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What is muscle?

One of the four basic tissues of the body (epithelial, connective and nervous tissue are the other three).
It is made up of cells that can shorten or contract.


What are the three types of muscle and what are some of the general characteristics of each type?

Skeletal, cardiac and smooth muscle.
Skeletal: voluntary, moves bones of skeleton
Cardiac: involuntary, found only in the heart
Smooth: involuntary, found all over body (e.g. eyes, lungs, stomach & intestines,...)


Difference between a tendon and an aponeurosis?

Tendon: attach muscle to bone; tough, fibrous connective tissue bands
Aponeurosis: broad sheets of fibrous connective tissue attaching muscles to bones or to other muscles; example the linea alba running lengthwise between the muscles on an animal's ventral midline. (common site for surgical entry into abdomen)


What is the origin of a muscle?
What is the insertion?

Origin: more stable (moves less); does not move when muscle contracts
Insertion: undergoes most of the movement when a muscle contracts
EXAMPLE: the origin of the triceps brachii muscle is on the scapula and proximal humerus; its insertion is on the olecranon process of the ulna. When triceps contracts, it pulls on olecranon process and straightens (extends) the elbow joint.


Describe a skeletal muscle cell in terms of cell size, shape, number of nuclei, and appearance under the microscope.

CELL SIZE & SHAPE: huge, not very wide, but long and thin. Can be several inches long. Threadlike or fiberlike shape.
NUMBER OF NUCLEI: not just one but many. Large ones can have 100 or more nuclei per cell, all located out at the edge of the cell just beneath the sarcolemma (muscle cell membrane).
LOW POWER MICROSCOPE: alternating dark and light bands
HIGHER MAGNIFICATION: thin, dark line in the center of the large, light band and a lighter band in the center of the dark band, which are the attachments and overlappings of the tiny actin and myosin filaments that make up the myofibril.


Differences between a skeletal muscle fiber, myofibril and protein filament?

MUSCLE FIBER: are the skeletal muscle cells
MYOFIBRIL: most of the volume of a skeletal muscle fiber is made up of hundreds or thousands of smaller myofibrils packed together lengthwise
PROTEIN FILAMENTS: myofibrils are made up of thousands of even tinier, contractile protein filaments. They are the thin actin filaments and the thick myosin filaments.


Which contractile protein filaments make up the dark bands and which make up the light bands?

DARK BANDS (A band): Thick myosin filaments
LIGHT BANDS (I band): Thin actin filaments


What is a sarcomere and what are its components?

It is the basic contracting unit of a skeletal muscle.
The dark line in the center of the I band (light band, actin filaments) is called the Z line. It's the attachment site for actin filaments. An area from Z line to Z line is called a SARCOMERE.


What ion, released from the sarcoplasmic reticulum by a nerve impulse, starts the contraction process in a muscle fiber?

Stored calcium ions (Ca++) are released into the sarcoplasm once the impulse reaches the sarcoplasmic reticulum. As the Ca++ diffuses into the myfibrils, it turns on the contraction process, powered by ATP.


What molecules in muscle act as the "batteries" to power the sliding of the actin and myosin filaments? What molecules function as the "battery chargers"?

BATTERIES: ATP, which is produced by the many mitochondria in muscle fibers. ATP molecules are like tiny batteries that can release energy and then be recharged.
BATTERY CHARGER: Creatine phospate (CP) splits and the released energy adds a phophate group to the ADP, converting it back to ATP.


If individual muscle fiber contractions obey the all-or-nothing principle, how does an animal control the size and strength of its muscular movements?

The body does so by carefully controlling the number of muscle fibers it stimulates for a particular movement. Small, fine movements require only a few muscle fibers to contract. Larger, more powerful movements require the contraction of many muscle fibers. The nervous system must predict how large and powerful a movement needs to be, and then it must send the appropriate nerve impulses down to the appropriate muscle fibers in the appropriate muscle(s) (this is what we call muscle memory).


What is myoglobin and why is it important?

