- Discuss the structures and functions of the nervous system.
- Discuss the structures and functions of the muscular system.
- Discuss the structures and functions of the skeletal system.
- Name the various types of connective tissues and their unique functions.
The human body is organized by increasing levels of complexity.
We have subatomic particles that make up atoms at a microscopic level.
Atoms form molecules when grouped together.
Molecules make up organelles, which are small structures performing specific functions.
Cells form tissues that make up organs and organ systems.
And the organ systems come together to make humans.
There are 11 organ systems that operate to keep us alive and healthy. The three most prominent ones will be the nervous, muscular, and skeletal systems. We will be discussing these systems throughout the chapter.
The Nervous System
The nervous system is the command center for the body. All movement is dictated by the nervous system.
Training adaptations and physical fitness can’t be well understood without some solid knowledge on the human nervous system and how it generates, propagates, and interprets neural signals.
The nervous system is made up of the nerves, spinal cord, and brain, and these are responsible for the control of voluntary and involuntary functions of the body.
Nervous tissue plays a role in the ability for us to sense, analyze, and interpret information and then respond to it. There are three types of nervous tissue.
Neurons are the nervous tissue responsible for transmitting signals to and from other neurons, glands, or muscles.
Neuroglia is also called glial cells, and these are neural tissues that support, protect, and insulate neurons.
Neurosecretory tissues are the neurons that translate neural signals into chemical stimuli.
The three main parts of the neuron will be the cell body, axon, and dendrites.
The cell body is the core part of the neuron and has a nucleus and other specialized organelles aiding the nervous system.
The axon is a thin tail-like structure of neurons which generate and conduct nerve impulses.
Dendrites are rootlike structures which branch from the cell body to receive and process signals from the axons of other neurons.
We can also categorize neurons based on three main jobs. Neurons can be sensory, motor, or interneurons.
- Sensory neurons are involved in communicating visual, auditory, and tactile info.
- Motor neurons initiate muscle contraction or activate the glands.
- Interneurons are nerve cells that connect neurons to each other.
Central Nervous System
The CNS is the first of two divisions of the nervous system. The other is the peripheral nervous system.
The CNS has the job of receiving the sensory inputs and functions so that the body can organize, analyze, and process information. It is made up of the brain and spinal cord.
The Brain and Brain Stem
The brain is made up of the cerebrum, cerebellum, and brain stem.
The cerebrum is the largest part of the brain and at the top, and it consists of the right and left hemispheres. Its role is to receive and process sensory information and control the body.
The cerebellum is the region of the brain responsible for conscious motor coordination.
The brain stem is the trunk of the brain, and it is made of the medulla oblongata, pons, and midbrain that continues down to form the spinal cord.
The cerebral cortex is also worth mentioning as this is where the most neural integration occurs.
The left hemisphere of the brain is in charge of language, logical processing, science, math, and controlling the muscles on the right side.
The right hemisphere of the brain is in charge of spatial perception, creativity, intuition, and controlling the muscles of the left side.
The Spinal Cord
The spinal cord is a tube of nervous tissue that extends from the brain to the bottom of the spine. This is the connection point between the brain and the body, as all nerve impulses travel through the spinal cord to and from the brain.
The Peripheral Nervous System
The PNS is the second division of the nervous system, and it is essentially all nervous tissue outside of the brain and spinal cord.
In total, we have 12 cranial nerves extending from the brain, and then 31 spinal nerves extending from the CNS to the peripheral organs and muscles.
These nerves serve the two main functions of receiving sensory information and relaying motor and autonomic signals between the body, brain, and spinal cord.
The PNS has afferent and efferent neurons.
Afferent neurons are sensory neurons that send information, also called stimuli, from the body to the CNS.
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Efferent neurons will be the neurons sending information from the CNS to the muscles so movement can happen.
The PNS has the sensory division and the motor division. The motor division divides into the somatic nervous system and the autonomic nervous system.
The somatic system is a part of the nervous system in charge of controlling voluntary movement. The autonomic nervous system is the part of the nervous system which is responsible for involuntary functions and movement.
Functions of the Central Nervous System
There are millions of sensory receptors in the body and they are all perceiving and communicating stimuli without stopping.
Internal stimuli are changes that happen inside the body, like temperature or pH.
External stimuli are messages from outside the body due to the environment, light, or sound.
Sensory input receptors are called mechanoreceptors.
The three types of mechanoreceptors are tactile receptors, proprioceptors, baroreceptors.
Integration of Sensory Input
The input collected by all of the receptors is translated into nerve impulses or electrical signals.
The brain interprets these impulses to perceive the sensations, have thoughts, or form memories.
Motor functions include both voluntary and involuntary muscle contractions. These contractions occur due to the firing of a motor unit.
