human anatomy and physio - module 5

musculoskeletal system

Functions of the skeletal system

1. Support

2. Protection

3. Assistance in movement

4. Mineral homeostasis

5. Blood cell production

6. Triglyceride storage

Functions of the skeletal system - support

• The skeleton provides a rigid framework for other elements to connect to. It therefore gives structural support to other anatomy

Functions of the skeletal system - protection

• Bone is hard, dense and strong. It commonly provides a shield for deeper, vital organs in order to protect them. Think of the skull surrounding the brain, or the rib cage surrounding the lungs and heart

Functions of the skeletal system - mineral homeostasis

• Osseous tissue has a high concentration of a few important minerals, such as calcium and phosphorus. When levels of these minerals in our bodily fluids are low, they can be released from the bone to boost their concentration in our body fluids

• When levels of these minerals are high in the bodily fluids, we can take these minerals from the fluids and store them in the bones

Functions of skeletal system - blood cell production

• Many bones have a type of tissue inside them called the red bone marrow. It is in this tissue that new blood cells are created

• Red bone marrow is red due to the high density of blood vessels in it

Functions of the skeletal system - triglyceride storage

• Some bones have another type of bone marrow called yellow bone marrow

• Yellow bone marrow is a very fatty tissue and largely functions to store a certain type of lipid called a triglyceride

• Triglycerides are a potential energy substrate our bodies can use to create ATP

Key cells in the skeletal system

• Osteoblasts are the cells that synthesise and secrete much of the extracellular matrix of bone (osteo = bone)

• Chondroblasts are the cells that synthesise and secrete much of the extracellular matrix of cartilage (chondro = cartilage)

• Fibroblasts are the cells that synthesise and secrete much of the extracellular matrix of dense connective tissue (which is softer than bone and cartilage)

• The final cell we need to know is an osteoclast. Remember that an osteoblast helps to make bone tissue. An osteoclast, on the other hand, breaks down bone tissue. A key feature of this cell is its ruffled border, which is highlighted in the picture

Classifications of bones

• Long bones - longer than they are wide

• Short bones - roughly as wide as they are long, and are somewhat cuboidal in shape

• Flat bones - these bones are largely flat

• Sesamoid - not a shape category, bones that grow inside tendons

Periosteum

• Most bones except for long bones are made of an outer shell of compact bone and are filled on the inside with spongy bone

• Outside the compact bone shell of all bones lies a connective tissue layer called the periosteum. This covers the entire bone except where there is a cartilage aspect of the bone

Features of a long bone

• Long bones have compact bone on the outside and some spongy bone on the inside

• Features of a long bone:

  • Epiphysis (plural = epiphyses)

  • Diaphysis

  • Metaphysis (plural = metaphyses)

  • Medullary cavity

  • Epiphyseal lines/plates

Long bone REVIEW

Compact bone

• Also called cortical bone tissue, it is more dense than spongy bone. It has fewer spaces and is the stronger tissue of the two

• It is found on the outer aspect of a bone, beneath the periosteum or articular cartilage. It consists of repeating units called osteons, which are concentric lamellae (rings of tissue) that surround a central canal, which contains vessels and nerves

• At various points in between the lamellae are osteocytes. Other types of lamellae fill the spaces between and around osteons

Spongy bone

• Also called trabecular or cancellous bone tissue, there are many more spaces between it. This type of osseous tissue is created by a network of spicules (trabeculae) that join together

• There are lots of spaces between these thin trabeculae, making this type of bone comparatively weaker. However, this structural make-up is important for a few reasons:

  1. The increased space between the trabeculae makes this bone much lighter than compact bone. This means that the bone moves faster and more easily when a muscle pulls on it

  2. The spaces between the trabeculae house either red or yellow bone marrow. This bone marrow contributes to two of the important functions of bone

• Although spongy bone can be considered weaker than compact bone, it can increase the strength of certain parts of the bone in many different directions

Which type of bone creates the outer shell of bones

Compact bone

Which type of bone tissue is found on the inside of bones?

Spongy bone

What are the other names for spongy bone?

Trabecular bone & Cancellous bone

What is often housed between the trabeculae of spongy bone?

