ANAT Week 4.docx

Week 4 **Module: Muscle Tissue** ========================= **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the primary germ layer that muscle tissue is derived from **LO2**: Describe the five properties of muscle tissue **LO3**: Descr...

Week 4 **Module: Muscle Tissue** ========================= **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the primary germ layer that muscle tissue is derived from **LO2**: Describe the five properties of muscle tissue **LO3**: Describe and compare the three different types of muscle tissue **LO4**: Describe the functions and organisation of skeletal muscle **LO5**: Describe the blood supply and innervation of skeletal muscle **Origin of Muscle Tissue** --------------------------- ***LO1: Name the primary germ layer that muscle tissue is derived from*** Recall from the Cells and Basic Tissues module that muscle tissue is one of the four basic tissue types in the body. Recall from the Embryology module that all tissues in the body are derived from the three primary germ layers of the embryo, with muscle tissue being derived from the mesoderm. **Properties of Muscle Tissue** ------------------------------- ***LO2: Describe the five properties of muscle tissue*** Muscle tissue has five properties, which are excitability, conductivity, contractility, extensibility and elasticity. Click on the hotspots below to learn more about these properties. **Types of Muscle Tissue** -------------------------- ***LO3: Describe and compare the three different types of muscle tissue*** Now that we have looked at the properties of muscle tissue in general, we are going to look at the different types of muscle tissue. There are three types of muscle tissue, which are skeletal muscle, cardiac muscle and smooth muscle. Click on the hotspots on the image below to find out where these three types of muscle tissue can be found in the body. Now we are going to look at the three types of muscle tissue in more detail. **Skeletal Muscle** ------------------- ***LO3: Describe and compare the three different types of muscle tissue*** ***LO4: Describe the functions and organisation of skeletal muscle*** The first type of muscle tissue is skeletal muscle, which is found in the muscles that attach to bones. It is voluntary, meaning that it is under our conscious control. It is also striated, meaning that it has a striped appearance due to the organisation of the contractile proteins in the muscle cells. Skeletal muscle has several functions, which are body movement, maintenance of posture, protection and support, regulation of the elimination of materials and heat production. Click on the hotspots below to learn more about these functions. Skeletal muscle has a highly organised structure that is supported by connective tissue. A whole skeletal muscle is surrounded by a layer of dense irregular connective tissue called epimysium (*'epi'* = above or upon). This often blends with an additional external layer of dense irregular connective tissue called deep fascia, which separates individual skeletal muscles and binds muscles with similar functions together. The skeletal muscle itself is composed of many fascicles, which are surrounded by a layer of dense irregular connective tissue called perimysium (*'peri'* = around). A fascicle is a bundle of skeletal muscle cells, which are known as muscle fibers or myofibers (*'myo'* = muscle) due to their long, cylindrical shape. Myofibers are surrounded by a layer of areolar connective tissue called endomysium (*'endo'* = inside), which contains reticular fibers to help bind neighbouring myofibers together and support surrounding capillaries. **Skeletal Myofibre Structure** ------------------------------- ***LO3: Describe and compare the three different types of muscle tissue*** ***LO4: Describe the functions and organisation of skeletal muscle*** Now that we have looked at the general organisation of skeletal muscle, we are going to look more closely at a skeletal muscle cell, or myofiber. A myofiber is a long, cylindrical cell with multiple peripherally located nuclei. It is composed of many myofibrils, which are the contractile elements of a myofiber. A myofibril is composed of bundles of myofilaments, which are contractile proteins. There are two types of myofilaments -- thick filaments composed of myosin molecules, and thin filaments composed of actin molecules. These myofilaments are arranged in repetitive units called sarcomeres, which are the basic functional contractile units of a myofiber. **Sarcomere** ------------- ***LO3: Describe and compare the three different types of muscle tissue*** ***LO4: Describe the functions and organisation of skeletal muscle*** Now we are going to have a closer look at a sarcomere. A sarcomere is composed of overlapping thick and thin filaments and has several regions. It is this arrangement of thick and thin filaments to form these regions that gives skeletal muscle its striated appearance. **Blood Supply and Innervation of Skeletal Muscle** --------------------------------------------------- ***LO5: Describe the blood supply and innervation of skeletal muscle*** Now we are going to look at the blood supply and innervation, or nerve supply, of skeletal muscle. There is an extensive network of blood vessels and nerves travelling through the epimysium and perimysium connective tissue layers of skeletal muscles. These structures travel together in groups called neurovascular bundles. When we look specifically at the innervation, or nerve supply, of skeletal muscle, we can understand why skeletal muscle is voluntary. Skeletal muscles are innervated by the somatic nervous system, which is the voluntary component of the peripheral nervous system. The word "soma" means "body", so the somatic nervous system innervates structures of the body wall, including skeletal muscles. The motor neurons, or nerve cells, that innervate skeletal muscles have long process called axons that terminate on individual myofibers. The junction between an axon and a myofiber is called a neuromuscular junction. At a neuromuscular junction, chemicals called neurotransmitters are released from the axon terminals of the motor neuron and these transmit the electrical impulse from the neuron to the myofiber, thus stimulating it. The property of excitability exhibited by muscle tissue allows the myofiber to respond to this stimulus by generating its own electrical impulse and the property of conductivity allows the plasma membrane, or sarcolemma, of the myofiber to conduct this electrical impulse, ultimately leading to skeletal muscle contraction. **Cardiac Muscle** ------------------ ***LO3: Describe and compare the three different types of muscle tissue*** The second type of muscle tissue is cardiac muscle, which is only found in the heart wall, specifically in the layer called the myocardium (*'myo'* = muscle; *'cardium'* = heart). It is involuntary, meaning that it is not under our conscious control. It is also straited, meaning that it has a striped appearance due to the organisation of the contractile proteins in the muscle cells. Now we are going to look more closely at the features of cardiac muscle. A cardiac muscle cell is a short, thick cell with one or two centrally located nuclei. Like skeletal muscle cells, cardiac muscle cells are surrounded by a layer of areolar connective tissue called endomysium, but cardiac muscle does not have the perimysium and epimysium connective tissue layers that are seen in skeletal muscle. However, like skeletal muscle, cardiac muscle is striated due to the arrangement of the contractile proteins in sarcomeres. A unique feature of cardiac muscle