Muscle fibers can also store glucose and oxygen.
Glucose is stored in the fibers in the form of glycogen, however, oxygen is stored attached to large protein molecules called MYOGLOBIN.
It is red and can store and release large quantities of oxygen. As long as the oxygen supply is adequate to keep up with energy needs, the process is known as aerobic metabolism (anaerobic metabolism is not as efficient, the need for oxygen exceeds the supply and results in lactic acid formation).


Why does an animal breathe heavily for a while after heavy exercise?

An animal may continue to breathe heavily for a while as its body repays its so-called "oxygen debt".


Describe a cardiac muscle cell in terms of size, shape, number of nuclei, and appearance under the microscope.

SIZE & SHAPE: much smaller than skeletal muscle cells; longer than they are wide and often have multiple branches
NUMBER OF NUCLEI: only one per cell
MICROSCOPIC APPEARANCE: firm, end-to-end attachments between cells appear as dark, transverse lines called intercalated disks.


What are intercalated disks and why are they important to the functioning of cardiac muscle?

Intercalated disks are the firm, end-to-end attachment sites of cardiac muscle cells.
They securely fasten the cells together and also transmit impulses from cell to cell to allow large groups of cardiac muscle cells to contract in a coordinated manner.


Describe the effect of a cardiac muscle's nerve supply on its functioning.
What is the general effect of sympathetic nervous system stimulation on cardiac muscle? What is the effect of parasympathetic nervous system stimulation?

The heart's nerve supply is not essential to its functioning.
The nerves to the heart are from both divisions of the autonomic portion of the nervous system, that is, the sympathetic and parasympathetic systems.
Sympathetic fibers stimulate the heart to beat harder and faster as part of the fight-or-flight response that kicks in when an animal feels threatened.
Parasympathetic fibers do the opposite, they inhibit cardiac function, thereby causing the heart to beat more slowly and with less force when the body is relaxed and resting.


Describe a smooth muscle cell in terms of size, shape, number of nuclei, and appearance under the microscope.

SIZE & SHAPE: small, spindle-shaped (tapered on the ends)
NUMBER OF NUCLEI: single nucleus in center
MICROSCOPIC APPEARANCE: smooth, homogeneous, because their filaments of actin and myosin are not arranged in parallel myofibrils. Their small, contractile units of actin and myosin filaments crisscross the cell at various angles and are attached at both ends to dense bodies that correspond to the Z lines of skeletal muscle.


What are the main differences between visceral smooth muscle and multiunit smooth muscle?
Describe the effect of nerve stimulation on the functioning of visceral smooth muscle and multiunit smooth muscle.

VISCERAL: Large and relatively powerful; cells are linked to form large sheets in the walls of organs such as the stomach, intestine, uterus, and urinary bladder. Fine movements not possible, instead large, rythmic waves of contraction. Its nerve supply is not necessary to initiate contractions, but serves to modify them.
MULTIUNIT: Small and delicate; made up of individual smooth muscle cells or small groups of cells. Found where small, delicate contractions are needed, such as the iris and ciliary body of the eye, the walls of small blood vessels, and around small air passageways in the lungs. Require specific impulses from autonomic nerves to contract.


What is the general effect of sympathetic nervous system stimulation on visceral smooth muscle? What is the effect of parasympathetic nervous system stimulation?

Sympathetic stimulation decreases visceral smooth muscle activity.
Parasympathetic increases it.


What are the main differences in the structures and functions of skeletal muscle, cardiac muscle, and smooth muscle?

SKELETAL MUSCLE: Voluntary, striated; moves the bones, generates heat; nerve supply is necessary for function
CARDIAC MUSCLE: Involuntary, striated; pumps blood through the heart; nerve supply modifies activity, but isn't necessary for function
SMOOTH MUSCLE: Involuntary, non-striated; produces movements in internal organs and structures; nerve supply in visceral smooth muscle modifies activity but is not necessary for function; nerve supply in multiunit smooth muscle is necessary for function.