MCAT Study Guide Biology Ch. 10 – Connective Tissues 2017-08-15T06:45:06+00:00

I.          10.1:  OVERVIEW OF MUSCLE TISSUE

 

II.          10.2:  SKELETAL MUSCLE

A.     MOVEMENT OF JOINTS

B.     STRUCTURE OF SKELETAL MUSCLE

1.     Muscle fibers (myofiber)– single muscle cell; multiple fibers make up a fascicle

2.     Sarcolemma – muscle cell membrane that has extra structure to it to allow the ends to form a fibrous tendon

3.     Myofibrils – smaller units inside a muscle cell that generate the contraction; made of thin filaments (actin) and thick filaments (myosin)

a)     Sarcomeres – units that are lined up end-to-end to form a myofibril

(1)   I band – portion of the sarcomere made of thin filaments

(2)   A band – portion of the sarcomere represented by the full length of the thick filament

(3)   H zone – region composed only of thick filament (no thin filament); present only at rest

(4)   Z lines – the ends of the sarcomeres, where the actin ends are attached

C.    SLIDING FILAMENT MODEL OF MUSCLE CONTRACTION

1.     There are head and tails to myosin; the heads (like golf clubs) attach to binding sites on the actin; contraction occurs when they are released, bind to a further binding site, then rest and contract

2.     Steps:

a)     Cross bridge formation – myosin heads bind to actin sites; bound with ADP and Pi

b)     Power stroke – this is the low energy conformation, where the myosin head pulls the actin towards it; ADP is released

c)     ATP binding releases head from current binding site

d)     ATP hydrolysis cocks myosin head (high-energy conformation) and allows it to bind to a new actin site

D.    EXCITATION-CONTRACTION COUPLING IN THE SKELETAL MUSCLE

1.     Contraction only occurs when cytoplasmic Ca++ is present

2.     Troponin-tropomysin complex – complex of protein that blocks actin binding sites unless Ca++ is there to unlock them

a)     Tropomysin is long and fibrous

b)     Troponin is globular

3.     Sarcoplasmic reticulum (SR) – stores the intracellular Ca++

E.     NMJ AND IMPULSE TRANSMISSION

1.     NMJ – large region along cell membrane; this allows depolarization of large area at once

2.     Motor end plate – the post-synaptic area of membrane

3.     EPP (end plate potential) – depolarization of the postsynaptic membrane

4.     T-tubules – deep invaginations in the cell membrane to help with depolarizing the entire cell

F.     MECHANICS OF CONTRACTION

1.     Force of contraction can be increased in 2 ways:

a)     Motor unit recruitment:  one motor neuron innervates multiple myfibers; a twitch is the smallest activity, is the activation of only 1 cell; a larger twitch or contraction can be activated by recruiting the rest of the cells in the motor unit

b)     Frequency of summation:  repeatedly stimulating the motor unit between impulses (after refractory period but before Ca++ is resequestered in sarcolemma) will results in a stronger contraction (maxing out at tetanus)

2.     Length-tension relationship – muscle fiber contracts the most strongly at a specific length, where there is max overlap between actin and myosin (2.2 microns)

G.    ENERGY STORAGE IN THE MYOFIBRIL

1.     Creatine phosphate – intermediate-term energy storage molecule utilized by muscle tissue; hydrolysis of this drives regeneration of ATP from ADP and Pi

2.     Myoglobin – takes oxygen away from hemoglobin and stores for reserve

III.          10.3:  CARDIAC MUSCLE COMPARED TO SKELETAL MUSCLE

A.     SIMILARITIES

1.     Thick and thin filaments are organized into sarcomeres; both are striated

2.     T-tubules are present in both

3.     Troponin-tropomyosin regulates contraction in the same way

4.     The length-tension relationship works the same way and it is more significant in cardiac muscles

B.     DIFFERENCES

1.     Cardiac muscles are not structurally syncytial (each only have 1 nucleus) while skeletal muscles are syncytials

2.     Cardiac muscle cells are connected by gap junctions known as intercalated disks which allow the action potential to propagate throughout the entire heart without sharing nuclei or cytoplasmic contents (functional syncytium)

3.     Connected to several neighbors by intercalated disks (branching?)

4.     Cardiac contraction does not depend on stimulation by motor neurons (stimulation by ACh is actually inhibitory

5.     AP in cardiac muscle depends on both voltage gated Na+ channels and Ca++ channels (which cause the plateau phase); plateau phase important for 2 reasons:

a)     Longer duration of contraction facilitates better EF

b)     Longer refractory periods helps prevent disorganized transmission of impulses and makes summation and tetanus impossible

 

