Understanding Skeletal, Smooth, and Cardiac Muscle Functions
Skeletal Muscle
Skeletal muscle is primarily responsible for keeping the bones together and facilitating their movement. This is achieved thanks to its fiber-shaped cells, which are multinucleated and elongated. These cells may exhibit the following three types of proteins:
- Structural proteins: Responsible for support and flexibility of the tissue.
- Regulatory proteins of contraction: They activate and inactivate muscle contraction. The main two are troponin and tropomyosin.
- Contractile proteins: Responsible for muscle contraction; myosin and actin.
1.1. Connective Tissue Components Surrounding the Skeletal Muscle
Surrounding the skeletal muscle are the following tissue layers, with some in particular standing out:
- Tendon: An extension of connective tissue that is beneath the muscle fibers and is responsible for maintaining the muscles attached to bones or other muscles.
- Epimysium: A layer that surrounds the entire muscle.
- Perimysium: A layer that surrounds muscle bundles.
- Endomysium: Surrounds individual muscle cells.
1.2. Nerve and Blood Supply
All skeletal muscles have a nerve, an artery, and two veins. A motor neuron can innervate several muscle fibers at the same time. We can state categorically that the muscles receive the blood and impulses required at any given time, although exceptions are always present.
1.3. Mechanisms of Skeletal Muscle Contraction
One of the main functions of skeletal muscle is to facilitate movement, commonly known as contraction. This can be involuntary or voluntary through nervous innervation. The reactions that lead to muscle contraction start when myosin pulls the thin filaments so that they slide into the I band. Once inside, the Z disk approaches or recedes depending on whether the muscle is contracting or relaxing.
It is important to note that there is a relationship between length and muscle tension, which depends on the length of the sarcomeres (the anatomical and functional unit of muscle limited by two Z disks). Before contraction occurs, it is a fact that if the sarcomere is short, the contraction will also be small and vice versa.
1.4. Muscle Fatigue
Muscle fatigue, commonly known as stiffness, occurs when there is an inability to contract the muscles normally after continuous activity, something we have all experienced in life. Some factors that contribute to the onset of fatigue include:
- Decrease of creatine phosphate.
- Reduction of Ca2+ in the sarcoplasm (cytoplasm).
- Lack of oxygen and glycogen.
- Failure of the neurotransmitter acetylcholine.
- Increase of lactic acid and ADP.
2. Smooth Muscle
Smooth muscle, also known as involuntary muscle, is composed of a series of spindle-shaped cells and is characterized by not being striated. It functions as a single unit and contains the proteins actin and myosin. Within this type of tissue, we can find two types of muscles:
- Single-unit smooth muscle: Contracts quickly, as seen in the gastrointestinal tract or uterus.
- Multi-unit smooth muscle: Contraction depends on nervous stimulation, occurring in the trachea, iris, and nictitating membrane of the eye.
Cardiac Muscle
Cardiac muscle is found only in the walls of the heart and is referred to as the myocardium. It is characterized by muscle fibers that are joined together by intercalated discs containing desmosomes and gap junctions. What most distinguishes it from other muscle types is that it works involuntarily and rhythmically.
Nerve Tissue
Nerve tissue comprises about one million neurons and an untold number of interconnections that are part of what we know as the nervous system.
Neurons
Neurons are the nerve cells of the nervous system and usually consist of three distinct parts:
- Cell body: The central portion of the nerve cell, which contains the nucleus and cytoplasm.
- Dendrites: Extensions specialized to receive stimuli from sensory cells, axons, and other neurons.
- Axon: An extension of variable size, measuring up to 1 meter long in some cases.
There are three types of neurons:
- Sensory neurons: Activated by receptor cells.
- Motor neurons: Transmit impulses received.
- Interneurons: Connect sensory neurons with motor neurons.
The main function of neurons is to receive external stimuli and convert them into nerve impulses to send to other neurons, muscles, or glands to elicit a response to that stimulus. Until recently, it was believed that neurons were the only cells in our body that do not reproduce or recover when damaged, but it has been shown that they can reproduce, albeit at an extremely slow rate.
Glial Cells or Neuroglia
Glial cells, commonly known as glia or neuroglial cells, are responsible for providing support and protection to neurons. There are dozens of these cells for each neuron.
3. Nervous System
The nervous system is a network of ectodermal tissues, which, as we saw before, are the basic units of neurons. Its principal objective is to capture and quickly process environmental signals to coordinate the organs that interact with the environment effectively.
3.1. Central Nervous System (CNS)
The central nervous system (CNS) consists of the brain and spinal cord. Here, we can find cells arranged in such a way that they lead to:
- Gray matter: Consists of the cell bodies of neurons.
- White matter: Consists of dendrites and axons of neurons.
This nervous system is responsible for receiving and processing stimuli gathered by the different senses and for transmitting orders to respond to effector organs.
3.2. Peripheral Nervous System (PNS)
The peripheral nervous system (PNS) consists of nerves and neurons found in all the limbs and organs of our body. Its main function is to coordinate, regulate, and integrate our internal organs. The main difference between the CNS and PNS is that the latter is not protected by bone or the blood-brain barrier, which increases its exposure to harmful toxins.