Red bone marrow in bones that produce blood cells, yellow bone marrow in other bones

Bones of the skeleton

• There are two sections of the skeletal system, called the axial and appendicular skeleton

• The axial skeleton consists of the skull, spine, ribs and sternum

• The appendicular skeleton consists of the bones of the upper and lower limbs, as well as the bones from the pectoral and pelvic girdles

• A girdle is something that encircles something else. The pectoral girdle encircles the upper part of the rib cage, and the pelvic girdle encircles the lower spine

Bony landmarks

• All bones have surface markings or particular aspects that allow specific functions. These markings/aspects of the bone are called bony landmarks

• Some of these are present at birth, however many develop in response to tendons, ligaments, or other elements pulling or pushing on certain parts of the bone

• Generally speaking there are 3 types of bony landmarks:

  • Projections

  • Depressions

  • Openings

• There are many different types of projections, depressions and openings, which have specialised terms associated with them

• For example, a roughly circular shaped opening is a called a foremen, however a long narrow opening is called a fissure

• The names of these specific types of bony landmarks are then given further detail which relates to exactly where in the body they are found

Axial skeleton overview

The axial skeleton includes:

1. Skull

2. Spine (vertebral/spinal column)

3. Thoracic cage

Thoracic cage components

Make up of:

• Ribs: 12 pairs = 24 total

• Costal cartilage: White connective tissue seen attached to ribs

• Thoracic spine: The part of the spine contributing to the rib cage

• Sternum: Central chest bone

The skull

Divided into:

1. Cranial bones:

• Make up the cavity (walls and roof) where the brain sits

2. Facial bones:

• All other bones that do not contribute to the cranial cavity (mostly face area)

Example:

• Mandible: The lower jawbone (a facial bone)

The spine (vertebral column)

• Made of individual bones called vertebrae (plural), vertebra (singular)

• Viewed from the front or back: appears straight

• Viewed from the side (lateral): shows natural curvatures which are important for weight distribution and movement

Spinal regions & vertebrae count

1. Cervical (neck) - 7 vertebrae

2. Thoracic (chest) - 12 vertebrae

• Each has ribs attached

3. Lumbar (lower back) - 5 vertebrae (common), sometimes 4 or 6

4. Sacrum - 5 fused vertebrae (in adults it’s one bone; separate in children)

5. Coccyx (tailbone) - most commonly 4 fused vertebrae

General structure of a vertebra

Two main parts:

1. Vertebral body (blue in diagram) - weight-bearing portion

2. Vertebral arch - forms the vertebral foramen (central hole) for the spinal cord

When vertebrae stack:

• The vertebral foramina align, forming the vertebral canal where the spinal cord passes

Bony landmarks of a vertebra

Vertebral body

• Large, anterior portion

Vertebral arch components:

• Spinous process: posterior, projects back (midline)

• Transverse processes (x2): Project laterally from the arch base

• Pedicles (x2): “Feet” of the arch connecting to the spinous process

• Laminae (x2): Upper parts of the arch connecting to the spinous process

• Articular processes (x4 total per vertebra):

  • Superior (2)

  • Inferior (2)

  • Allow articulation between vertebrae

Opening:

• Vertebral Foramen: Central hole for the spinal cord

Intervertebral discs

• Located between vertebral bodies (except between sacrum & coccyx)

Two parts:

1. Nucleus Pulposus:

• Inner, gelatinous core (pulp-like)

2. Annulus Fibrosus:

• Outer, tough cartilage in rings

Appendicular skeleton overview

The appendicular skeleton consists of four main parts:

1. Pectoral girdle

2. Upper limbs

3. Pelvic gridle

4. Lower limbs

• A girdle is a structure that encircles part of the body

Pectoral girdle

• Location: Encircles the upper part of the thorax (rib cage)

• Bones involved (2 bones per side):

  • Clavicle (collarbone): A Long bone running in the transverse plane. Connects:

    • Medially to the sternum

    • Laterally to the scapula

  • Scapula (shoulder blade): Broad, flat bone on the posterior side of the rib cage. Articulates with the humerus

