Human Processes 1 Exam Revision

Blood, Skeletal, Muscular, Nervous, Endocrine

When and how are erythrocytes produced?

When low oxygen levels are detected, the kidneys release erythropoietin which is responsible for synthesising erythrocytes. Iron (from stored ferritin or dietary intake) is transported in the blood by transferrin and released to cells in the bone marrow. In addition to iron, folic acid and vitamin B12 are required for erythropoiesis. Approximately 200 billion erythrocytes are produced daily.

What happens as erythrocytes age? How are they broken down?

They last around 120 days. As they age, they lose structural integrity. Old cells undergo phagocytosis by macrophages in the spleen and liver. As they are broken down, haemoglobin is released and metabolised to bilirubin. The bilirubin enters the blood stream and travels to the liver where it’s further metabolised and the by-products are secreted in bile and the small intestine to be eliminated in the faeces.

What is haemostasis and its 3 phases?

The process of blood clotting in response to damaged blood vessels. Vascular (spasm), platelet (plug) and coagulation phases.

what is meant by a “committed cell” ?

path of specialisation is determined and cannot regress to its precursor (stem cell)

why might those with kidney disease suffer from anaemia?

when the kidney cells fail, they do not produce enough erythropoietin to sustain erythropoiesis in bone marrow

Explain the process of haemostasis (3 phases)

Vascular phase: trauma to the blood vessel stimulates nociceptors which trigger the smooth muscle to contract, causing vasoconstriction of the injured blood vessel. Endothelial cells become sticky. This allows time for clotting to occur, lasting 30 seconds.

Platelet phase: due to injury, collagen fibres are exposed. Platelets arrive at the site and attach to the sticky basal lamina and collagen fibres. Platelets release chemicals that make them sticky to other platelets (platelet aggregation), forming a plug.

Coagulation phase: blood becomes a gel. prothrombin is converted to thrombin which converts soluble fibrinogen to insoluble fibrin to form a mesh that traps erythrocytes and platelets to form a clot. Takes 30 seconds. Clot retraction is when platelets contract within fibrin mesh, reducing size of clot (30-60 min).

List the functions of blood

distribution (O2, waste, hormones)

regulation (pH, temperature, fluid/water)

protection (clotting, immunity)

Describe the composition and physical characteristics of whole blood

plasma (55%) and formed elements (1% Buffy coat of leukocytes and platelets; 45% erythrocytes)

Discuss the composition and functions of plasma

- Plasma proteins; Albumin, Globulins - α, β transport proteins; γ antibodies; Clotting proteins - Fibrinogen; Prothrombin

• Non-proteins - Urea, Lactic Acid, Creatinine

• Nutrients - Carbs, amino acids, fatty acids, Glycerol, Vitamins

• Electrolytes - Na+, K+, Ca2+, Mg2+, Cl-, HCO3-

• Gases - Respiratory O2 and CO2

• Other - Enzymes; Antigens; Antibodies; Hormones

Describe the structure and function of erythrocytes. What’s the benefit of its shape?

biconcave, anucleate, flattened, rich in haemoglobin. To transport gases (O2 and CO2).

→ Increased surface to volume ratio

→ All areas of cytoplasm close to membrane

→ Ability to squeeze through small vessels

Describe the composition of haemoglobin

Made up of 4 polypeptide chains (2 alpha 2 beta) and 4 heme groups containing iron. Can carry up to 4 oxygen molecules

List the classes, structural characteristics, and functions of leukocytes

(3) granulocyte (-phil), (2) agranulocyte (-cyte).

Defence against disease

Granulocytes (Bound granules)

• Neutrophils - Phagocytise bacteria, multilobed nucleus

• Eosinophils - Kill parasitic worms, bilobed nucleus, red granules

• Basophils - Mediators of inflammation (heparin, histamine), bilobed nucleus

Agranulocytes (Lack granules)

• Lymphocytes - Immune response via attack or antibodies

• Monocytes - Migratory phagocyte, kidney shaped nucleus

How are leukocytes are produced

Occurring in the bone marrow, they begin as haemocytoblast stem cells which divide and turn into myeloid or lymphoid stem cells. Then they divide and turn into one of 3 committed cells: myeloblast, monoblast or lymphoblast then follow their developmental pathway and mature in the bone marrow, with the exception of T cells that mature in the thymus (lymphocytes)