IV.          10.4:  SMOOTH MUSCLE COMPARED TO SKELETAL MUSCLE

A.     SIMILARITIES

1.     Four-step contractile cycle is the same

2.     Contraction is triggered by cytoplasmic influx of Ca++

3.     No branching

B.     DIFFERENCES

1.     Smooth muscle cells are narrower and shorter than skeletal muscles

2.     No T-tubules present (unnecessary due to small size)

3.     Each muscle cell is connected to another by gap junction; is functional syncytium

4.     Thick and thin filaments are not organized into sarcomeres; just dispersed in the cytoplasm

5.     No troponin-tropomyosin complex; instead, camodulin and myosin light-chain kinase (MLCK)

6.     More heavily relies on extracellular Ca++ (poorly developed SR)

7.     AP depends on location of smooth muscle cell; can elicit spike potentials (above threshold), but it is more difficult because there are no fast Na+ channels, only slow; takes much longer to propagate AP than in skeletal muscle

8.     Smooth muscle can sustain prolonged contraction

9.     Have constantly fluctuating resting potential (slow waves); when RP decreases and a spike potential occurs, this is when an AP gets propagated (ACh is released in response to local stimuli to cause the spike potential; NE inhibits amplitude of slow waves

10.  Innervated by autonomic motor neurons

Feature Skeletal muscle Cardiac muscle Smooth muscle
Appearance Striated Straited No stration
Upstroke of AP Inward Na+ current Inward Ca++ (SA node)Inward Na+ (elsewhere) Inward Na+
Plateau No Yes No
Duration of AP 2-3 msec 150 msec (SA node)300 msec (other cells) 20 msec
Ca++ from voltage gated Ca++ channels, Ca++ from SR voltage gated Ca++ channels, inward Ca++ during plateau, Ca++ from SR voltage gated Ca++ in membrane
Molecular basis for contraction Ca++ troponin binding Ca++ troponin binding Ca++ calmodulin binding, MLCK activation
Fuctional syncytium No Yes Yes
Contraction dependent on extracellular Ca++ No Partially Yes

 

V.          10.5:  OVERVIEW OF THE SKELETAL SYSTEM

A.     Axial skeleton – skull, vertebral column, ribcage

B.     Appendicular skeleton – the remainder (including the pelvis, but not the sacrum)

 

VI.          10.6:  CONNECTIVE TISSUE

A.     Fibroblast

The single progenitor of connective tissue; can secrete collagen and elastin (adipocytes, chondrocytes, osteocytes)

B.     Connective tissue

Different from other tissues because of large extracellular matrix to cell ratio

1.     Extracellular matrix – consists of fibers and ground substance (thick, viscous material)

2.     Ground substance – made of proteoglycans (protein/carbohydrate complexes) that are very hydrophilic

3.     Loose CT – packing tissues, including areolar tissue and adipose

4.     Dense CT – contains a large amound of fibers, like bone, cartilage, ligaments

 

VII.          10.7:  BONE STRUCTURE

A.     MACROSCOPIC

1.     Red marrow – found in spongy bone, site of hematopoesis

2.     Yellow marrow – inactive fatty marrow found in diaphysis

B.     MICROSCOPIC

1.     Bone composition – composed of collagen and hydroxyapetite (Ca2PO4 crystals)

2.     Spongy bone – made of spicules or trabeculae

3.     Compact bone unit – osteon (consists of central Haversian canal surrounded by lamellae

a)     Canniculi – tiny canals that branch out from central canals into spaces called lacunae which contain 1 osteocyte

b)     Volkmann’s canals – canals that run perpendicularly to Haversian canal

 

VIII.          10.8:  TISSUES FOUND AT JOINTS

A.     CARTILAGE – very flexible tissue secreted by chondocytes; avascular

1.     Hyaline cartilage – strong and somewhat flexible; rings of trachea, lining of joints

2.     Elastic cartilage – has more flexibility than hyaline cartilage; contains elastin (ear, epiglottis)

3.     Fibrous cartilage – very rigid; pubic symphisis, vertebral disks

B.     LIGAMENTS, TENDONS, AND JOINTS

1.     Amphiarthroses – slightly movable joints (vertebrae)

2.     Synarthroses – immovable joints (skull bones)

3.     Diarthrosis – freely movable joints (hips)

 

IX.          10.9:  BONE GROWTH AND REMODELING; THE CELLS OF THE BONE

A.     Endochondral ossification

Hyaline cartilage is produced then replaced by bone

B.     Intramembranous ossification

Synthesis of bone from an embryonic mesenchyme

Hormone Effect on bone Effect on kidneys Effect on intestines
PTH ↑ osteoclasts ↑ reabsorption of Ca++, stimulates conversion of vit D into calcitriol indirectly (via calcitriol) ↑ intestinal Ca++ absorption
Calcitriol minor ↑ osteoclasts ↑ reabsorption of phosphorus ↑ intestinal absorption of Ca++
Calcitonin ↓ osteoclasts ↓ reabsorption of Ca++ n/a

MCAT Study Guide Biology - Kim Matsumoto


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