Questions and Answers
Which germ layer contributes to the development of the neural tube?
At what stage of gestation does the formation of the neural tube begin?
What is the most common anomaly of the CNS?
What is the significance of maternal folate deficiency in relation to NTD?
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What is the primary method of diagnosis for NTD?
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What is the time frame for surgical repair of meningocele and myelomeningocele?
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What is the term for the closure of the neural tube in two directions?
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What is the name of the structure formed by the embryo by day 14 of gestation?
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What is the characteristic of Spina Bifida Occulta?
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What is the typical complication of Myelomeningocele?
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What is the characteristic of Type II Arnold Chiari Malformation?
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What is the purpose of Maternal serum alpha fetoprotein (MSAFP) testing?
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What is the detection rate of spina bifida using fetal ultrasound examination in a high-risk population?
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What is the characteristic of Anencephaly?
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What is the ideal gestation period for Maternal serum alpha fetoprotein (MSAFP) testing?
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What is the percentage of cases of neural tube defects that occur in pregnancies without any risk factors?
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Which of the following types of receptors is responsible for detecting pain stimuli in the viscera?
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Which of the following is a characteristic of the sympathetic nervous system?
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Which of the following is a target tissue of the autonomic nervous system?
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What is the function of the preganglionic neuron in the autonomic nervous system?
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What is the main difference between the autonomic nervous system and the somatic nervous system?
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Which of the following is a characteristic of the parasympathetic nervous system?
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What is the function of the mechanoreceptors in the autonomic nervous system?
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Which of the following receptors is responsible for bronchodilation?
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What is the effect of nicotine on nicotinic receptors?
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What is the primary function of muscarinic receptors in the GI tract?
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What is the effect of atropine on the cardiac conduction system?
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What is the primary function of β1-adrenergic receptors in the heart?
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What is the effect of α2-adrenergic receptors on neurotransmitter release?
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What is the mechanism of action of Varenicline in smoking cessation?
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What is the location of the sympathetic trunk in the body?
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Which neurotransmitter is released directly into the bloodstream by the adrenal medulla?
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What is the role of the parasympathetic nervous system?
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Which of the following organs is NOT innervated by the parasympathetic nervous system?
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What is the characteristic of the postganglionic fibers of the parasympathetic nervous system?
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What is the receptor type for the neurotransmitter acetylcholine in the autonomic nervous system?
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What is the effect of α1-adrenergic receptors on the body?
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What is the primary neurotransmitter released by the sympathetic nervous system?
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What is the anatomical difference between the sympathetic and parasympathetic nervous systems?
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What is the effect of the parasympathetic nervous system on cardiac conduction?
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Where does the spinal cord originate from?
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What is the function of the ventral horn in the spinal cord?
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What is the term for the nerve fibers projecting from the end of the spinal cord beyond L1?
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What is the neurotransmitter used by upper motor neurons?
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What is the characteristic of spinal reflexes?
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What is the location of the cell bodies of lower motor neurons?
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Which type of reflex arc is responsible for maintaining posture?
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What type of muscle recruitment occurs in the stretch reflex arc?
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What is the purpose of the Golgi Tendon Reflex?
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What type of fibers decussate immediately at their level of entry into the spinal cord?
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What is the significance of dermatomes and myotomes in spinal cord organization?
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What is the primary function of ascending tracts?
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What is the clinical significance of the stretch reflex arc?
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What is the purpose of reciprocal innervation in the stretch reflex arc?
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In the corticospinal tract, which neurons are medial and which are lateral?
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What is the main difference between a complete and incomplete spinal cord injury?
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What is the primary mechanism of injury in a cord concussion?
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What is the characteristic clinical presentation of central cord syndrome?
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What is the primary cause of cord compression?
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What is the effect of excitotoxicity on neurons?
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What is the primary location of hemorrhage in spinal cord injuries?
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What is the primary difference between primary and secondary spinal cord injury?
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What is the effect of infarction on the spinal cord?
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Why do arms tend to be more affected than legs in central cord syndrome?
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What is the most common cause of Anterior Cord Syndrome?
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In Brown-Sequard Syndrome, which type of sensations would be preserved below the level of injury?
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What is the underlying cause of Subacute Combined Degeneration?