Motor units are the single motor neurons and the muscle fibers it controls.
Motor unit pools are groups of motor units that work together.
To turn an electrical impulse into a mechanical response, there needs to be a process known as excitation-contraction coupling.
Nerve impulses sent to skeletal muscle fibers are called action potentials.
The all-or-none principle states that the strength of a neural electrical signal is independent of the magnitude of the stimulus so long as the neural threshold is achieved.
The Muscular System
There are over 600 muscles within the body that contribute to locomotion.
These muscles are able to pull via contraction and are going to be found in pairs or groups to allow for the dynamic movement that humans are capable of.
Types of Muscle Tissue
We have three types of muscle tissue. These are skeletal, smooth, and cardiac muscles. They will vary in their function, location, and cellular structure.
Skeletal muscle is the most common type of muscle in the body. These muscles are striated and voluntary. They attach to bones via tendons, and are found intrinsically in the eyeballs and the upper third part of the esophagus.
Smooth muscle is found within certain organs and organ systems in the body and here it is used to push food and other substances throughout the body. These muscles will contract slowly, operate involuntarily, and they do not fatigue quickly.
Smooth muscle contraction is triggered by hormones, neural signals from the ANS, and local factors.
Cardiac muscle tissue is striated and involuntary. It primarily makes up just the wall of the heart.
The cardiac muscle functions to contract the heart and pump blood throughout the body. Cardiac muscle cells will often branch and fuse into one another.
Cardiac muscle does not fatigue very easily and the rest period between contractions is all it really ever needs.
Structure of Skeletal Muscle
Skeletal muscle allows a person to move, exercise, and perform ADL, or activities of daily living.
Skeletal muscle is going to consist of muscle tissue, connective tissue, nerve tissue, and vascular tissue.
Activities of daily living are tasks that are done in the course of a regular day, like brushing teeth, going to the bathroom, dressing, and bathing.
Instead of a cytoplasm, muscles have a sarcoplasm, which holds more oxygen-binding proteins and granules of stored glycogen.
Myofilaments are found in this sarcoplasm. These are both actin and myosin.
Actin is where myosin binds to contract the muscles, and myosin is the myofilament with a fibrous head neck, and tail that binds to actin.
Skeletal muscles can have thousands of muscle fibers, which are bundled together to give support and structure to the muscle.
Each muscle fiber is surrounded by the endomysium, which is a covering connective tissue.
The epimysium is the connective tissue that surrounds the entire muscle, and the perimysium surrounds bundles of around 150 muscle fibers at a time.
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Connective tissues of the body meet at a site of muscle and tendon connection, which is called the myotendinous junction.
The tendons connect to the periosteum of the bones.
Skeletal Muscle Contraction
Muscle contraction requires a signal from the CNS. This travels from the nervous system and connects to the muscles via motor neurons.
From there, a multistep reaction occurs in which calcium is released into the muscle fibers, and this calcium along with the ATP is going to drive the binding of actin and myosin for contraction.
When the contractions occur and the myosin heads pull across actin filaments, there is a filament sliding that occurs and the muscle contracts.
This process of muscle contraction is known as the sliding filament theory.
Types of Muscle Fibers
We place skeletal muscle fibers in two different categories, and these two categories will have different energy needs, capabilities, and purposes in human movement.
The two categories of muscle fibers are known as slow-twitch and fast-twitch.
Slow-twitch fibers are also called type I fibers. These are seen to have a lot of mitochondria, which is the component of the cell that is noted for energy production.
Slow-twitch muscle fibers will derive energy from aerobic metabolism and they are ideal for endurance and lower intensity activity.
These type I fibers will contract somewhat slow and are very resistant to fatigue, also known as oxidative fibers due to their use of oxygen for their energy pathway.
Fast-twitch fibers are ones that contract quickly and give off greater force than the slow-twitch. These fibers can be either Type IIA or Type IIx fibers.
Type IIa fibers fatigue somewhat fast, but they have a moderate density of mitochondria, which means they can contract through most intermittent sports activity and still recover.
Type IIx fibers are the fast-twitch fibers that fire with the most force and strength. They are also going to fatigue quicker than the Type IIa fibers.
People who participate in endurance sports will oftentimes have more type I muscle fibers, and people who participate in more intermittent sports will have more type II fibers.
Size Principle of Motor Recruitment
The force output of a muscle is related to the stimulus it has received.
The size principle of fiber recruitment states that gibers with high levels of liability are first recruited and then those with lower levels are recruited last.
They are also recruited in their order according to set recruitment thresholds and firing rates.
To pick up a cell phone, we would recruit the lower threshold muscle fibers. To pick up a 100-pound dumbbell we would recruit our highest threshold fibers.