cells is that they are branched and they are joined to adjacent cardiac muscle cells at junctions called intercalated discs. These are composed of desmosomes and gap junctions. Desmosomes are a type of cell junction that provide strong adhesion between cells, which is important in cardiac muscle as it allows multiple cardiac muscle cells to act together as one unit. Gap junctions are a type of cell junction that connect the cytoplasm of adjacent cells together via a small tunnel or pore, allowing small molecules to travel between them. This is important in cardiac muscle, as it allows for fast transmission of electrical impulses between cardiac muscle cells to coordinate cardiac muscle contraction **Smooth Muscle** ----------------- ***LO3: Describe and compare the three different types of muscle tissue*** The third and final type of muscle tissue is smooth muscle, which is found lining the walls of blood vessels and hollow organs, as well as in the eye. It is involuntary, meaning that it is not under our conscious control. It does not have striations, as the contractile proteins in the muscle cells are not precisely organised. Now we are going to look more closely at the features of smooth muscle. A smooth muscle cell is a short, fusiform-shaped cell with a single, cigar-shaped, centrally located nucleus. Like both skeletal and cardiac muscle cells, smooth muscle cells are surrounded by a layer of areolar connective tissue called endomysium and like cardiac muscle, smooth muscle does not have the perimysium and epimysium connective tissue layers that are seen in skeletal muscle. However, unlike both skeletal and cardiac muscle, smooth muscle does not have striations as the contractile proteins are not arranged in sarcomeres. **Comparison of Muscle Tissue Types** ------------------------------------- ***LO3: Describe and compare the three different types of muscle tissue*** Now that we have looked at all three types of muscle tissue, here is a summary table comparing the main features of each that we have covered in this module. **Module: Muscular System 1 - Introduction** ============================================ **Learning Outcomes** --------------------- By the end of this module you should be able to: **LO1**: Describe the functions of the muscular body system **LO2**: Describe the gross anatomy of a skeletal muscle **LO3**: Define the motor unit **LO4**: Define the origin and insertion of a skeletal muscle **LO5**: Define the types of muscle contraction and muscle actions **LO6**: Explain the criteria used in naming skeletal muscles **Muscular System Functions** ----------------------------- ***LO1: Describe the functions of the muscular body system*** Skeletal muscle is striated, voluntary muscle that makes up the gross skeletal muscles of the body. The muscular body system is composed of over 700 voluntary skeletal muscles that produce movements and stabilise bone, joints and other structures. In addition, there are many other functions of skeletal muscles and special characteristics of muscle tissue. Click on the hotspots below to learn more about muscle functions (please note that these are also covered in the Muscle Tissue module). **Skeletal Muscles** -------------------- ***LO2: Describe the gross anatomy of a skeletal muscle*** ### **Structural Organisation of a Skeletal Muscle** Skeletal muscles are considered **organ**s because they are composed not only of muscle tissue but also epithelial, connective and nervous tissue. From the gross anatomy point of view, a typical skeletal muscle has a muscle **belly** (contractile portion) and **tendons** that attach it to the bone at points that are called attachments, or origins and insertions. The most common appearance of muscle tendons is rope-like, being somewhat round or oval in cross-section. However, tendons of some muscles (usually flat muscles) form **flat tendons** that are called '**aponeuroses**', for example the aponeurosis of the external abdominal oblique muscle shown on the image below. Click on the hotspots below to learn more about muscle tendons and aponeuroses. **Tendon** Typically, a tendon is a fibrous connective tissue band that attaches a muscle to a bone. However, tendons can also attach muscles to other structures such as cartilage (e.g. laryngeal muscles), fascia (e.g. abdominal muscles), organs (e.g. extrinsic muscle of the eye), skin (e.g. muscles of facial expression) and even mucous membranes (tongue). The tendon serves as a \"mechanical bridge\" that allows the transmission of muscle strength to the bones and joints. Therefore, it enables the contraction of the muscle to make the tangible movement of a bone or a structure. Tendons are stiffer than muscles, have greater tensile strength, and can withstand very large loads with minimal deformations. Tendon tissue is not just about the initial or terminal area of each muscle, but involves the entire muscular tissue. The connective tissue layers surrounding the muscle (epimysium, perimysium and endomysium) merge into a single organisation to contact one or more fixed osseous (bony) points. Please note that in contrast to a tendon, a ligament is a fibrous connective tissue band that attaches bone to bone, and thus, usually serves to hold structures together and keep them stable. **Aponeurosis** An aponeurosis is a flattened tendon by which muscle attaches to a bone or fascia. This type of muscle tendon is found in flat-shaped muscles such as the anterolateral abdominal muscles (external abdominal oblique, internal abdominal oblique, transversus abdominis) and on the surface of some pennate muscles. Like a tendon, an aponeurosis transmits forces and length changes from the muscle fascicles to the skeleton. In addition to transmitting these forces, aponeuroses play a role in modulating the direction of muscle shape changes during contraction. ### **Fascia** Please recall from the Muscle Tissue module that each skeletal muscle is surrounded by a connective tissue sheath called the **epimysium**. **Fascia** is a thin layer of connective tissue outside the epimysium that surrounds individual muscles. However, fascia is **a lot more than a simple covering of a muscle**: - - - - - Fascia can be classified as **superficial** and **deep**. The **superficial fascia** is found directly under the skin and within the superficial adipose layers. Located deep to the superficial fascia is the **deep fascia** that surrounds and subdivides the muscular compartments. On the image below, identify the fascial layers on a cross-section through the arm. ### **Patterns of Fascicle Arrangement in Muscles** Please also recall from the Muscle Tissue module that the striated muscle belly of a skeletal muscle has a highly organised internal structure. Skeletal muscle cells are called **muscle fibers** because they are long and narrow. Muscle fibers are organised into bundles or **fascicles**, which are large enough to be seen by the naked eye. In different skeletal muscles, fascicles are aligned in different patterns. **Fascicle arrangement** and the number of fascicles affect range of motion and muscle power. For example, multipennate muscles shorten very little but tend to be very powerful, while muscles with a parallel pattern produce a larger distance of movement but are typically not the most powerful. Now, let's look at **different fascicle arrangements** found in skeletal muscles. Click on the hotspots on the image below to learn more about common patterns of fascicle arrangement in skeletal muscles. **Circular** Muscles with a circular pattern of fascicle arrangement usually surround external body openings, which they close by contracting. Therefore, they act as sphincters, e.g. the orbicularis oris muscle ('orbicularis' = circular; 'oris' = mouth). **Convergent** In the convergent pattern of fascicle arrangement, the origin of the muscle is broad and fascicles converge towards the insertion. An example is the pectoralis major muscle ('pectoralis' = belonging to the chest). **Parallel** In the parallel pattern of fascicle arrangement, the long axes of the fascicles are parallel to the long axis of the muscle. The muscle fibers extend all the way from the origin to the insertion. Muscles with this arrangement can be either fusiform (e.g. the biceps brachii muscle) or strap-like (e.g. the sartorius muscle). Now that we have looked at the gross anatomy of a skeletal muscle, let's look at how skeletal muscles work and what is the functional motor unit. **Motor Unit** -------------- ***LO3: Define the motor unit*** Every skeletal muscle is richly supplied by blood vessels for nourishment, oxygen delivery and waste removal. In addition, **every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron**, which signals the fiber to contract. Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system. We will learn more about the anatomy of the nervous system in later modules. For now, let's define the motor unit. **The functional unit of a muscle is called the motor unit.** It consists of a **motor neuron** and **all muscle fibres it controls**. An impulse from a motor neuron generates a simultaneous contraction of all muscle fibres supplied by the branching terminals of its axon. The **number of muscle fibres in a motor unit varies** from one to several hundred depending on how precise the produced movements are. Large motor units are found in trunk or thigh muscles, while small motor units are found in hand muscles. We will learn more about neurons in later modules. Now, let's look at how muscles attach and why it is important to learn muscle origins and insertions. **Origins and Insertions** -------------------------- ***LO4: Define the origin and insertion of a skeletal muscle*** Skeletal muscles function by contracting and by common sense, when a muscle contracts, it shortens. Muscles pull and never push. Each skeletal muscle has at least two attachments, called an origin and an insertion. The end of the muscle attached to the bone being pulled during muscular contraction is called the **insertion**, and the end of the muscle attached to a fixed, or stabilised, bone is called the **origin**. In the anatomical position (when the hands and feet are free to move), the **origin is usually the proximal attachment** and the **insertion is usually the distal attachment**. However, this is not always the case, as many muscles can act in both directions under different circumstances. For example, doing push-ups reverses the origin and insertion for upper limb muscles, as the movable part is the trunk rather than the upper limb. Since **humans are so dynamic outside of the anatomical position** and undertake a range of movements where our limbs are not fixed to the ground, the use of **proximal and distal attachment** is increasingly used. In summary, it\'s important to acknowledge that the both ends of a muscle can become the origin or insertion, or indeed both ends can become the insertion when the muscle contracts from both ends. Click on the hotspots on the image below to learn more about the definitions of the origin and insertion of a skeletal muscle, using the brachialis muscle as an example Upon activation, the muscle pulls the insertion towards the origin. When learning about muscle actions, it is important to pay attention to the disposition of the attachments regarding a given joint. **To act on a joint, the muscle must cross the joint plane** at a particular aspect. To provide a simplified example, the **brachialis** muscle originates at the anterior aspect of the humerus and inserts at the anterior aspect of the proximal ulna. Therefore, this muscle crosses the plane of the elbow joint anteriorly -- thus, it produces elbow flexion. The triceps brachii muscle crosses the elbow joint at the posterior aspect, thus extending it. Please note that muscles can have **more than one attachment**. For example, the **triceps brachii** has three proximal attachments: one at the scapula and two at the humerus. Its distal attachment is in the forearm, at the posterior aspect of the proximal ulna. Therefore, the muscle bulk crosses the elbow joint posteriorly and when the muscle contracts, it extends the elbow joint from the flexed position. Now, please watch the animations below demonstrating the actions of the brachialis and triceps brachii muscles. **Muscle Contraction and Actions** ---------------------------------- ***LO5: Define the types of muscle contraction and muscle actions*** ### **Types of Active Muscle Contraction** There are **two main types of active (phasic) muscle contraction**: - - **Isotonic** contraction occurs in **two ways**: concentric and eccentric. To illustrate the types of muscle contraction, let's use the biceps brachii muscle as an example. The two origins of this muscle are at the scapula and the insertion is in the forearm, at the proximal radius. One of main functions of this muscle is elbow flexion (i.e. same function as the brachialis muscle). Click on the hotspots on the tree diagram below to learn more about each type of muscle contraction. **Isotonic** Isotonic contraction produces movement that is accompanied by changes in muscle length. They are two types of isotonic contractions; concentric and eccentric **Concentric** **Eccentric** Most physical activities involve a combination of both isotonic and isometric muscle contraction, although one form usually predominates. Usually, the main muscle producing a particular movement (prime mover, or agonist) undergoes concentric contraction, while an antagonist muscle (a muscle that acts in the opposite way to the prime mover) simultaneously undergoes coordinated eccentric contraction. The result is a smooth, coordinated movement e.g. walking, bringing a glass of water to the mouth etc. Now, let's define the different types of muscle actions. ### **Different Types of Muscle Actions** Muscles serve specific functions in moving and positioning the body. Let's consider elbow movements as an example. As we already know, the muscles that cross this joint anteriorly, which are the biceps brachii and brachialis muscles, flex this joint. Opposite to this, the triceps brachii muscle crosses the elbow joint posteriorly, and therefore extends it. The principal muscle involved in a given joint action is called the prime mover, or agonist. During elbow flexion, for example lifting a cup, the brachialis muscle is the prime mover. Because it is assisted by the biceps brachii muscle, biceps brachii is called a synergist in elbow flexion. The triceps brachii muscle is an antagonist (works against the agonist) in elbow flexion. However, if we consider elbow extension, the triceps brachii muscle is the prime mover while the brachialis and biceps brachii muscles are antagonists in this movement. Please note that the biceps brachii muscle crosses more than one joint. In addition to the elbow joint, it also crosses the shoulder joint. Therefore, this muscle acts on both the elbow and shoulder joints. Click on the hotspots below to learn the definitions of the different muscle actions. **Prime Mover (Agonist)** A prime mover, or agonist, is the main muscle producing a specific movement at a given joint. It contracts concentrically. These usually produce no or little unwanted actions. For example, the brachialis muscle is the agonist in flexing the elbow joint in a neutral