Upper limb bones

• Connected to the pectoral girdle

a. Arm

• Humerus: The single long bone of the upper arm

b. Forearm

• Radius: Lateral bone (thumb side)

• Ulna: Medial bone (pinky side)

c. Wrist

• Carpal bones: 8 small bones grouped together

d. Hand

• Metacarpals: 5 bones forming the structure of the hand

• Phalanges: Bones of the fingers

  • Same name and spelling as the toe bones

Pelvic girdle

• Location: Encircles and supports the lower abdominal organs

• Primary bones:

  • Hip bones (also called pelvic bones):

    • Each side consists of 3 fused bones (in adults):

      1. Ilium: Superior part

      2. Ischium: Posterior part

      3. Pubis: Inferior anterior part

• Connections:

  • Posteriorly attached to the sacrum

  • Anteriorly joined to each other at the pubic symphysis

Lower limb bones

a. Thigh

• Femur: Single long bone in the upper leg

b. Leg (Below the Knee)

• Tibia: Larger, medial bone

  • Spelling: Tibia, not Tivia

• Fibula: Smaller, lateral bone

  • Common mistake: Fibia is incorrect; the correct spelling is Fibula

c. Kneecap

• Patella: Small, triangular bone at the front of the knee

d. Ankle and Foot

• Tarsal bones: Equivalent to wrist bones (carpals), 7 in total

• Metatarsals: Long bones in the middle of the foot

• Phalanges: Toe bones (same name as finger bones)

  • Tip: "T for Toes" can help remember the difference between carpal (upper limb) and tarsal (lower limb)

Summary of bone name comparisons

Upper Limb	Lower Limb
Clavicle	Pelvic bone
Scapula
Humerus	Femur
Radius & Ulna	Tibia & Fibula
Carpals	Tarsals
Metacarpals	Metatarsals
Phalanges	Phalanges
	Patella (knee)

Cartilage

• Like bone tissue, cartilage is a type of connective tissue. It is less rigid than bones and is mostly found in areas of the skeleton where more flexibility is required

• A good example of this is in the rib cage, which is required to constantly expand and contract as you breathe. Cartilage also forms a major part of many joints

• Cartilage can also be found in areas of the body that are not part of the skeletal system. The epiglottis is a good example of this. It is a plate of cartilage in the back of your throat that has roles consistent with both the digestive and respiratory systems

Types of cartilage

3 types of cartilage can be found in the human body:

• Hyaline cartilage - shiny, smooth, white type of cartilage that is the most abundant type of cartilage in the body. This type of cartilage is flexible but relatively weak

• Fibrocartilage - a very strong and rigid type of cartilage. The many thick collagen fibres present give it these properties

• Elastic cartilage - somewhat rigid, yet flexible and elastic. The elastic fibres present give it these properties

Joints

• Movement is permitted through the formation of joints. Joints, also known as articulations or arthroses, are points where two or more bones meet

• Although bones are a key component of joints, there are other contributing elements such as cartilage, ligaments and other membranes or tissues

• There are a few different types of joints in the body, each with a unique structural make-up that permits a specific function. Some allow a significant degree of movement, whereas others are strong structural unions between bones

joint classifications

• The functional classification relates to how much movement is available for the joint

• The structural classifications relative to whether or not a joint cavity (space) is present, and the type of tissue that binds the joints if there is no cavity

• The three structural classifications of joints are:

  • Fibrous joints

  • Cartilaginous joints

  • Synovial joints

fibrous joints

• Do not have a joint cavity. The bones that participate in a fibrous joint are held together by strong connective tissue elements that are rich in collagen fibres

• There are fibrous joint subtypes:

  • Sutures

  • Syndesmosis

  • Gomphosis joints

Cartilaginous joints

• Lack a joint cavity. Consists of bones (or parts of bones) that are united by a cartilage plate. There are two subtypes of cartilaginous joints:

  • Primary cartilaginous joints (also called synchondrosis joints)

  • Secondary cartilaginous joints (also called symphysis joints)

Synovial joints

• DO have a joint cavity and are the most movable type of joint. They contain several common features that allow for this free movement