Describe the structure and function of platelets (thrombocytes)

2-4 um in animated, rough discs.

mature platelets are:

• bound by plasma membrane

• anucleate

• age rapidly (degenerate after 10 days)

• rich in serotonin granules to aid clotting process

Describe how platelets are produced

Haemeocytoblast → megakaryoblast → promegakaryoblast → megakarycyte → platelets

What is the body’s natural anticoagulant?

heparin (produced in the granules of mast cells and basophils)

Discuss the difference between monocytes and lymphocytes in their differentiation

They both start from a haemocytoblasts but the haemocytoblast differentiates into either a myeloid stem cell or lymphoid stem cell.

myeloid = monocyte (much larger)

lymphoid = lymphocyte (travel between blood and lymphatic organs)

Oxygen and carbon dioxide are poorly soluble in plasma so they require erythrocytes for their transport in blood. Describe how erythrocytes transport these gases.

Erythrocytes contain two proteins necessary for the transport of oxygen and carbon dioxide: hemoglobin and carbonic anhydrase.

Hemoglobin is a protein that contains four subunits, each of which has a heme group that can bind oxygen reversibly. Thus, hemoglobin can transport up to four oxygen molecules. Hemoglobin can also transport carbon dioxide, but the carbon dioxide binds reversibly to amino acids within the polypeptide chains, not to the heme group.

Carbonic anhydrase is an enzyme that catalyzes the reversible reaction converting carbon dioxide and water to carbonic acid. The carbonic acid can then dissociate into a hydrogen ion and bicarbonate ion. Most of the carbon dioxide transported in blood is transported as bicarbonate ion.

Describe the general format/prefix/suffix for cell differentiation

-blast (initial stem cell) → -blast (committed cell; first half of stem cell + -blast) → pro-(previous cell name)-cyte (drop -blast) → drop pro- → full cell name

e.g. haemocytoblast → myeloid stem cell → myeloblast → promyelocyte → myelocyte → neutrophil/eosinophil/basophil

e.g. hemocytoblast → myeloid stem cells → monoblast → promonocyte → monocyte

List the functions of haemoglobin

→ Hb loads O2 in the lungs and transports it to

the tissues where it is offloaded for cellular use

→ Hb loads CO2 (waste) and transports it

to the lungs where it is offloaded and exhaled to the

atmosphere (maintains acid-base homeostasis)

→ Hb has a role in vasodilation through its release

of small amounts of nitric oxide

Explain how blood clots are removed from the body

Plasminogen is converted to plasmin, which is fibrinolytic (dissolves clots)

Describe the functions of the skeleton and of bone tissue

support (hard framework to support soft organs)

protect (casing for soft organs against external forces)

movement (for muscles to use as anchors and levers)

mineral storage (deposit and withdrawal of Ca2+ and K+)

blood cell formation (synthesis occurring in marrow)

Name the major regions of the skeleton and describe their relative functions

Axial Skeleton (skull, vertebral column, thoracic cage) - 80 bones

• protection, support, carrying

Appendicular Skeleton (limbs, shoulders, hips) - 126 bones

• locomotion, environmental manipulation

Compare and contrast the four bone classes and provide examples of each class

Long Bones

• Consists of an elongated long shaft and two ends

• e.g: Finger bones, Femur

Short Bones

• Generally cube-shaped, and stubby

• e.g: Wrist, Ankles, Patella (Sesamoid)

Flat Bones

• Thin, flattened, and slightly curved

• e.g: Skull, Ribs, Sternum, Shoulder blades

Irregular Bones

• Complicated shapes not fitting any classes

• e.g: Vertebrae, Pelvis

Indicate the locations and functions of red and yellow marrow

Red:

• in flat bones (sternum, skull, ribs, spongy ends of long bones)

• produces RBCs, WBCs and platelets

• gets colour from haemoglobin in developing RBCs

Yellow:

• found in medullary cavities of long bones

• stores lipids

• gets colour from carotenoids in fat cells

Indicate the locations and functions of articular cartilage

• A form of hyaline cartilage (collagen)

• ribs, joint surfaces, diaphyses of long bones

• resists outward expansion due to water component

• support, flexibility, resilience

Distinguish between the epiphyseal disc and epiphyseal line

The epiphyseal disc is hyaline cartilage and the site of linear growth. When the cartilage is replaced by bone, a line of fusion remains