wrist position. Brachialis in this example can be called the primary agonist (prime mover). Some textbooks list both brachialis and biceps brachii as agonists in elbow flexion - in this case, brachialis is considered the primary agonist and biceps brachii is considered the assistant agonist. In some movements, gravity can play the role of the prime mover as well. **Synergist** A synergist compliments the action of an agonist. It may directly assist the prime mover or may assist indirectly. For example, the biceps brachii muscle assists the brachialis muscle in flexing the elbow joint. However, the biceps brachii have unwanted actions for our movement of interest. Biceps brachii also supinates the wrist and flex the shoulder with increasing recruitment. To prevent these unwanted movements from occurring, we need synergistic muscles that assist indirectly. For example, the pronator teres can prevent the supination keeping the wrist in a neutral position. Thus, this muscle can be called the indirect synergist because it faciliates the desired movement by preventing or cancelling out unwanted movements produced by the agonists. **Antagonist** An antagonist opposes the action of a prime mover and therefore undergoes eccentric contraction. For example, the triceps brachii muscle undergoes a controlled lengthening while the brachialis and biceps brachii muscles perform flexion of the elbow. **Fixator** A fixator steadies the proximal parts of a limb through isometric contraction while movements occur in the distal parts of the limb. For example, rotator cuff muscles steady the shoulder joint while the brachialis and biceps brachii muscles flex the elbow joint. ### **Muscle Names** ***LO6: Explain the criteria used in naming skeletal muscles*** Skeletal muscles are named according to several criteria, each of which are descriptive of a particular muscle based on location, shape, size and other characteristics. Being aware of this fact helps with learning muscles. Look at the diagram below to see some of these criteria and examples of how they are applied in naming muscles. Taking the time to learn the Latin and Greek roots of anatomical terms is crucial to understanding the vocabulary of anatomy. The best way to learn is not memorising muscle names mechanically, but instead understanding their meanings. For example, the adductor pollicis brevis muscle -- the name of this muscle can be translated as "a short muscle that moves the thumb towards the palm". The word 'adductor' has two Latin roots: 'ad' = to, and 'duct' = move; the word 'pollicis' means the thumb; and the word 'brevis' means 'short'. The latter also implies that there must be a long muscle ('longus') that adducts the thumb. The table below summarises the meanings of, and some mnemonics for, the common Latin/Greek roots of anatomical terms. **Module: Muscular System 2 - Axial Muscles** ============================================= **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the main groups of axial muscles **LO2**: Name representative muscles of the head, neck and trunk **LO3**: Explain muscle actions based on their attachments and locations **Axial Muscles** ----------------- ***LO1: Name the main groups of axial muscles*** In this module, we will study axial muscles. Please recall from the Skeletal System 1 module that the skeleton can be divided into two parts: the axial and appendicular skeleton. We will use these two skeleton subdivisions to break body muscles into two distinctive groups: the axial and appendicular muscles. The axial muscles have both their attachments located on different parts of the axial skeleton. Therefore, axial muscles support and move the axial skeleton. Depending on their locations, the axial muscles can be further divided into five main functional groups: As we are going to learn individual muscles, it is important that you use a functional anatomy approach rather than memorising muscles mechanically. Firstly, pay attention to the muscle group a given muscle belongs to, as muscles are grouped according to their common attachments, locations, functions, innervation and blood supply. Therefore, knowing the muscle group gives you a lot of useful information instantly. Secondly, pay attention to the name of a given muscle, as it is descriptive of some muscle features such as gross anatomy, attachments or functions. Identifying muscle attachments and location in relation to body joints provides a clear basis for the understanding (not memorising!) of muscle functions. Use atlas illustrations and anatomical software such as Complete Anatomy to locate muscles, followed by identifying these muscles on yourself. When using anatomical software such as Complete Anatomy, please note that muscles are often are arranged in layers and some muscles are only visible when more superficial muscles are removed. In this unit, we pay significant attention to the understanding of muscle groups, and we will locate and identify some representative muscles in each group. Before we move forward, let's define **the anatomical term 'trunk'**. The trunk or torso refers to the central part of the human body, which includes all body regions except the head, neck and limbs. Therefore, the trunk includes the thorax, abdomen, pelvis and back. Each of these regions has muscles that belong to the axial group. Now, let's describe each muscle group, name some representative muscles and discuss their functions. **Muscles of the Head and Neck** -------------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The muscles of the head and neck can be divided into several groups based on their locations and general actions: Most of these muscles have their attachments on the skull and the hyoid bone. We will start with the muscles of facial expression. ### **Muscles of Facial Expression** The muscles of facial expression attach to bone or fascia and produce effects by pulling the skin. These muscles are located within the subcutaneous space. They convey facial expressions to indicate mood and they also close openings such as the eyes and the mouth. Click on the hotspot next to each number on the image below to learn more about some major muscles of facial expression. The muscles of facial expression are all innervated by **cranial nerve number VII, the facial nerve**. Damage to the facial nerve on its way from the brainstem to the muscles it innervates is called Bell's palsy. It produces paralysis of the facial muscles on the same side as the damage. You may like to watch the short video below to learn more about Bell's palsy (please note that this is for interest only). To consolidate your understanding of the muscles of facial expression, have a go at matching each picture below demonstrating an action of a muscle of facial expression with the corresponding muscle. **Extrinsic Eye Muscles** ------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** Six strap-like extrinsic eye muscles control the movement of each eyeball. There are four 'rectus' muscles (labelled 1 to 4 on the image below) and two 'oblique' muscles (labelled 5 and 6 on the image below). Click on the hotspot next to each number on the image below to learn the name of each muscle. Please note that the rectus muscles run straight (rectus = straight), while the oblique muscles have an oblique course in the orbit. Most of these muscles originate from a structure called the common tendinous ring, which is located around the apex of the orbit. The extrinsic eye muscles insert into the eyeball, therefore allowing the eyes to follow a moving object and providing external support to maintain the eyeball shape and its position within the orbit. The extrinsic