• There are 6 subtypes of synovial joints:

  • Plane (also called gliding) joint

  • Hinge joint

  • Pivot joint

  • Ball and socket joint

  • Condyloid joint

  • Saddle joint

fibrous joints REVIEW

cartilaginous joints REVIEW

synovial joint features

• Composed of two (or sometimes more) bones that are covered in articular cartilage where they contact each other, a joint capsule which encloses the joint to create a joint cavity (space), synovial fluid which occupies the joint cavity and supporting ligaments

• Many synovial joints also have additional structures that aid in their stability and function

articular cartilage

• Made from hyaline cartilage. This smooth, glass cartilage reduces friction between the contact surfaces. It also acts as a shock absorber in some of the major weight-bearing joints of body

joint capsule

• Contains two layers: an inner synovial membrane and an outer fibrous capsule. The inner synovial membrane synthesises and secretes synovial fluid into the joint cavity

• The outer fibrous aspect is flexible yet strong, which allows movement in the joint while also giving it stability to prevent dislocation

• Collectively, the joint capsule also makes the joint cavity fluid-tight

joint cavity

• The space between the two bones (and associated articular cartilage) that is inside the joint capsule. It is filled with synovial fluid

synovial fluid

• A thin, slippery fluid that is similar in appearance and consistency to uncooked egg white

• It occupies the joint cavity and acts as a lubricant between the joint surfaces (surfaces of the articular cartilage)

• The primary purpose of the synovial fluid is to reduce friction between the articulating surfaces; however, it also nourishes the articular cartilage

supporting ligaments

• The primary role of these connective tissue structures is to support the stability of the joint and reduce the likelihood of dislocation

• Ligaments can be a thickened part of the fibrous joint capsule (capsular ligaments), or they can be their entity, either located inside the joint cavity (intracapsular) or completely outside the joint (extracapsular)

additional synovial joint features

• Some also contain labrum, meniscus or bursa

Ligaments can be:

• Intracapsular - meaning they are inside the fibrous joint capsule (however outside the synovial membrane)

• Capsular - meaning they are thickened parts of the fibrous joint capsule

• Extracapsular - meaning they are completely outside the joint and have no attachment to the fibrous capsule

Axial skeleton joints

• Intervertebral joints - symphysis joint

• Facet joints

• Temporomandibular joints

pectoral girdle & upper limb joints

• Sternoclavicular joint

• Acromioclavicular joint

• Glenohumeral joint

• Radiohumeral joint

• Humeroulnar joint

• Proximal radioulnar joint

Pelvic girdle & lower limb

• Sacroiliac joint - Compound joint: synovial and syndesmosis  

• Pubic symphysis joint - Symphysis joint

• Hip joint (also called the femoroacetabular joint)

• Tibiofemoral joint

• Patellofemoral joint

• Proximal tibiofibular joint

• Talocrural joint

• Subtalar joint

SHOULDER

• Highlight the clavicle bone and on the model's right hand side

Q1 - Which two other bones does the clavicle form a joint with?

Q2 - What are the names of these joints?

Q3 - You have just listed two of the three shoulder joints. What is the name of the third shoulder joint (between the scapula and the humerus)?

Q1 - Scapula & Sternum (manubrium is the specific part of the sternum)

Q2 - Acromioclavicular joint & Sternoclavicular joint

Q3 - Glenohumeral joint

SPINE

• Select the L3 vertebra of the spine. A tab will appear on the left.

• Tap the 'Parts' icon (next to the highlighted 'Info' icon.

• Use the aspects to learn the locations of the following bony landmarks of the vertebrae:

  • Vertebral body

  • Transverse processes

  • Spinous process

  • Pedicles

  • Lamina

  • Superior articular processes

  • Inferior articular processes

  • Vertebral foramen

Q4 - Which above bony landmark participates in the intervertebral joint?

Q5 - What type of joint is the intervertebral joint?

Q6 - What are the names of the two different parts of the intervertebral disc? 

• Tap 'Back' (top left corner of screen)

• Rotate the model and zoom to identify the facet joint between the L3 vertebra and the L2 vertebra on the left side.