Indicate the locations and functions of periosteum and endosteum

Periosteum

• dual-layer covering external bone surface

  • outer layer (thick collagen fibres/fibrous) - anchor tendons, muscles and contains most blood vessels and nerves

  • inner layer (contains osteo stem cells)

• innervated, highly vascular and lymphatic

• cell repair, bone growth, anchor point for tendons and ligaments

Endosteum

• found internally, surrounding the trabeculae and medullary cavity

• single layer membrane

• internal bone remodelling (breakdown and growth) and reabsorption of internal bone tissue

Describe the histology of compact and spongy bone

Compact Bone (Hard)

• Osteon (Haversian System) - Mineralised, structural unit

• Haversian Canal - Contains capillary and nerve supply

• Volkmann’s Canal - Link blood and nerves to periosteum

Spongy Bone (Cancellous)

• Trabeculae - Align along stress lines

• Lamellae - Deposited by osteocytes in rings

• Canaliculi - Canals joining lamellae and osteocytes

• Endosteum - Surrounds trabeculae providing nutrition

Discuss the chemical composition of bone and the advantages conferred by its organic and inorganic components

• Osteoid - Organic; Collagen; Flexibility

• Mineral Salts - Inorganic; Ca, (PO4)2; Durability

Compare and contrast intramembranous ossification and endochondral ossification

Describe how epiphyseal plates allow long bones to grow

Compare the locations and remodeling functions of the osetoblasts, osteocytes, and osteoclasts

• Osteocytes - Mature bone cells; Former osteoblasts (lacunae, canaliculi)

• Osteoblasts - Bone ‘forming’ cells; Add matrix (endosteum, periosteum)

• Osteoclasts - Bone ‘degrading’ cells; Remove matrix (on hard bone matrix at sites of breakdown/repair); stem from WBCs

Explain how hormones and physical stress regulate bone remodeling

Parathyroid Hormone (PTH)

• Blood Ca2+ levels ↓; Bone matrix degraded; Ca2+ released

Calcitonin

• Blood Ca2+ levels ↑; Ca2+ deposited in bones; Ca2+ absorbed

Describe the steps of fracture repair

1. Haematoma Formation

• Torn vessels, clotting, phagocytes clear debris

2. Fibrocartilaginous Callus

• Collagen secreted, broken ends joined, spongy bone

3. Bony Callus

• Trabeculae appear in callus, compact bone forms

4. Bone Remodelling

• Excess materials removed, shaft walls reconstructed

Classify joints by structure and by function

Structurally:

• Fibrous - Immoveable or slightly moveable

• e.g: Skull sutures; Periodontal ligaments

• Cartilaginous - Immoveable and immobile

• e.g: Rib and sternum joints; Intervertebral discs

• Synovial - Highly moveable with fluid

• e.g: Majority of joints; Patella; Shoulder

Functionally:

• Syntharthroses - Immovable

• Amphiarthroses - Slightly moveable

• Diarthroses - Freely moveable

Describe the structural characteristics of synovial joints

→ Freely moveable joints

→ They have two articulating surfaces covered in hyaline cartilage

→ Surrounded by a fluid-filled joint capsule (synovial fluid)

→ Comprised of fibrous connective tissue

→ Joint capsule is lined with a synovial membrane

Name and provide examples of the six types of synovial joints based on the movement(s) allowed

→ Saddle (first carpometacarpal/thumb base)

→ Hinge (elbow, interphalangeal, ankle)

→ Plane (inter carpal joints)

→ Condyloid (wrist, knuckles)

→ Ball-and-socket (shoulder, hip)

→ Pivot (proximal radioulnar joints)

What is osteoporosis?

→ Bone density regulated by sex hormones

→ ↓ oestrogen = ↑ osteoclast activity

→ Bone matrix is degraded, bone density ↓

→ Increased risk of fractures (porous bones)

What is rickets?