eye muscles are among the most precisely and rapidly controlled skeletal muscles in the entire body. These muscles are innervated by **three cranial nerves: numbers III (oculomotor nerve), IV (trochlear nerve) and VI (abducens nerve)**. You are not required to learn the precise innervation of each muscle. **Muscles of Mastication** -------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The muscles of mastication facilitate movement of the mandible to bring the maxillary and mandibular teeth into occlusion (contact) for the mechanical breakdown of food. To enable this, these muscles act on the temporomandibular joint -- a modified synovial hinge joint between the head of the mandible and the temporal bone (mandibular fossa and articular tubercle). This muscle group includes four pairs of muscles: temporalis (1), masseter (2) and two pterygoid muscles (3). Click on the hotspot next to each number on the image below to learn more about these muscles. The muscles of mastication are innervated by **cranial nerve number V, the trigeminal nerve**. Please note that some of the muscles of facial expression also assist in mastication. For example, the buccinator helps in chewing and the positioning of the food bolus inside the oral cavity. Please watch the two animations below demonstrating the primary muscles of mastication in action. **Muscles Moving the Tongue** ----------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The tongue is composed of 'intrinsic' muscle fibres that curl, squeeze and fold the tongue during speaking and chewing. These muscles change the shape of the tongue. The movement of the tongue is facilitated by the 'extrinsic' muscles of the tongue that anchor it to various surrounding bony and soft tissue structures, such as the hyoid bone, mandible, skull base, soft palate and pharynx. Most of the extrinsic tongue muscles are innervated by **cranial nerve number XII, the hypoglossal nerve** (glossal = tongue). **Muscles of the Pharynx** -------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** During swallowing, the buccinator muscle and tongue squeeze the food back from the oral cavity via the soft palate into the pharynx. We will discuss this process when we study the digestive system in later modules. For now, please note that the pharynx is a muscular tube located posterior to the nasal and oral cavities and the larynx. Therefore, the pharynx helps you to breathe as well as ingest food. The food is propelled through the pharynx into the oesophagus inferiorly by muscles in the wall of the pharynx called the pharyngeal constrictor muscles. On the image below, note these three constrictors: superior, middle and inferior. These muscles attach anteriorly at structures of the skull and neck (mandible, base of skull, hyoid bone and laryngeal cartilages). Posteriorly, the muscle fibres merge to form a seam-like structure called the pharyngeal raphe. The pharyngeal raphe joins the right and left pharyngeal constrictors into one muscular organ. As the name suggests, the pharyngeal constrictors constrict the pharynx during swallowing. Note how the inferior constrictor is directly continuous with the muscular wall of the oesophagus. The superior oesophageal sphincter is located at the intersection between these two organs. The pharyngeal constrictor muscles are innervated by **cranial nerve number X, the vagus nerve**. **Muscles Moving the Head and Neck** ------------------------------------ ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** Muscles that move the head and neck arise from the vertebral column, thoracic cage and pectoral girdle and attach to the bones of the skull and cervical spine. These muscles can be divided into the posterior neck and anterolateral neck groups. On the image below, note the multiple muscles arranged into superficial and deep layers within the anterolateral neck muscle group. The anterolateral neck muscles can be further subdivided into two distinctive sub-groups: the suprahyoid muscles, located above the hyoid bone (supra = above), and the infrahyoid muscles, located below the hyoid bone (infra= below). Accordingly, the suprahyoid muscles elevate the hyoid bone (and depress the mandible), while the infrahyoid muscles depress the hyoid bone (and move the laryngeal cartilages). In addition, neck muscles include powerful movers of the head and neck such as the right and left sternocleidomastoid muscles. Let's look at the sternocleidomastoid muscle in more detail. ### **Sternocleidomastoid Muscle** The sternocleidomastoid (SCM) muscle is a broad muscle that runs obliquely on the side of the neck. It has two heads corresponding to two origins: the sternal head and the clavicular head. The sternal head originates at the manubrium of the sternum, while the clavicular head originates at the medial third of the clavicle. The two heads unite as they pass upward towards the superior attachment of this muscle at the mastoid process of the temporal bone. As you can see, the name of this muscle literally spells out its attachments (sterno = sternum, cleido = clavicle, mastoid = mastoid process). When both the right and left SCM muscles contract simultaneously (bilateral contraction), they flex the neck forward so that the chin approaches the manubrium. Acting alone (unilateral contraction), the SCM flexes the neck to the same side and rotates the head to the opposite side due to the oblique direction of its fibres. For example, if the right SCM muscle contracts, it rotates the head to the left side of the body. The SCM muscle is innervated by **cranial nerve number XI, the accessory nerve.** Please watch the first one minute of the video below to see how the SCM muscle is tested. **Muscles of the Back** ----------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The back muscles are organised in several layers. Click on each layer below to learn more about them. **Superficial Back Muscles** ---------------------------- **Intermediate Back Muscles** ----------------------------- **Deep Back Muscles** --------------------- **Muscles of the Thorax** ------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The primary function of the deep muscles of the thorax is to promote respiratory movements necessary for breathing. Breathing involves phases of inspiration (inhalation) and expiration (exhalation) that are accompanied by phasic changes in the thoracic cavity volume. In this module, we will look at several primary respiratory muscles: intercostal muscles and the diaphragm. Please note that in addition to the primary muscles, we also have accessory respiratory muscles that are mainly recruited when we require extra effort to breathe (e.g. physical activity). Click on the hotspot next to each number on the image below to learn more about the primary respiratory muscles. Please watch the two short videos below to learn more about respiratory muscles and movements. The first video discusses the role of the diaphragm in breathing. The second video discusses respiratory muscles overall. **Muscles of the Abdominal Wall** --------------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** Unlike in the thorax, the abdominal wall has no bony reinforcement anteriorly and laterally. On each side, the anterolateral abdominal wall is formed by four paired muscles and their aponeuroses. Directly anteriorly, there are two rectus abdominis muscles spanning the distance between the pelvis and sternum/rib cartilages. Lateral to each rectus abdominis, there are three broad flat muscles that are layered. From superficial to deep, these muscles are the external abdominal oblique, internal abdominal oblique and transversus abdominis. As