• This is a plane/gliding type of synovial joint. Can you see how the joint surfaces are two flat areas in contact with each other?

Q4 - Vertebral body

Q5 - Symphysis, also known as a secondary cartilaginous joint

Q6 - Annulus fibrosus & Nucleus Pulposus 

ELBOW

• The three joints around the elbow are the radiohumeral joint (between the humerus and the radius), the humeroulnar joint (between the humerus and the ulna), and the superior radioulnar joint (between the proximal aspects of the radius and ulna). 

Q7 - Which of these two joints contributes to the hinge joint of the elbow?

• Try and appreciate that the ulna hooks onto the humerus whereas the radius just sits below it

• Identify the left superior radioulnar joint (also called the proximal radioulnar joint)

• Tap the search button near the top right of screen. Search for 'annular ligament of radius (left)' and tap on this to add it to the model.  

Q8 - Which of the three elbow joints is a pivot type of synovial joint?

• Can you see how the annular ligament that you added circles around the proximal radius? This essentially forms most of the ring that makes this joint a pivot joint. The radius then spins within this ring as you twist your forearm.

Q7 - Radiohumeral joint (between the humerus and the radius) & Humeroulnar joint (between the humerus and the ulna)

Q8 - Superior radioulnar joint (also called the proximal radioulnar joint)

HIP

• Tap on the 'Connective T.' icon at the bottom of the screen and add up to layer 3.

• Locate the right hip joint

Q9 - What type of joint is the hip joint?

• Tap on the fibrous layer of the joint capsule of the hip to select it. Remove this structure by tapping 'HIDE' (this is near the top of the info box that appears when you select the structure).

• Underneath the fibrous layer of the joint capsule is the synovial membrane layer of the joint capsule.

Q10 - What is the role of the synovial membrane?

Q9 - Synovial, ball & socket joint

Q10 - The synovial membrane synthesises and secretes synovial fluid into the joint cavity. This fluid lubricates the joint surfaces to reduce friction when the joint moves.

KNEE

• Remove all of the Connective Tissue layers

• Locate the left knee joint

Q11 - What are the names of the two joints that are part of the modified hinge joint of the knee?

• Identify these two joints

Q12 - What is the name of the other joint at the knee region?

Q11 - Tibiofemoral joint, Patellofemoral joint

Q12 - Superior tibiofibular joint (also known as the proximal tibiofibular joint)

Flexion & Extension

• Flexion = movement that results in decreasing the angle between 2 body parts

• Extension = movement that results in increasing the angle between 2 body parts

Medial/Internal rotation & lateral/external rotation

• Medial rotation = anterior aspect rotating towards the midline

• Lateral rotation = anterior aspect rotating away from the midline

Abduction & Adduction

• Abduction = distal aspect moving away from the midline

• Adduction = distal aspect moving towards the midline

Dorsi & plantar flexion

Movement Term	Area term applies	Image	Description of movement
Dorsi flexion	Ankle		Dorsum of the foot moving superiorly
Plantar flexion	Ankle		Plantar aspect of the foot moving inferiorly

Inversion

Movement Term	Area term applies	Image	Description of movement
Inversion	Foot		The sole is moved towards the midline (inwards)

Eversion

Movement Term	Area term applies	Image	Description of movement
Eversion	Foot		The sole is moved away from the midline (outwards)

Elevation

Jaw and shoulder

The body part moves vertically superiorly

Depression

Jaw and shoulder

The body part moves vertically inferiorly

Protraction

Jaw and shoulder

Body part moves horizontally anteriorly

Retraction

Jaw and shoulder

Body part moves horizontally posteriorly

Lateral flexion (also called side bending)

Trunk

Movement of the trunk towards one side

Supination & Pronation

Supination	Forearm/hand		The forearm/hand moves to face anteriorly
Pronation	Forearm/hand		The forearm or hand moves to face posteriorly

Opposition

Thumb and little finger

Specialised movement where the pad of the thumb moves towards the the pad of another finger to meet it (complex and important movement!)

functions of the muscular system - producing body movements

• When skeletal muscles activate, they generate a force that pulls on bones, creating the capacity to move joints