→ Poorly mineralised bones occurring in children

→ Causes bow legs and bone deformities

→ Decreased calcium or Vitamin D in diet

→ Vitamin D promoted intestinal calcium absorption

Compare and contrast the three basic types of muscle tissue

• skeletal

  • striated, bundles, multiple nuclei in cells, attached to skeleton and skin, voluntary

• cardiac

  • striated, sheets, one nucleus, blunt-ended, joined by intercalated discs, found in heart walls, involuntary

• smooth

  • non-striated, sheets, once central nucleus, spindle shaped, walls of hollow organs, involuntary

List four important functions of muscle tissue

• thermoregulation (heat generation, piloerection)

• movement (locomotion, fine motor, digestion)

• posture & stability

• circulation and respiration

Describe the gross structure of a skeletal muscle

• Connective tissue coverings

  • Epimysium

    • Outer layer surrounding whole muscle

  • Perimysium

    • Middle layer surrounding muscle fibre bundles

  • Endomysium

    • Inner layer surrounding each muscle fibre

  • Continuous with tendons

    • Attach muscles to bones → bone is moved

• Fascicles are formed by bundled muscle fibres, composed of myofibrils

• Muscle fibres (often called cells)

• Myofibrils multinucleate contractile cells

  • Composed of myofilaments (molecular structure)

  • Defined by sarcomeres (contractile units)

• Nerve Supply

  • Muscle fibres supplied with motor units for contractibility

• Blood Supply

  • Waste removal, ATP delivery, O2 delivery

Describe the microscopic structure and functional roles of the myofibrils

• myofibrils are made of myofilaments

• Molecular structure of muscles

• composed of sarcomeres

• Thick filaments (myosin)

  • Two polypeptide chains, two globular heads, forming a myosin molecule

  • Heads contain ATP binding sites

  • contain 200 myosin molecules

• Thin filaments (actin)

  • Globular proteins linked into chains

  • These form actin molecule

  • Strengthened by tropomyosin protein strands

  • Contains myosin bindings sites (troponin)

Describe the functional roles of the sarcoplasmic reticulum

• specialised endoplasmic reticulum

• is the network surrounding each myofilament

• store, release and reabsorb Ca2+ (into cytosol) for muscle contraction and relaxation

• Terminal cisternae

  • Longitudinal cross channels

• T-tubules

  • Elongated tubes entering cell depths

Discuss the role of ionic calcium in muscle contraction

Actin filaments contain binding sites composed of troponin and tropomyosin (troponin complex)

When Ca2+ levels are low → Tropomyosin ‘blocks’ actin binding sites

When Ca2+ levels increase → Ca2+ binds to troponin causing shape change

Tropomyosin moves away from site → Binding sites free for myosin

Describe the sliding filament model of muscle contraction

Thick Filaments: globular heads

Thin Filaments: binding sites

Ca2+ ‘unlocks’ Actin binding sites causing:

1. Cross-Bridge Attachment

• Actin and Myosin bind

2. ‘Power’ Stroke

• Myosin pivots and pulls on Actin filament

3. Cross-Bridge Detachment

• Actin and Myosin detach

4. ‘Cocking’ of Myosin

• Myosin head resumes initial position

Filaments overlap causing contraction

What are the 4 fibre arrangements?

Parallel (Fusiform)

• Long, parallel, strap-like; Cover long distances

Convergent

• Fan-shaped; Converge at insertion

Pennate (Pinnate)

• Short; Many fibres per unit area

Circular

• Circular fibre bands; Surround body openings

What is the function of cholinesterase and the advantage of having it at the neuromuscular junction?

• To degrade acetylcholine in the synapse

• To prevent multiple impulse transmission

Describe the structural and functional relationships between the hypothalamus and the pituitary gland

→ Hypothalamus is connected to pituitary gland by infundibulum

→ Infundibulum contains neural tracts that carry hormones from the hypothalamus to the posterior pituitary (neural tissue)

→ Hypothalamus connects to anterior pituitary via hypophyseal portal system (glandular tissue)

→ Capillaries in the hypothalamus are connected to capillaries in the anterior pituitary via the infundibulum

Describe how hormones are classified chemically

lipid-soluble/steroid and water-soluble/protein/non-steroid

Describe the two major mechanisms by which hormones bring about their effects on their target tissues

Discuss the structure of the posterior pituitary and describe the effects of the two hormones it releases

List and describe the chief effects of anterior pituitary hormones

Identify the key differences between the posterior and anterior pituitary gland

Describe the effects of the two groups of hormones produced by the thyroid gland

Explain the process of thyroxine formation and release

Describe the hormonal functions of parathyroid hormone and calcitonin on bone homeostasis (feedback loops when too high/low)

Describe the function and regulation of the thyroid hormones T3 and T4

Thyroid hormones influence protein, carbohydrate, and lipid and vitamin metabolism, as well as heat production and cardiac function. Thyroid stimulating hormone, which is made and stored in the anterior pituitary, acts on the thyroid gland, stimulating the secretion of T3 and T4 as needed. Protein-bound T3 and T4 are circulated in the plasma and released as needed for metabolic processes. This process is regulated by a negative feedback loop involving thyroid releasing hormone, the anterior pituitary and the hypothalamus.