a group, the anterolateral abdominal muscles span the space between the costal margin and the bony pelvis. Click on the hotspot next to each number on the image below to identify these muscles and learn more about them. The four muscles of the anterolateral abdominal wall function in anterior and lateral flexion as well as rotation of the trunk. They also pull the ribs inferiorly and compress the abdominal contents. This action pushes the diaphragm superiorly and aids in forced expiration. Contraction of abdominal muscles with the diaphragm with closed airways produces the so-called Valsalva manoeuvre that increases intra-abdominal pressure. This mechanism assists in expelling urine, defecation, vomiting, childbirth, sneezing, coughing, laughing, screaming and nose blowing. The abdominal muscles also contract in heavy lifting. Additionally, they support viscera and strengthen the trunk. Please note that there are also posterior abdominal muscles (such as quadratus lumborum and psoas major) that we may observe on the models during the practical classes. **Trunk Movements** ------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** Smooth and controlled movements of the trunk involve the cooperative actions of multiple muscles of the neck, back, thorax, abdomen and limbs. Study the images below and note how different muscles work together in producing a variety of complicated movements of our trunk (please note that these details are not examinable) **Muscles of the Pelvic Floor** ------------------------------- ***LO2: Name representative muscles of the head, neck and trunk*** ***LO3: Explain muscle actions based on their attachments and locations*** The pelvis and the pelvic cavity are the lowest parts of the trunk. You may have noticed that the pelvic outlet is an open space and unless it is enclosed by a structure, the pelvic and abdominal cavity contents would be unsupported from below the trunk. The pelvic diaphragm (also referred to as the pelvic floor) seals the inferior opening of the bony pelvis, supports the pelvic organs, lifts the pelvic floor superiorly to release faeces and resists increased intra-abdominal pressure, assisting in expelling the contents of the urinary bladder, rectum and uterus. This funnel-shaped muscular formation consists of several muscles. The pelvic diaphragm is pierced by the rectum and urethra in both sexes, and the vagina in females. The body region inferior to the pelvic diaphragm is called the perineum. The perineum contains the external openings of the anal canal and urethra in both sexes, and the vaginal opening in females. This space also contains the attachments of the external genitalia, supported by the urogenital diaphragm, and specialised muscles including the external urethral and anal sphincters. **Module: Muscular System 3 - Appendicular Muscles: Upper Limb** ================================================================ **Learning Outcomes** --------------------- By the end of this module, you should be able to: **LO1**: Name the main groups of appendicular muscles **LO2**: Name representative muscles of the upper limb and explain their actions based on their attachments and locations **Upper Limb Appendicular Muscles** ----------------------------------- ***LO1: Name the main groups of appendicular muscles*** In this module, we will study the appendicular muscles. We will start with the muscles of the upper limb. The upper limb appendicular muscles control the movements of the upper limbs and stabilise and control the movements of the shoulder girdles. Please recall from the Skeletal System 3 module that the upper limb appendicular skeleton can be divided into two parts: the girdle and the free limb, that is further divided into three segments. We will use these subdivisions to navigate main muscle groups based on their locations. Please recall from the Muscular System 1 module that a muscle must cross a given joint to act on it, meaning that at least one of the muscle attachments must be proximal to the joint, while the other must be distal to the joint. The position of the muscle relatively the joint and the joint structural type dictate precise muscle functions. Before we look at the muscles of the upper limb, let\'s briefly revise the skeletal anatomy of the upper limb. The upper limb girdle (called the shoulder or pectoral girdle) includes the scapula and clavicle. The free upper limb is further subdivided into three segments united by major joints. The three segments of the free upper limb are: - - - Considering the upper limb appendicular skeleton subdivisions, the muscles of the upper limb can be divided into the following groups There are also alternative ways to divide the appendicular muscles into groups. For example, the upper limb muscles can be divided based on their main actions relative to major joints, such as muscles that move the pectoral girdle, muscles that move the shoulder joint, muscles that move the elbow joint, etc. Please recall that muscles are organised into groups based on their location, and these groups have common functions and share common neurovasculature. Now, let's study the upper limb muscle groups and look at the important representative muscles in these groups. **Axio-Appendicular Muscles** ----------------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The axio-appendicular muscles have their proximal attachments on the axial skeleton and their distal attachments on the appendicular skeleton. Therefore, depending on their precise attachments, these muscles move the shoulder girdle and/or shoulder joint. We can divide the axio-appendicular group further into the anterior and posterior sub-groups: - - Click on the hotspot next to each number on the image below to identify these muscles. Now, let's look at some of these muscles in more detail to understand their actions. **Axio-Appendicular Muscles** ----------------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** **Anterior Axio-Appendiular Muscles** ------------------------------------- ### **Pectoralis Major** The pectoralis major muscle belongs to the anterior axio-appendicular group. It is a large, fan-shaped muscle that covers the superior aspect of the anterior thorax. According to its two main origins, this muscle has two heads: the clavicular head and the sternocostal head. From the origins, muscle fibres converge towards its insertion at the anterior aspect of the proximal humerus, in the region of the bicipital groove. You can feel the free lower edge of the pectoralis major muscle, as it forms the anterior axillary fold. Considering the attachments of pectoralis major and its position anterior to the shoulder joint, try to work out its function. Click on the card below to check whether you are correct. Pectoralis Major : The pectoralis major muscle flexes, adducts and medially rotates the arm at the shoulder joint. Note that the pectoralis minor muscle, which is located deep to the pectoralis major muscle, does not act on the shoulder joint, as it does cross the plane of the shoulder joint. Please watch the animation below demonstrating the actions of the pectoralis major muscle. **Posterior Axio-Appendicular Muscles** --------------------------------------- ### **Trapezius** The trapezius muscle belongs to the posterior axio-appendicular group. It is a large, triangular, superficial muscle covering the posterior aspect of the neck and the superior half of the trunk. The right and left trapezius muscles form the shape of trapezium, hence the name of the muscle. Trapezius connects the axial skeleton (skull and vertebral column) with the pectoral girdle (scapula and clavicle). Therefore, this muscle