Biology: Muscles and Locomotion

Question 1

Vertebrate Skeleton

Answer 1

An endoskeleton serves as the framework within all vertebrate organisms. Muscles are attached to the bones, permitting movement. The endoskeleton also provides protection by surrounding delicate vital organs in bone. The rib cage protects the thoracic organs (heart and lungs), whereas the skull and vertebral column protect the brain and spinal cord. The two major components of the skeleton are catilage and bone.

Question 2

Cartilage

Answer 2

A type of connective tissue that is softer and more flexible than bone. Cartilage is retained in adults in places where firmness and flexibiltiy are needed. For example, in humans, the external ear, nose, walls or larynx and trachea, and skeletal joints contain cartilage. Chrondytes are cells responsible for synthesizing catilage.

Question 3

Bone

Answer 3

A specialized type of mineralized connective tissue that has the ability to withstand physical stress. Ideally adapted for body support, bone tissue is hard and strong while at the same time somewhat elastic and lightweight. There are two types of bone, compact and spongy.

Question 4

Compact Bone

Answer 4

Dense bone that does not appear to have any cavities when observed with the naked eye. The bony matrix is deposited in dtructural units called osteons (Haversian systems). Each osteon consists of a central microscopic channel called a Haversian canal, surrounded by a number of concentric circles of bony matrix (calcium phosphate) called lamellae.

Question 5

Spongy Bone

Answer 5

Much less dense and consists of an interconnecting lattice of bony spicules (trabeculae); the cavities between the spicules are filled with yellow or red bone marrow.

Yellow marrow is inactive and infiltrated by adipose tissue.

Red marrow is involved in blood cell formation.

Question 6

Osteocytes

Answer 6

Two other types of cells found in bone tissue are osteoblasts and osteoclasts. Osteoblasts synthesize and secrete the organic constituents of the bone matrix; once they have become surrounded by their matrix, they mature into osteocytes. Osteoclasts are large, multinucleated cells involved in bone resorption.

Question 7

Bone Formation

Answer 7

Bone formation occurs by either endochondral ossification or by intramembranous ossification. In endocondral ossification, existing cartilage is replaced by bone. Long bones arise primarily through endochondral ossification. In intramembranous ossification, mesenchymal (embryonic or undifferentiated) connective tissue is transformed into and replaced by bone.

Question 8

Organization of Vertebrate Skeleton: Axial vs. Appendicular

Answer 8

The axial skeleton is the basic framework of the body, consisting of the skull, vertebral column, and the rib cage.

It’s the point of attachment of the appendicular skeleton, which includes the bones of the appendages (limbs) and the pectoral and pelvic girdles.

Question 9

Oraganization of Veterbrate Skeleton: Held Together

Answer 9

Bones are held together in a # of ways.

Sutures or immovable joints hold the bones of the skull together.

Bones that move relative to one another are held together by movable joints and are additionally supported and strengthened by ligaments. Ligaments serve as bone-to bone-connectors.

Question 10

Organization of Vertebrate Skeleton: Skeletal Muscle

Answer 10

Tendons attach skeletal muscle to bones and bend the skeleton at the movable joints.

The point of attachment of a muscle to a stationary bone (the proximal end in limb muscles) is called the origin.

The point of attachment of a muscle to the bone that moves (distal end in limb muscles) is called the insertion.

Extension indicates a straightening of a joint, whereas flexion refers to a bending of a joint.

Question 11

Muscular System

Answer 11

Muscle tissue consists of bundles of specialized contractile fibers held together by connective tissue. There are three morphologically and functionally distinct types of muscle in mammals: skeletal, smooth, and cardiac.

Nervous control of the muscular system involves the axons of the pyramidal cells of the motor cortex, which descend to synapse on lower motor neurons in the brain stem and the spinal cord. Because there are no intervening synapses, the pyramidal system is able to provide rapid commands to the skeletal muscles and various other organs.

Several other centers can issue somatic motor commands as a result of processing performed at the unconscious , involuntary level. These centers and their associated tracts comprise the extraphyramidal system. The red nucleus, located in the mesencephalon, is the component of the extrapyramidal system primarily in control of skeletal muscle tone.

Question 12

Skeletal Muscle

Answer 12

Reponsible for voluntary movements and is innervated by the somatic nervous system.

Each fiber is a multinucleated cell created by the fusion of several mononucleated embryonic cells. Embedded in the fibers are filaments called myofibrils, which are further divided into contractile units called sacromeres. The myofibrils are enveloped by a modified endoplasmic reticulum that stores calcium ions and is called the sarcoplasmic reticulum.

The cytoplasm of a muscle fiber is called sarcoplasm, and the cell membrane is called the sarcolemma. The sarcolemma is capable of propagating an action potential and is connected to a system of transverse tubules (T system) oriented perpendicularly to the myofibrils. The T system provides channels for ion flow throughout the muscle fibers and can also propagate an action potential.

Because of the high energy requirements of contraction, mitochondria are abundant in muscle cells, all along the myofibrils. Skeletal muscle has striations of light and dark bands and is therefore also referred to as striated muscle.

Question 13

Sarcomere Structure

Answer 13

Composed of thin and thick filaments.

The thin are chains of actin molecules. Thick are composed of organized bundles of myosin molecules.

Electron microscopy reveals that the sarcomere is organized as follows. Z lines define the boundaries of a single sarcomere and anchor the thin filaments. The M line runs down the center of the sarcomere. The I band is the region containing thin filaments only. The H zone is the region containing thick filaments only. The A band spans the entire length of the tick filaments and any overlapping portions of the thin filaments.