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What is the primary method of treatment for Autonomic Dysreflexia?
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Which type of spinal cord injury is characterized by motor impairment greater in the arms and sacral sparing?
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What is the primary function of Vitamin B12 in the spinal cord?
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Which of the following is NOT a clinical manifestation of Autonomic Dysreflexia?
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In Brown-Sequard Syndrome, which tracts are damaged on the opposite side of the injury?
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What is the level of spinal cord injury above which Autonomic Dysreflexia is more likely to occur?
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What is the primary method of distinguishing between incomplete and complete Spinal Cord Injury (SCI)?
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What is the primary mechanism by which autoimmune diseases originate?
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What is the primary mechanism by which immune complexes cause injury in SLE?
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What is the primary reason why SLE is difficult to diagnose?
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Which of the following organs is least likely to be affected by SLE?
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What is the characteristic of the primary pathogenesis in osteoarthritis?
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What is the most common joint disease?
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What is the leading cause of disability in middle-aged and older populations in the US?
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What is the characteristic of proteoglycans in articular cartilage?
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What is the effect of the loss of proteoglycans from articular cartilage?
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What is the characteristic of the relationship between age and osteoarthritis?
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What is the effect of obesity on the risk of osteoarthritis?
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What is the characteristic of Ankylosing Spondylitis?
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What is the characteristic of Rheumatoid Arthritis?
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What is the characteristic of Gout?
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What percentage of the body's phosphate is found in the bones?
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Which type of bone is characterized by being longer than they are wide?
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What is the primary function of osteoblasts?
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Which of the following bones protects the spinal cord?
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What is the function of the skeletal system in movement?
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What percentage of bone cells are osteocytes?
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Which type of tissue is found in bones, supporting the bone cells?
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Which of the following bones is an example of a flat bone?
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What is the primary function of canaliculi in spongy bone?
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What is the process by which the bony skeleton is formed in embryos?
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What is the function of osteoblasts in intramembranous ossification?
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What is the characteristic of trabeculae in spongy bone?
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What is the purpose of step 3 in intramembranous ossification?
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What is the primary function of osteoclasts in bone growth, development, maintenance, and repair?
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What is the characteristic of endochondral ossification?
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What is the function of the periosteum in intramembranous ossification?
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What type of bone marrow is responsible for producing blood cells?
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What is the term for the delicate mesh of reticular tissue saturated with immature red blood cells and scattered adipocytes?
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What is the characteristic of compact bone formed through intramembranous ossification?
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What is the purpose of the periosteum?
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What is the name of the structural units in compact bone?
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What is the function of Volkman's canals?
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What is the term for the incomplete lamellae that fill the gaps between osteons or are remnants of bone remodeling?
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What is the primary component of the inorganic part of bone matrix?
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What is the delicate connective tissue membrane that covers internal bone surfaces?
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What occurs when large chondrocytes grow in endochondral ossification?
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What happens to the cartilage near the epiphysis in endochondral ossification?
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What is the function of the epiphyseal plate?
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What is the process by which diaphyseal cartilage is transformed into bone?
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What is the purpose of the secondary ossification center in endochondral ossification?
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What is the result of the division of epiphyseal cartilage in endochondral ossification?
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What is the primary function of osteoblasts beneath the periosteum?
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What is the result of the combined activities of growth hormone, thyroid hormone, and the sex hormones during puberty?
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What is the purpose of Wolff's law in bone remodeling?
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Which of the following nutrients is necessary for bone growth and maintenance?
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What is the function of osteoclastic cells in bone remodeling?
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What is the percentage of calcium found in bone and teeth?
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What is the purpose of calcitriol in the intestine?
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What is the sequence of bone remodeling?
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What type of fracture is common in the elderly due to brittle bones?
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What is the term for the new connective tissue and microscopic blood vessels that form on the surfaces of a wound during the healing process?
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What is the result of avascular necrosis in the proximal scaphoid bone?
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What is the typical timeframe for the conversion of the fibrocartilaginous callus into a bony callus of spongy bone?
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What type of fracture occurs when the bone breaks incompletely, with one side bent and the other side broken?
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What is the significance of the periosteum/endosteum in fracture repair?
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What is the result of vascular interruption in fracture repair?