does not cross the shoulder joint and does not act on it directly. The origin of the muscle is at the posterior aspect of the skull, nuchal ligament and spinous processes of C7-T12 vertebrae. This makes the line of attachment quite extensive. All muscle fibres con around the shoulder - the insertion of trapezius is at the lateral end of the clavicle and the spine and acromion of the scapula. Therefore, this muscle has three large parts: descending (or upper), middle (that has mostly horizontal fibres), and ascending (or inferior) parts. Considering the attachments of trapezius and its position, try to work out its function. Click on the card below to check whether you are correct. Trapezius : The trapezius muscle actions depend on which part of the muscle contracts. The descending part elevates, the ascending part depresses, and the middle part (or all three parts together) retracts the scapula. Actions of the trapezius are important for repositioning of the glenoid cavity of the scapula, therefore, increasing the range of movement at the shoulder joint. However, please note that the trapezius does not cross the shoulder joint plane and therefore, it does not produce direct movements at the shoulder joint. Note that deep to the trapezius, you will find the rhomboid and levator scapulae muscles. These may be pointed out on the models during the practical classes, but are for interest only. Please watch the animation below demonstrating the actions of the trapezius muscle. **Axio-Appendicular Muscles** ----------------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** **Anterior Axio-Appendiular Muscles** ------------------------------------- ### **Pectoralis Major** The pectoralis major muscle belongs to the anterior axio-appendicular group. It is a large, fan-shaped muscle that covers the superior aspect of the anterior thorax. According to its two main origins, this muscle has two heads: the clavicular head and the sternocostal head. From the origins, muscle fibres converge towards its insertion at the anterior aspect of the proximal humerus, in the region of the bicipital groove. You can feel the free lower edge of the pectoralis major muscle, as it forms the anterior axillary fold. Considering the attachments of pectoralis major and its position anterior to the shoulder joint, try to work out its function. Click on the card below to check whether you are correct. The pectoralis major muscle flexes, adducts and medially rotates the arm at the shoulder joint. Turn Retry Card 1 of 1 Note that the pectoralis minor muscle, which is located deep to the pectoralis major muscle, does not act on the shoulder joint, as it does cross the plane of the shoulder joint. Please watch the animation below demonstrating the actions of the pectoralis major muscle. **Posterior Axio-Appendicular Muscles** --------------------------------------- ### **Trapezius** The trapezius muscle belongs to the posterior axio-appendicular group. It is a large, triangular, superficial muscle covering the posterior aspect of the neck and the superior half of the trunk. The right and left trapezius muscles form the shape of trapezium, hence the name of the muscle. Trapezius connects the axial skeleton (skull and vertebral column) with the pectoral girdle (scapula and clavicle). Therefore, this muscle does not cross the shoulder joint and does not act on it directly. The origin of the muscle is at the posterior aspect of the skull, nuchal ligament and spinous processes of C7-T12 vertebrae. This makes the line of attachment quite extensive. All muscle fibres con around the shoulder - the insertion of trapezius is at the lateral end of the clavicle and the spine and acromion of the scapula. Therefore, this muscle has three large parts: descending (or upper), middle (that has mostly horizontal fibres), and ascending (or inferior) parts. Considering the attachments of trapezius and its position, try to work out its function. Click on the card below to check whether you are correct.  The trapezius muscle actions depend on which part of the muscle contracts. The descending part elevates, the ascending part depresses, and the middle part (or all three parts together) retracts the scapula. Actions of the trapezius are important for repositioning of the glenoid cavity of the scapula, therefore, increasing the range of movement at the shoulder joint. However, please note that the trapezius does not cross the shoulder joint plane and therefore, it does not produce direct movements at the shoulder joint. Turn Retry Card 1 of 1 Note that deep to the trapezius, you will find the rhomboid and levator scapulae muscles. These may be pointed out on the models during the practical classes, but are for interest only. Please watch the animation below demonstrating the actions of the trapezius muscle. ### **Latissimus Dorsi** The latissimus dorsi muscle belongs to the posterior axio-appendicular group. This fan-shaped muscle covers a large superficial area of the back (muscle name translates from Latin as "the broadest muscle of the back"). It originates at the spinous processes of lower thoracic and lumbar vertebrae, thoracolumbar fascia, lower ribs and iliac crest of the hip bone. It inserts at the anterior aspect of the proximal humerus. Therefore, the latissimus dorsi fibres cross the shoulder joint plane from the posterior aspect as it courses from the posterior to anterior direction. Considering the attachments of latissimus dorsi and its position, try to work out its function. Click on the card below to check whether you are correct. Latissimus Dorsi : The latissimus dorsi muscle extends, adducts and medially rotates the arm at the shoulder joint. It can also raise the body towards the arms during climbing. Please watch the animation below demonstrating the actions of the latissimus dorsi muscle. **Scapulohumeral Muscles** -------------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** As the name suggests, the scapulohumeral muscles connect the scapula and the humerus. Therefore, these muscles cross the plane of the shoulder joint and thus, act on this joint. Their precise functions depend on which aspect of the shoulder joint a given muscle crosses. For example, if a muscle is located above the shoulder joint, it abducts the arm at the shoulder joint. The scapulohumeral muscles are relatively short and powerful. There are six muscles in this group. We have already discussed the dynamic stabilisers of the shoulder joint in the Articular System 2 module -- the four rotator cuff muscles. These are supraspinatus, infraspinatus, teres minor and subscapularis. In addition to the rotator cuff, there are two more powerful muscles -- deltoid and teres major. Click on the hotspot next to each number on the image below to identify these muscles. Now, let's look at these muscles in more detail. We will start with revising the rotator cuff muscles. **Rotator Cuff** ---------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The rotator cuff includes three muscles originating at the posterior aspect of the scapula and one at the anterior aspect. All four insert at the proximal humerus. Click on the hotspots on the image below to revise the attachments and function of these muscles. Please watch the short video below to learn a useful mnemonic for the rotator cuff muscles. You may also like to watch this second video below to learn more about the rotator cuff muscles. **Deltoid** ----------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The deltoid muscle is a powerful muscle covering the shoulder and forming its rounded contour. Its proximal attachment is at the shoulder girdle: clavicle (its lateral part) and