• The picture shows the biceps brachii muscle contracting to pull on the radius and flex the elbow joint

Functions of the muscular system - stabilising body positions

• Skeletal muscle contractions can also stabilise joints. For example, when you are standing up, gravity is constantly trying to collapse you

• Many of your skeletal muscles are resisting the force of gravity to stabilise your posture and keep you upright

• The picture shows an example where this person’s muscles are activating to stabilise her in a pose

functions of the muscular system - storing substances in the body

• Muscles often work to close off the opening of an organ to ensure substances are temporarily stored in this organ until it is time for them to be released

• For example, the picture shows the bladder and highlights its sphincter. The urethral sphincter is a ring of muscle that surrounds one of the openings of your bladder

• When the muscle is contracted, this opening is closed off, and urine is stored in the bladder

• When this muscle relaxes, the urine flows through the opening and is excreted from your body. Usually, it is smooth muscle tissue that is involved in this function

functions of the muscular system - moving substances around the body

• The cardiac muscle of the heart, which pumps the blood around the body, is a great example of this

• The heart muscle contracts, which forces blood through the body’s vessels

• Another example is the smooth muscle in the wall of the intestines, which helps propel food through them

functions of the muscular system - generating heat

• Some of the chemical reactions involved in the overall muscle contraction process release heat as a by-product

• This is why you get hot when you exercise, or why your body automatically decides to start shivering when you get cold

common muscle tissue properties

1. Electrically excitable - like neurons, the plasma membrane of muscle cells can establish an action potential that can move across the entire plasma membrane of that cell

2. Contractility - once excited, muscle cells can generate force by significantly altering their size and shape

3. Extensibility - muscle tissue can be stretched (to a point) without being damaged, sort of like an elastic band

4. Elasticity - the muscle tissue springs back to its original length once the force that stretches the muscle tissue is removed. Again, think of an elastic band springing back to its original shape after it has been stretched

muscle tissue comparsion

Muscle Tissue	Appearance	Description	Voluntary/Involuntary
Skeletal		Cells and tissues have a striated appearance Multiple nuclei per cell are located near the edge of the cell Cylindrical-shaped cells Can be very long cells - the cells can be 40cm long	Voluntary - the activation and relaxation of skeletal muscle tissue is under conscious control
Cardiac		Cells have a striated appearance Branched cells that usually have one central nucleus	Involuntary - you cannot choose to contract or relax your heart wall
Smooth		NO straited appearance, hence the name 'smooth' muscle Single central nucleus Thin, spindle-shaped cells	Involuntary - you cannot choose to contract or relax smooth muscles

gross anatomy of skeletal muscles

• The two main parts of a skeletal muscle are the muscle belly and the tendons. The muscle belly contains the muscle fibres, which are the cells capable of activating

• The tendons are connective tissue structures that attach the muscle belly to a bone. Tendons achieve this by blending with the periosteum of a bone

• The muscle belly and its fibres are arranged into structures called fascicles. This arrangement is achieved through 3 layers of connective tissue sheaths called the:

  • Epimysium - surrounds the whole muscle

  • Perimysium - surrounds a fascicle

  • Endomysium - surrounds each muscle fibre

• These connective tissue sheaths converge to form the tendon at either end of the muscle belly

skeletal muscle fibres (cells)

• The functional unit of a skeletal muscle. They are long cells (hence the name ‘fibre’) that can generate a pulling force when excited by a motor neuron

• Skeletal muscle fibres are multinucleated cells with lots of mitochondria

• They have many unique components, with each of these unique components having an important function:

  • Sarcolemma - the plasma membrane

  • Transverse tubules - tube-like extensions of the plasma membrane that penetrate through the fibre

  • Sacroplasm - the cytoplasm of the muscle fibre

  • Sacroplasmic reticulum - fluid-filled membranous sacs that store calcium ions (Ca²⁺)

  • Terminal cisterns - enlarged parts of the sarcoplasmic reticulum that are nested against the transverse tubules

  • Triad - a transverse tubule with a terminal cistern on either side

  • Myofibrils - microfilament components that move to generate the pulling force by changing the shape of a muscle fibre

sarcomeres

• Each cylindrical myofibril contains lots of small protein filaments, with the two primary filaments being the thick filament and the thin filament