Note that during contraction, the A band isn’t reduced in size, but H zone and I band are.

Question 14

Sarcomere Contraction

Answer 14

Muscle contraction is stimulated by a message from the SNS sent via a motor neuron.

The link between the nerve terminal (synaptic bouton) and the sarcolemma of the muscle fiber is called the neuromuscular junction. The space between the two is known as the synapse, or synaptic cleft.

Depolarization of the motor neuron results in the release of neurotransmitters (e.g., acetylcholine) from the nerve terminal. The neurotransmitter diffuses across the synaptic cleft and binds to special receptor sites on the sarcolemma. If enough of these receptors are stimulated, the permeability of the sarcolemma is altered and an action potential is generated.

Once an action potential is generated, it is conducted along the sarcolemma and the T system and into the interior of the muscle fiber. This causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. Calcium ions initiate the contraction of the sarcomere. Actin and myosin slide past each other, and the sarcomere contracts.

Question 15

Isotonic Contraction

Answer 15

Occurs when a muscle shortens against a fixed load while the tension on that muscle remains constant.

Question 16

Rigor Mortis

Answer 16

An interesting point is that several hours after death, all of the muscles in the body go into a state or rigor mortis.

In this condition, the muscles contract and become rigid, even w/o action potentials. The rigidity is caused by an absence of adrnosine triphosphate, which is required for the myosin heads to be released from the actin filaments.

The muscles typically remain rigid for 12-24 hours after death until the muscle proteins are destroyed.

Question 17

Concentric Contration

Answer 17

A type of dynamic contraction where the muscle fibers shorten and the tension on the muscle increases.

Question 18

Dynamic Contraction

Answer 18

Includes both concentric and eccentric types of contractions. In general, a dynamic contraction results in the change in length of the muscle with a corresponding change in tension on that muscle.

Question 19

Eccentric Contraction

Answer 19

A type of dynamic contraction where the muscle fiber lengthens and the tension on the muscle increases.

Question 20

Isometric Contraction

Answer 20

Occurs when both ends of the muscle are fixed and no change in length occurs during contraction, but the tension increases.

Question 21

Stimulus and Muscle Response

Answer 21

Individual muscle fibers generally exhibit an all-or-none response; only a stimulus above a minimal value called the threshold value can elicit contraction.

The strength of the contraction of a single muscle fiber cannot be increased, regardless of the strength of the stimulus. However, the strength of contraction of the entire muscle can be increased by recruiting more muscle fibers.

Question 22

Simple Twitch

Answer 22

The response of a single muscle fiber to a brief stimulus at or above the threshold stimulus, and consists of a latent period, a contraction period, and a relaxation period.

The lalent period is the time between stimulation and the onset of contraction. During this time lag, the action potential spreads along the sarcolemma, and calcium ions are released.

After the contraction period, there is a belief relaxation period in which the muscle is unresponsive to a stimulus; this period is known as the absolute refractory period.

Question 23

Temporal Summation

Answer 23

When the fibers of a muscle are exposed to very frequent stimuli, the muscle cannot fully relax. The contractions begin to combine, becoming stronger and more prolonged.

The contractions become continuous when the stimili are so frequent that the muscle cannot relax. This type of contraction is known as tetanus and is stronger than a simple twitch of a single fiber. If tetanus is maintained, the muscle will fatigue, and the contraction will weaken.

Question 24

Tonux

Answer 24

State of partial contraction. Muscles are never completely relaxed and maintain a partially contracted state at all times.

Question 25

Cori Cycle

Answer 25

During periods of strenuous activity, skeletal muscles convert glucose to pyruvic acid through the process of glycolysis. This process enables skeletal muscles to continue contracting, even in the absence of oxygen.

Lactic acid is generated when pyruvic acid is reacted with the enzyme lactate dehydrogenase. This process allows the pyruvate to enter the Krebs cycle. The purpose of the Cori cycle during periods of strenuous activity is to convert lactic acid in the liver to glucose for discharge into the bloodstream. Once the glucose is in the blood, the muscles are then able to use the glucose as an immediate source of energy or rebuild their glycogen reserves.

Conversion of glucose into pyruvate in the muscle cells is necessary for the creation of ATP during periods of strenuous exercise and does not involve the Cori cycle. During periods of intense exercise, the production of lactic acid is increased, and glycogen is broken down into glucose; however, that is not by means of the Cori cycle.

Question 26

Smooth Muscle

Answer 26

Responsible for involuntary actions and is innervated by the ANS.

Smooth muscle is found in digestive tract, bladder, uterus, and blood vessel walls, among other places.

Smooth muscle cells possess one centrally located nucleus.

Smooth muscles lack the striations of skeletal muscle.

Question 27

Cardiac Muscle

Answer 27

The muscle tissue of the heart is composed of cardiac muscle fibers.

These fibers possess characteristics of both skeletal and smooth muscle fibers.

As in skeletal muscle, actin and myosin filaments are arranged in sarcomeres, giving cardiac muscle a striated appearance.

However, cardiac muscle cells generally have only one or two centrally located nuclei.

Question 28

Creatine Phosphate

Answer 28

In vertebrates energy can be temporarily stored in this high-energy compound.

Question 29

Myoglobin

Answer 29

A hemoglobin-like protein found in muscle tissue. Has a high oxygen affinity and maintains the oxygen supply in muscles by binding oxygen tightly.