scapula (its spine and acromion). Its distal attachment is at the deltoid tuberosity on the lateral aspect of the proximal humerus. Therefore, the entire muscle is located above the shoulder joint and thus, it abducts the arm at the shoulder joint. However, deltoid can be divided into three parts: anterior, middle and posterior. Each part can act separately. Click on the hotspots on the image below to identify these parts and learn their functions. Please watch the animation below demonstrating the actions of the deltoid muscle. **Teres Major** --------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The teres major muscle (from the Latin word 'teres' meaning 'rounded') is a thick muscle passing laterally from the inferior angle of the scapula to the anterior aspect of the proximal humerus, near the attachment of the latissimus dorsi. Therefore, this muscle courses from posterior to anterior, crossing the plane of the shoulder joint. As such, it adducts and medially rotates the arm at the shoulder joint. **Muscles of the Arm** ---------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The arm has two muscular compartments: anterior and posterior. The anterior compartment is the flexor compartment and includes the brachialis and biceps brachii muscles. The posterior compartment is the extensor compartment and consists of the triceps brachii muscle. We have discussed these muscles already in the Muscular System 1 module, but let's now revise them and discuss them in more detail. ### **Anterior Muscles of the Arm** The superficial muscle in the anterior (flexor) compartment of the arm is biceps brachii. As the name suggests, this muscle has two heads (bi = two; ceps = heads): short and long. It crosses both the shoulder and elbow joints, as well as the proximal radioulnar joint. Therefore, it acts on all three joints. The brachialis muscle is located deep to biceps brachii. It is significantly shorter than biceps brachii and it only crosses the elbow joint. Therefore, it acts exclusively on the elbow joint. Click on the hotspots on the image below to learn the attachments and functions of these muscles. Please watch the animations below demonstrating the actions of the biceps brachii and brachialis muscles. ### **Posterior Muscles of the Arm** As the name suggests, the triceps brachii muscle has three heads (tri = three; ceps = heads): long, lateral and medial. The proximal attachments of the lateral and medial heads are at the lateral and medial surfaces of the humerus, respectively, while the proximal attachment of the long head is at the infraglenoid tubercle of the scapula. The distal attachment of triceps brachii is at the olecranon of the ulna. Therefore, this muscle crosses the elbow joint at the posterior aspect and is the prime extensor of the forearm. As the long head also crosses the plane of the shoulder joint, it supports the shoulder at the posterior aspect, resisting dislocation. Please watch the animation below demonstrating the actions of the triceps brachii muscle. **Muscles of the Forearm** -------------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The forearm extends from the elbow to the wrist. It contains two bones (the radius on the lateral side and the ulna on the medial side) united by an interosseous membrane. Like the arm muscles, the forearm muscles with similar functions and innervations are grouped within the same compartments. Like the arm, the forearm has two muscular compartments: anterior and posterior. The anterior compartment contains the flexors and posterior compartment contains the extensors. In each compartment, the muscles are arranged in several layers from superficial to deep. Functionally, in addition to the radius and ulna, the forearm also includes the distal humerus because the superficial forearm muscles originate at the distal humerus. Therefore, these muscles cross and act at the elbow joint. Muscles in the deeper forearm layers originate more distally than the superficial muscles (i.e. at the radius, ulnar and/or interosseous membrane). The distal attachments of the forearm muscles are mostly located within the hand. Therefore, these muscles act at the wrist and hand joints Click on the hotspots on the image below to learn more about the two forearm muscle compartments. **Muscles of the Hand** ----------------------- ***LO2: Name representative muscles of the upper limb and explain their functions based on their attachments and locations*** The hand is the manual part of the upper limb. Its movements are controlled by two groups of muscles. The extrinsic hand muscles are the forearm muscles. Their long tendons stretch to attach within the hand, while their proximal attachments and muscle bellies are located within the forearm. The intrinsic hand muscles originate and insert within the hand. This group of muscles is located on the palmar (anterior) surface of the hand and can be further subdivided into three groups: - - - Please note that there are no intrinsic muscles on the dorsal (posterior) aspect of the hand. The dorsal side of the hand contains the tendons of the extrinsic hand muscles (forearm extensors). **Muscular System 3 Module Crossword** ====================================== **Module: Muscular System 4 - Appendicular Muscles: Lower Limb** ================================================================ **Muscles of the Leg** ---------------------- ***LO2: Name representative muscles of the lower limb and explain their functions based on their attachments and locations*** The leg extends from the knee to the ankle. It contains two bones (the tibia on the medial side and the fibula on the lateral side) united by an interosseous membrane. The leg includes three muscular compartments: **anterior, posterior** and **lateral.** The muscles within each compartment share common functions and innervation. - - - Study the images below to revise the dorsiflexion and plantarflexion movements at the ankle joint (image 1) and the inversion and eversion movements at the intertarsal joints of the foot (image 2). Now, let\'s look at the superficial group of posterior leg muscles in more detail. ### **Posterior Leg Muscles: Superficial Group** The muscles collectively referred to as **triceps surae** ('sura' = calf) are the most superficial posterior leg muscles. The name implies that triceps surae has three heads (tri = three; ceps = heads): two heads belong to the gastrocnemius muscle and one head is of the soleus muscle. Gastrocnemius is superficial to soleus. All three parts merge to form one tendon called the calcaneal or 'Achilles' tendon that attaches at the calcaneus of the foot. Therefore, all three parts of triceps surae work together to plantarflex the foot at the ankle joint. Click on the hotspots on the image below to learn more about these muscles **Muscles of the Foot** ----------------------- ***LO2: Name representative muscles of the lower limb and explain their functions based on their attachments and locations*** Like the hand, the foot has both extrinsic and intrinsic muscles. The extrinsic foot muscles have their proximal attachments and muscle bellies located in the leg and their long tendons cross both the dorsal and plantar aspects of the foot to insert within the foot. The intrinsic foot muscles have their origins and insertions located within the foot. Unlike in the hand (where there are no dorsal intrinsic muscles), there is a dorsal group of muscles in the foot. These are extensors of the digits. The plantar group of intrinsic foot muscles includes several layers of short muscles that in addition to the tendons of the extrinsic foot muscles, move the digits and support the foot arches.