• These filaments (and other myofibril elements) are arranged into repeating subunits called sarcomeres

• It is the overlapping and non-overlapping parts of these filaments within a sarcomere that create the striated appearance of skeletal muscle tissue

• Each sarcomere is separated from its neighbours by narrow plates of dense protein material called Z-discs

• There is another protein plate in the centre of the sarcomere called the M-line

• The thin filaments are attached to the Z-disc, and the thick filaments are attached to the M-line

sarcomere - thick filament

• Primarily composed of a long contractile proteins called myosin molecules

• Myosin is the structure with the ability to convert energy liberated from ATP hydrolysis into mechanical energy - it is the actual structure that generates the force when a muscle activates

• Each myosin molecule contains 2 myosin tails which are twisted together. A head then extends outwards from the molecule (towards an adjacent thin filament)

sarcomere - thin filament

• Has a contractile protein component, which is called actin

• However, the thin filament also contains other parts we need to know about, including the troponin complex and tropomyosin

sarcomere - thin & thick filament REVIEW

Skeletal muscle activation

• Muscles can activate to move joints or stabilise joints. To achieve these functions, a muscle activates (contracts) in one of three ways:

  • Isometric activation - the muscle stays the same length as it activates

  • Isotonic concentric activation - the muscle shortens as it is activated

  • Isotonic eccentric activation - the muscle lengthens as it is activated

neuromuscular junction

• The point where a neuron meets a muscle cell. At this junction, an action potential is essentially transferred from a motor neuron to a muscle fibre, electrically exciting it

• Neuromuscular junction components involved include:

  • Somatic motor neuron

  • Axon terminals

  • Action potential

  • Motor end plate

  • Synaptic cleft

  • Voltage-gated calcium channels

  • Acetylcholine vesicles

  • Ligand-gated cation channels

process at the neuromuscular junction

1. Action Potential Arrival: An action potential travels down the motor neuron to the axon terminal

2. Calcium Channel Activation: Voltage-gated calcium channels open, allowing calcium ions to enter the axon terminal

3. Acetylcholine Release: Calcium entry triggers synaptic vesicles to release acetylcholine (ACh) via exocytosis

4. ACh Binding: ACh diffuses across the synaptic cleft and binds to receptors on the muscle fibre’s motor end plate

5. Channel Opening: ACh receptor binding opens ligand-gated cation channels

6. Ion Movement: Sodium enters and potassium exits the muscle fibre, leading to membrane depolarisation

7. Muscle Action Potential: If the threshold is reached, an action potential propagates along the sarcolemma (muscle membrane)

8. Signal Termination: ACh is removed from the synapse by diffusion and breakdown via acetylcholinesterase; choline is recycled

neuromuscular junction process Qs

1. Which ion enters the muscle fibre in great quantities during this process?

Sodium

 

2. The action potential in the axon terminal of the motor neuron triggers the _______ gated _______ channels to open.

voltage, calcium

 

3. True or False - Acetylcholine exits the axon terminal via a process called exocytosis. This is triggered by calcium ions attaching to the acetylcholine vesicles in the axon terminal.

True

 

4. What is the name of the enzyme that breaks down acetylcholine once it is in the synaptic gap?

Acetylcholinesterase

excitation contraction coupling

• We learned that an action potential is eventually simulated on the plasma membrane (sarcolemma) of the skeletal muscle fibre

• Now, we will learn how that action potential spreads across the plasma membrane and what impact it ends up having on the cell

• This process looks at how the initial excitation of the muscle fibre (which occurred at the neuromuscular junction) ends up causing the contraction of the muscle fibre

  1. Action potentials occur on a plasma membrane and move across this plasma membrane once they are initiated

  2. Transverse tubules are tube-like extensions of the sarcolemma (plasma membrane of the muscle fibre) that penetrate through the fibre

