BIOD27H3 Lecture Notes - Anaerobic Respiration, Sarcomere, Cramp

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Chapter 12- Muscles
x Three types of muscle tissue occur in the human body: skeletal muscle, cardiac muscle, and smooth muscle
x Most skeletal muscles are attached to the bones of the skeleton, enabling these muscles to control body movement
x Cardiac muscle is found only in the heart and is responsible for moving blood through the circulatory system.
x Skeletal and cardiac muscles are classified as striated muscles b/c of their alternating light and dark bands under the
light microscope
x Smooth muscle is the primarily muscle of internal organs and tubes, such as the stomach, urinary bladder, and blood
x Its primary function is to influence the movement of material into, out of, and within the body
x Skeletal muscles are often described as voluntary muscles, and smooth and cardiac muscle as involuntary
x Skeletal muscles can contract w/o conscious direction, and we can learn a certain degree of conscious control
over some smooth and cardiac muscle
x Skeletal muscles are unique in that they contract only in response to a signal from a somatic motor neuron; they cannot
initiate their own contraction, nor is their contraction influence directly by hormones
x Although their primary extrinsic control arises through autonomic innervation, some types of smooth and cardiac
muscle can contract spontaneous, w/o signals from the CNS
x In addition, the activity of cardiac and some smooth muscle is subject to modulation by the endocrine system
Skeletal Muscle
x Skeletal muscles make up the bulk of muscle in the body and constitute about 40% of total body weight and they are
responsible for positioning and moving the skeleton
x Skeletal muscles are usually attached to bones by tendons made of collagen; the origin of muscle is the end of the
muscle that is attached closest to the trunk or to the more stationary bone; the insertion of the muscle is the more distal
or more mobile attachment
x When the bones attached to a muscle are connected by a flexible joint, contraction of the muscle moves the skeleton
x If the centres of the connected bones are brought closer together when the muscle contracts, the muscle is called a
flexor, and the movement is called flexion
x If the bones move away from each other when the muscle contracts, the muscle is called an extensor, and the
movement is called extension
x Most joints in the body have both flexor and extensor muscles, b/c a contracting muscle can pull a bone in one direction
but cannot push it back
x Flexor-extensor pairs are called antagonistic muscle groups b/c they exert opposite effects
x When the biceps muscle contracts, the hand and forearm move toward the shoulder but when the triceps contracts, the
flexed forearm moves away from the shoulder
Skeletal Muscles Are Composed of Muscle Fibres
x A skeletal muscle is a collection of muscle cells, or muscle fibres (Fig 12-3 pg. 400)
x Each skeletal muscle fibre is a long, cylindrical cell w/ up to several hundred nuclei on the surface of the fibre.
x Skeletal muscles fibres are the largest cells in the body, created by the fusion of many individual embryonic
muscle cells
x The fibres in a given muscle are arranged w/ their long axes in parallel and each skeletal muscle fibre is sheathed in
connective tissue
x Groups of adjacent fibres are bundled together into units called fascicles with collagen, elastic fibres, nerves, and blood
vessels are found b/w the fascicles
x The entire muscle is enclosed in a connective tissue around the muscle fibres and fascicles and w/ the tendons holding
the muscle to underlying bones
Muscle Fibre Anatomy
x The cell membrane of a muscle fibre is called the sacrolemma and the cytoplasm is called the sarcoplasm
x The main intracellular structures in striated muscles are myofibrils, highly organized bundles of contractile and
elastic proteins that carry out the work of contraction
x Skeletal muscles also contain extensive sarcoplasmic reticulum (SR), a form of modified endoplasmic reticulum
that wraps around each myofibril like a piece of lace
x The sarcoplasmic reticulum consists of longitudinal tubules, which release Ca2+ ions, and the terminal cisternae,
which concentrate and sequester CA2+
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x A branching network of transverse tubules (t-tubules) is closely associated w/ the terminal cisternae; one t-
tubule w/ its two flanking terminal cisternae is known as a triad
x The membranes of t-tubules are a continuation of the muscle fibre membrane and this makes the lumen of the t-
tubules continuous w/ the ECF
x T-tubules rapidly move APs that originate at the neuromuscular junction on the cell surface into the interior of
the fibres
x W/o t-tubule, the AP could reach the centre of the fibre only by the diffusion of +ve charge through the
cytosol, a slower process that would delay the response time of the muscle fibre
x The cytosol b/w the myofibrils contains many glycogen granules and mitochondria
Myofibrils Are the Contractile Structures of a Muscle Fibre
x One muscle fibre contains a thousand or more myofibrils that occupy most of the intracellular volume, leaving little
space for cytosol and organelles
x Each myofibril is composed of several types of proteins: the contractile proteins myosin and actin, the regulatory
proteins tropomyosin and troponin, and the giant accessory proteins titin and nebulin
x Myosin is the motor protein of the myofibril
x Each myosin molecule is composed of protein chains that intertwine to form a long tail and a pair of tadpolelike
x In a skeletal muscle, about 250 myosin molecules join to create a thick filament; each filament is arranged so
that the myosin heads are clustered at the ends of the filament, and the central region of the filament is a bundle
of myosin tails
x The rodlike core of the thick filament is stiff, but the protruding myosin heads have an elastic hinge region
where the heads join the rods and this hinge allows the heads to swivel around their point of attachment
x Actin is a protein that makes up the thin filaments of the muscle fibre
x Usually multiple G-actin molecules polymerize to form long chains or filaments called F-actin
x In skeletal muscle, two F-actin polymers twist together like a double strand of beads, creating the thin filaments
of the myofibril
x Most of the time, the parallel thick and thin filaments of the myofibril are connected by crossbridges that span
the space b/w the filaments and these crossbridges form when myosin heads of the thick filaments bind loosely
to actin in the thin filaments
x The arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternating light and
dark bands
x One repeat of the pattern forms a sacromere, a unit or segment which has the following elements: (Fig 12-5
1. Z disks: One sarcomere is composed of 2 Z disks and the filaments found b/w them. Z disks are zigzag
protein structures that serve as the attachment site for thin filaments
2. I band: These are the lightest colour bands of the sarcomere and represent a region occupied only by thin
filaments. A Z disk runs through the middle of every I band, so each half of an I band belongs to a different
At the outer edges of the A band, the thick and thin filaments overlap. The centre of the A band is occupied
by thick filaments only. The protein fibres in this region scatter light unevenly
4. H zone: This central region of the A band is lighter than the outer edges of the A band b/c the H zone is
occupied by thick filaments only
5. M line: This band represents proteins that form the attachment site for thick filaments, equivalent to the Z
disk for the thin filaments. Each M line divides an A band in half
x The proper alignment of filaments within a sarcomere is ensured by two types of proteins: titin and nebulin
x Titin is a huge elastic molecule and the largest known protein, composed of more than 25 000 AA.
x A single titin molecule stretches form one Z disk to the neighbouring M line
x Titin has two functions: (1) it stabilizes the position of the contractile filaments and (2) its elasticity returns
stretched muscles to their resting length
x Titin is helped by nebulin, an inelastic giant protein that lies alongside thin filaments and attaches to the Z disk
x Nebulin helps align the actin filaments of the sarcomere
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