Muscle Fiber Arrangement
Fusiform muscles are spindle-shaped and they have a large muscle belly like we see with the biceps.
Convergent muscles are broad on one end with fibers converging and narrowing on the other, like with the pectoralis major.
Circular muscles are going to surround the external openings of the body, in what is known as sphincters.
Some common definitions and arrangements:
- Parallel muscles run along parallel to the axis of the muscle.
- Pennate muscles have fascicles that attach diagonally.
- Penniform muscles are muscle fibers that will run diagonally in respect to the tendon.
- Unipennate muscle extends from one side of a central tendon.
- Bipennate muscle extends from both sides of a central tendon.
- Multipennate muscle fibers extend from both sides of multiple central tendons.
There are three types of muscle actions possible: concentric, eccentric, and isometric.
Concentric muscle actions are when there is a shortening of the muscle as it produces tension, and it is overcoming the resistance.
Eccentric muscle action is when there is an overall lengthening of a muscle as it is producing tension, and the resistance is not being overcome.
Isometric muscle action is when there is no change in length of the muscle as it produces tension.
There is a built-in mechanism to amplify concentric contractions, which is known as the stretch-shortening cycle.
The Skeletal System
This is the last major organ system, and it is used for the structure and support of the human body. The human body has around 206 bones.
The bones give a framework that allow for the attachment of muscle tissue, which generates joint movement required for locomotion.
The bones will gain and lose density depending on many factors like age, activity, and nutrition.
The Axial Skeleton
The skeleton is divided into two parts, this is the first of them.
The axial skeleton has about 80 bones, and this includes the skull, spine, and ribs.
The Appendicular Skeleton
This is the second division of the skeletal system.
There are a total of 126 bones in the appendicular skeleton. These bones include the bones of the shoulder girdle, pelvic girdle, and limbs.
Categories and Functions of Bones
Flat bones are used for the protection of the internal organs and they provide a great amount of surface area for muscles to attach.
Short bones are the ones that are more cube-like and provide stability and a limited level of movement.
Long bones support body weight and facilitate movement. These bones will be longer than they are wide.
Sesamoid bones are smaller, rounder bones, and they are found in the joints and within tendons. They are used for reinforcement and protection of tendons from stresses.
Irregular bones are there to serve many purposes, like protecting vital organs.
Bone is made up of 50 – 70 percent minerals, 20 – 40 percent organic matrix, 5 – 10 percent water, and less than 3 percent lipids.
Bone is structured so that it may provide support and protection and also store calcium and bone marrow.
This is a constant process. Old bone is constantly being replaced with newer bone.
This happens through the process of osteogenesis.
Wolff’s law states that bone adaptation will occur and be based on the different loads placed on it. Strength training will help to build stronger bones, essentially.
Joints In The Human Body
Joints are the articulation point between two bones. The joints in the body are what allow movement to actually occur.
Fibrous joints are the joints with fibrous connective tissue joining two bones that allow for a little movement.
Fibrous joints can be stutures, syndesmoses, and gomphosis joints.
Cartilaginous joints ate joined by fibrocartilage, which is the strongest form of cartilage, or hyaline cartilage, which is softer and more widespread.
Fibrous joints can be primary, like epiphyseal growth plates, or secondary, like intervertebral discs.
Synovial joints are the most common and movable joints. They are also known as diarthrodial joints.
These joints are filled with synovial fluid which works to reduce friction and form a film over the joint surfaces.
Ball-and-socket joints are the joints that allow for a wide range of movement in many directions.
Saddle joints are like ball-and-socket joints but do not allow for rotation.
Hinge joints are going to allow for wide movements, but only in one plane.
Gliding joints are where two flat bones press up to one another.
Pivot joints are going to rotate around a long axis.
Condyloid joints are going to allow two directions of movement, with one direction being great and one being lesser.
Tendons connect the muscle to bone and serve as a mechanical bridge to transmit force created through the contraction of muscles.
Tendons are strong, mostly inflexible, and can withstand great force without getting injured.
These are the tough bands of collagen and elastin connecting bone to bone, which form joints. These help to prevent excessive movement in a joint which may cause damage,
There are extrinsic ligaments, intrinsic ligaments, and capsular ligaments.
The knee is one of the only joins with all types of ligaments.
Cartilage is used to resist compressive forces, make bones resilient, and offer support and flexibility in some areas.
Articular cartilage is a connective tissue that covers the end of long bones and provides smooth contact between bones.
Hyaline cartilage is the deformable, but elastic type. It is found in the nose, trachea, larynx, bronchi, and ends of the ribs.
Fibrocartilage is the tough tissue found in the intervertebral discs and insertions of tendons and ligaments.
Elastic cartilage is the most pliable type, and it gives shape to the external ear, an auditory tube of the middle ear, and the epiglottis.