Describe the physiological processes involved in antidiuretic hormone (ADH) secretion

The secretion of ADH is primarily accomplished by osmoreceptors in the hypothalamus but also by baroreceptors in the left atrium, the carotids and the aortic arches. The posterior pituitary releases ADH. Mediators are cholinergic and adrenergic neurotransmitters. ADH secretion is increased by changes in intravascular volume and decreased plasma osmolality (increased solute concentration). Stress, trauma, pain, exercise, nausea, nicotine, exposure to heat and drugs can also increase secretion. Decreased plasma osmolality, increased intravascular volume, hypertension and alcohol ingestion decreases the ADH secretion.

List hormones produced by the adrenal gland, and cite their physiological effects

• aldosterone - Na+ control via kidneys

• cortisol - Immune response and alertness (ie waking)

• adrogens - Gonad development

• adrenaline - fight or flight (sympathetic NS stimulation)

Compare and contrast the two effects of the two major pancreatic hormones

Identify factors that influence activation of a target cell by a hormone

Which hormones are secreted by the posterior pituitary gland?

oxytocin and antidiuretic hormone

Differentiate between a nucleus, a ganglion, a nerve, and a tract

Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems

Explain the structural and functional divisions of the nervous system

List the types of neuroglia and cite their locations

Classify neurons by structure and by function

Define resting membrane potential and describe its electrochemical basis

Explain how action potentials are generated and propagated along neurons

Define absolute and relative refractory periods

Distinguish between electrical and chemical synapses by structure and by the way they transmit information

Define neurotransmitter and classify neurotransmitters by chemical structure

Explain lateralisation of cortical function

Describe the location of the diencephalon, and name its subdivisions and functions

List the major lobes, fissures, and functional areas of the cerebral cortex

Name and locate the ventricles of the brain

Lateral (2) in either hemisphere

Third (in diancephalon)

Fourth (between brain stem and cerebellum)

Identify the three major regions of the brain stem, and note the functions of each area

Describe how meninges, cerebrospinal fluid, and the blood brain barrier protect the CNS

Meninges (physical and structural support)

• dura, arachnoid, pia mater (connective tissue)

• Together, these layers act as a tight physical sheet and shock absorber that restrains the brain, keeping it from slamming against hard bone during sudden physical impact

Cerebrospinal Fluid (CSF): Mechanical Cushioning & Waste Removal

• flows through brains ventricles into subarachnoid space

• buoyant suspension, shock absorption, supplies nutrients, clear metabolic waste and lymphatic drainage

BBB: biochemical defence (internal environment protection from threats in the bloodstream)

• selective permeability

• presence of tight endothelial junctions to prevent large molecules and blood borne pathogens to enter

• last line of defence against erratic nerve impulses or stray toxins and neurotransmitters (break down via enzymes)

Explain how cerebrospinal fluid is formed and describe its circulatory pathway

• formed by choroid plexus in all ventricles

Describe the gross and microscopic structure of the spinal cord

List the sensory receptors, their functions and locations

Describe sensory perception and adaptation

Name the 12 pairs of cranial nerves; indicate the body region and structures innervated by each

Name the major plexuses and describe the function of the peripheral nerves arising from each plexus

Cervical

Brachial

Lumbar

Sacral

Name the components of a reflex arc and distinguish between autonomic and somatic reflexes

1. Receptor: Site of stimulus action

2. Sensory Neuron: Afferent pathway toward CNS

3. Integrator: Synaptic interchange within CNS

4. Motor Neuron: Efferent pathway away from CNS

5. Effector: Muscle or gland response

Autonomic:

• smooth muscle, cardiac muscle and glands

• inhibitory OR excitatory/stimulatory

• ACh (nerves) OR norepinephrine (blood)

Somatic:

• skeletal muscle

• stimulatory only

• ACh only

• single neuron from CNS to effectors

Define cholinergic and adrenergic receptors (ANS)

cholinergic:

• binds to ACh

• 2 types:

  • nicotinic (always stimulatory)

  • muscarinic (inhibitory or stimulatory)

adrenergic:

• binds to adrenaline (dumped directly into blood as hormone) and noradrenaline (neurotransmitter)

• can be inhibitory or stimulatory

• 2 types:

  • alpha

  • beta

Compare and contrast the functions of the parasympathetic and sympathetic divisions

• both part of autonomic nervous system

• 
  

Sympathetic: emergency situations

• ‘Fight or flight’ (positive feedback)

Parasympathetic: energy conserving

• ‘Rest and digest’ (negative feedback)

Define autonomic nervous system and explain its relationship to the peripheral nervous system

The myelination of central and peripheral axons occurs rapidly through the first few years of life. How can this developmental process explain the improved motor abilities of infants and toddlers?

Without full myelination, information about limb movement and body position moves slowly to the CNS, and motor commands move slowly and erratically to the muscles. By the time the brain is aware of movement or position and can issue a motor command, the limb has already moved. When the motor command reaches the skeletal muscle, the response is no longer appropriate. As the neurons become fully myelinated, information processing speeds up, so we observe improved balance, coordination, and movement.

Mary, age 15, is watching a scary movie late at night. She notices that her heart is racing and she is beginning to feel cold and clammy. From what you have read in this chapter, what is causing her to feel this way?

Mary is experiencing stimulation of her sympathetic nervous system. Neurotransmitters such as adrenaline and noradrenaline are being released to prepare her for a ‘fight or flight’ response. This is the same response that Mary would experience if she were in a true life-threatening situation.

What 3 elements make up the neuromuscular junction?

presynaptic membrane

synaptic cleft

postsynaptic membrane

What occurs BEFORE the muscle contracts? (Hint: calcium)

Calcium comes in and binds to TROPONIN → TROPOMYOSIN can roll off → expose myosin binding sites and actin

Explain step-by-step how muscle contraction occurs

1. Action potential arrives at neuromuscular junction via motor neuron.

2. neurotransmitter acetylcholine (ACh) is released into synaptic cleft/synapse

3. ACh binds to post synaptic membrane (the sodium channels on the motor end-plate of sarcolemma), creating an end-plate potential.

4. Depolarisation of sarcolemma allows action potential to be propagated across the sarcolemma

5. Ca2+ is released into the sarcoplasm.

6. at rest, tropinin and tropomysoin wrap around actin, blocking the myosin binding sites

7. the Ca2+ ions bind to the troponin, allowing the tropomysoin to roll away and expose the myosin binding sites on the actin strands

8. myosin head attaches to the actin myofilament, forming a cross bridge

9. inorganic phosphate generated from previous contraction cycle is released, initiating the power stroke. Myosin head pivots and bends as it pulls on actin filament sliding towards the m line. ADP is released

10. As new ATP attaches to myosin head, the myosin-actin link weakens → cross-bridge detaches

11. ATP splits into ADP and inorganic phosphate (ATP hydrolysis), the myosin head is energised, ready for cycle to occur again.

What is the cross-bridge formation?

Calcium ions bind to troponin, moving tropomyosin away and exposing the binding sites on actin. The energised myosin head (holding ADP and inorganic phosphate) attaches to actin;

With the actin-binding site unblocked and the myosin head primed with ADP and inorganic phosphate still attached, the myosin head reaches out and binds firmly to the actin filament, forming a physical cross-bridge.

What is the power stroke? What happens?

When the myosin head releases the inorganic phosphate, the myosin head pivots and pulls the actin filament toward the centre of the sarcomere (the M-line). ADP is released during or after this pivoting motion.

The release of inorganic phosphate from the myosin head triggers the power stroke

Why does rigor mortis occur?

• Loss of ATP: When a person dies and breathing stops, cells are deprived of oxygen and can no longer produce ATP, the energy molecule required for muscle relaxation.

• Actin and Myosin Locking: Without ATP to detach them, the actin and myosin bind together and lock in place.

• Calcium Leaks: As cell membranes break down postmortem, calcium floods the muscle fibers, intensifying the contraction and rigidity

compare the endocrine and nervous systems