further, each team is subdivided into over 20 individual players and coaches.
4. Muscular Contraction
To kickstart muscle contraction, muscle fibers receive signals from somatic neurons. These neurons create a specialized connection known as the neuromuscular junction, similar to a regular neuronal synapse, except here, the muscle fiber takes the place of the postsynaptic neuron dendrites.
In the process of innervation, an excitatory action potential travels down axons until it reaches the presynaptic axon terminal. This prompts the release of the neurotransmitter acetylcholine, which diffuses across the synapse to bind with receptors on the muscle fiber.
This binding leads to the depolarization of the muscle fiber, initiating further intracellular processes that eventually result in contraction.
On a more detailed level, contraction occurs as sarcomeres, the primary contractile units of muscle fibers, shorten. Sarcomeres consist of thin actin and thick myosin protein filaments. The sliding filament theory explains this shortening: upon innervation, myosin filaments’ heads interact and bind with actin filaments. Subsequently, the myosin heads “pull” the actin filaments towards the center, shortening the sarcomere and causing contraction.
5. Anatomy of the Skeletal System and Bone Structure
The skeletal system can be divided into two main parts: the axial and the appendicular skeleton.
The axial skeleton includes the skull, thoracic cage, and vertebral column. On the other hand, the appendicular skeleton consists of bones extending from the axial skeleton, like those in the upper arms and lower legs.
A helpful way to visualize this is to think of the axial skeleton as the protector of the central nervous system!
While the skeletal system comprises various bone types, you’ll likely be tested on the anatomy of appendicular long bones, such as the femur. Long bones have two primary parts: the diaphysis, the bone’s long shaft, and the epiphysis, the bone’s ends.
There are two types of bone tissue: compact and spongy bone. Both contain osteocytes (the main bone cell) and bone matrix but differ in their arrangements.
Compact bones consist of osteons, where osteocytes and bone matrix form concentric circles, creating a cylindrical tube. These tubes have channels for blood vessels to supply nutrients and remove waste, providing structural rigidity.
In contrast, spongy bones don’t have osteons. Instead, their osteocytes and bone matrix form lattice-like trabeculae. The trabeculae of spongy bone contain more crevices, housing red bone marrow and blood vessels.
Remember that bone is a type of connective tissue, with a hydroxyapatite matrix (serving as the ground substance) and collagen proteins. Osteoblasts and osteoclasts, embedded within the bone matrix, play crucial roles in bone remodeling.
Osteoblasts contribute to bone formation (bone deposition) by secreting collagen and inorganic molecules for the matrix. In contrast, osteoclasts perform bone degradation (bone resorption), countering the bone-forming process.
To learn more on this topic, read Jack Westin’s Skeletal System Guide.
6. Endocrine Regulation of the Skeletal System
Beyond providing support and facilitating movement, the bones of the skeletal system play a crucial role in maintaining blood calcium levels, contributing to overall bodily balance. This is achieved through their function in storing calcium within the hydroxyapatite bone matrix.
Remember that calcitonin and parathyroid hormone (PTH) are the primary hormones involved in regulating blood calcium levels. Calcitonin works to decrease these levels, while PTH acts to increase them.
Understanding the actions of these hormones is straightforward: Calcitonin promotes bone deposition by facilitating the uptake of calcium from the bloodstream, storing it within the hydroxyapatite matrix. Conversely, PTH encourages bone resorption, releasing calcium ions into the bloodstream to elevate blood calcium levels.
Essential Definitions and Key Terms for Musculoskeletal System Review
| Term | Definition |
| Fascicle | Bundles of muscle fibers; multiple fascicles combine to form a muscle |
| Myofibril | Primary contractile structure located within muscle fibers |
| Sarcomere | Fundamental contractile unit present within myofibrils |
| Neuromuscular Junction | Specialized synaptic connection formed between the axon of a somatic neuron and a muscle fiber |
| Sliding Filament Theory | Explanation of muscle contraction involving the shortening of sarcomeres through the interaction of actin and myosin filaments |
| Epiphysis | Term denoting the bone’s end, predominantly composed of spongy bone and red bone marrow |
| Diaphysis | Term referring to the bone’s long shaft, mainly composed of compact bone and yellow bone marrow |
| Osteons | Concentric, tubular arrangement of osteocytes and bone matrix within compact bones |
| Osteoblast | Bone cell responsible for promoting bone formation and deposition |
| Osteoclasts | Bone cell responsible for promoting bone degradation and resorption |
Sample MCAT Questions
Question 1:
Which hormone plays a key role in promoting bone deposition by facilitating the uptake of calcium from the bloodstream and storing it within the hydroxyapatite matrix of bones?
- A) Thyroxine
B) Calcitonin
C) Parathyroid Hormone (PTH)
D) Insulin
Explanation:
The correct answer is B) Calcitonin. Calcitonin is responsible for decreasing blood calcium levels by promoting bone deposition. It facilitates the uptake of calcium from the bloodstream and stores it within the hydroxyapatite matrix of bones.
Question 2:
What is the primary function of the neuromuscular junction in the context of muscle contraction?
- A) Synthesis of neurotransmitters
B) Transmission of electrical signals between muscles
C) Connection between bones and muscles
D) Transmission of signals from somatic neurons to muscle fibers
Explanation:
The correct answer is D) Transmission of signals from somatic neurons to muscle fibers. The neuromuscular junction is a specialized synaptic connection where signals from somatic neurons stimulate muscle fibers to contract. It plays a crucial role in initiating the process of muscle contraction.
Question 3:
Which type of bone tissue is characterized by concentric, tubular arrangements of osteocytes and bone matrix within compact bones?
- A) Spongy bone
B) Yellow bone marrow
C) Osteons
D) Trabeculae
Explanation:
The correct answer is C) Osteons. Osteons are found in compact bones and have a concentric, tubular arrangement of osteocytes and bone matrix. These structures provide structural rigidity to compact bones and contain channels for blood vessels to supply nutrients and remove waste.
Conclusion
In conclusion, a thorough understanding of the musculoskeletal system is crucial for success in the MCAT, particularly in the biology section. Given the significant representation of musculoskeletal content across various sections of the MCAT, investing time in mastering these concepts is not only beneficial but also strategic for achieving a competitive edge in the examination.
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