  3. The inside of a transverse tubule is filled with extracellular fluid

  4. Calcium ions are stored in the sarcoplasmic reticulum of a muscle fibre

process of excitation contraction coupling

• There is only one step in this process, which is that the action potential travelling down a transverse tubule causes a voltage-sensitive protein to change shape, opening a calcium release channel in the sarcoplasmic reticulum, which allows lots of Ca²⁺ to enter the sarcoplasm

sliding filament mechanism

• Up until this point, the activation process for all types of contraction (isometric, concentric and eccentric) is the same. However, once excitation-contraction coupling occurs, the process for each contraction type changes slightly. We are just going to look at the mechanisms that occur in a muscle fibre during an isotonic concentric contraction. This mechanism is called the sliding filament mechanism

• The sliding filament mechanism involves the thin filaments sliding along the thick filaments. This occurs because the myosin heads of the thick filaments 'walk' along the thin filaments, pulling them closer to the centre of the sarcomere. This process causes the Z-discs to be pulled towards the M-line, shortening the overall length of the sarcomere

cross bridge cycle

Each 'step' a myosin head takes as it 'walks' along the thin filament is known as a cross bridge cycle. This is because the myosin head forms a bridge between the two filaments with each of these 'steps'. In order to understand this cycle we need to remind ourselves of ... fundamental pieces of information:

1. The thin filament contains actin, troponin complexes and tropomyosin

2. Ca²⁺ ions that have been released from the sarcoplasmic reticulum during excitation contraction coupling bind to troponin complexes

3. Actin monomers have a myosin binding site on them which is covered by tropomyosin when the muscle fibre is at rest

4. ATP hydrolysis is a reaction that releases energy which the body can use

steps occurring in the cross bridge cycle - preparation phase

Preparation Phase (Before the Cycle Begins):

• Calcium Release: Calcium ions are released from the sarcoplasmic reticulum

• Troponin Activation: Calcium binds to troponin, causing a shape change

• Tropomyosin Movement: This shifts tropomyosin, exposing myosin-binding sites on actin

• Myosin Head Activation: ATP binds to the myosin head, is hydrolyzed to ADP + Pi, energizing (cocking) the head

steps occurring in the cross bridge cycle - main steps

Cross Bridge Cycle (4 Main Steps):

1. Cross Bridge Formation

   • Activated myosin head binds to actin.

   • Inorganic phosphate (Pi) is released, strengthening the bond

2. Power Stroke

   • ADP is released

   • Myosin head pivots, pulling the actin filament toward the sarcomere centre

3. Cross Bridge Detachment

   • A new ATP binds to the myosin head

   • This causes the myosin head to detach from actin

4. Reactivation of Myosin Head

   • ATP is hydrolysed again to ADP + Pi

   • Energy from hydrolysis re-cocks the myosin head

steps occurring in the cross bridge cycle - cycle continuation/end

Cycle Continuation or End:

• As long as calcium remains and ATP is available, the cycle repeats

• Muscle contracts as sarcomeres shorten through repeated cycling

• When calcium is actively pumped back into the sarcoplasmic reticulum:

  • Troponin returns to its original shape

  • Tropomyosin covers the binding sites on actin

  • Cross-bridge cycling stops, and the muscle relaxes

Which which part of the cross bridge cycle process actually uses the energy liberated by ATP hydrolysis?

• Cocking of the myosin head

• It is NOT the power stroke that uses the energy

True or False? Each cross bridge cycle results in the two M-lines being pulled towards the central Z-disc

• False - it is the Z-discs that are pulled towards the centre of the sarcomere which is where the M-line is

• This occurs because the Z-discs are attached to the actin filaments which are the elements being pulled in each cross bridge cycle

Which part of the myofibril do the calcium ions attach to?

The troponin complexes

What is meant by the term 'cross bridge'?

• A cross bridge means a bridge being formed between the thick and thin filaments

• That is, these two elements are normally not attached however they become attached momentarily when this cross bridge forms

motor units

• Defined as a somatic motor neuron with all the muscle fibres it supplies

• A motor unit consists of:

  • One motor neuron (originating in the spinal cord)

  • All the muscle fibres (cells) that it innervates

  So, a single motor neuron may control several muscle fibres, but each muscle fibre is only innervated by one motor neuron