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What percentage of calcium in the blood is ionized and metabolically active?
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What is the primary function of parathyroid hormone?
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What is the characteristic of osteomalacia?
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What is the effect of glucocorticoids on bone?
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What is the primary cause of osteoporosis in postmenopausal women?
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What is the effect of calcitonin on bone?
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What is the primary function of insulin in bone?
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What is the characteristic of rickets?
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What is the effect of growth hormone on bone?
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What is the primary function of thyroid hormone in bone?
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Study Notes
Nervous System Development
- Formation of the neural tube occurs between 3-4 weeks of gestation
- Development of the forebrain from the neural tube occurs between 2-3 months of gestation
- Neuronal proliferation and migration occur between 3-5 months of gestation
- Formation of network connections and synapses occurs from 5 months gestation to many years postnatal
- Myelination occurs from birth to many years postnatal
Embryonic Development
- By day 14 of gestation, the embryo develops into a disc-shaped structure called the Prechordal Plate
- The Prechordal Plate consists of three main layers: Ectoderm, Mesoderm, and Endoderm
- The neural tube develops from the Ectoderm
- The Mesoderm contributes to the cardiovascular tube
- The Endoderm contributes to the gut tube
Neural Tube Defects (NTD)
- NTDs are the most common anomaly of the CNS
- They originate during the first month of embryonic development when the neural tube fails to close completely
- Maternal folate deficiency is associated with a significant risk of NTD
- Other risk factors include maternal diabetes and maternal use of some seizure medications like valproic acid
Types of NTD
- Spina Bifida: the most common NTD, characterized by a small opening in the vertebrae
- Spina Bifida Occulta: a small opening in the vertebrae, but the defect is fully covered by skin and there is no visible exposure of meninges or neural tissues
- Meningocele: a saclike cyst of meninges filled with spinal fluid that protrudes through a defect in the posterior arch of the vertebra
- Myelomeningocele: a herniation of a saclike cyst containing meninges, spinal fluid, and a portion of the spinal cord with its nerves through a defect in the posterior arch of a vertebra
Complications of Myelomeningocele
- Hydrocephalus occurs in 85% of infants with myelomeningocele
- Almost always associated with Type II Arnold Chiari malformation
- Spinal cord and nerve roots may be malformed distal to the lesion, resulting in loss of motor, sensory, reflex, and autonomic function
Arnold Chiari Malformation
- A complex malformation of the brainstem and cerebellum
- Type I: does not involve the brainstem and may be asymptomatic
- Type II: associated with hydrocephalus from pressure that blocks the flow of CSF
- Type III: the brainstem or cerebellum extends into a high cervical myelomeningocele
- Type IV: the cerebellum does not develop
Anencephaly
- A condition where the soft, bony component of the skull and part of the brain are missing
- Infants are stillborn or die within the first few days of life
- Diagnosis is usually made during prenatal ultrasound or with screening maternal serum alpha fetoprotein levels
Prenatal Screening for NTD
- All pregnant people are offered screening for NTDs
- 90-95% of cases occur in pregnancies without any risk factors
- Screening tests include:
- Maternal serum alpha fetoprotein (MSAFP): a fetal-specific globulin that is synthesized by the yolk sac, GI tract, and liver; elevated MSAFP levels are associated with NTDs
- Fetal ultrasound: detection of NTDs on ultrasound examination depends on the size and location of the defect, the position of the fetus, the volume of amniotic fluid, maternal body habitus, and the skill and equipment of the sonographer/sonologist
Autonomic Nervous System (ANS)
- Function: Maintains steady state among organs; regulates cardiac muscle, smooth muscle, and glands
- Targets: smooth muscles, glands, cardiac muscle, and cardiac electrical conduction system
Target Tissues
- Smooth muscles: blood vessel walls, alimentary canal, and urinary bladder
- Glands: sweat glands, respiratory and GI tract glands
- Cardiac muscle and cardiac electrical conduction system
Peripheral Nerves
- Special sensory receptors: gather and communicate information from target tissues, including skin, internal organs, and associated blood vessels
- Receptors: chemoreceptors, mechanoreceptors, and nociceptors
Sensory Component (Afferent)
- Long processes: from specialized receptors in peripheral tissues to cell bodies in the dorsal root ganglia of the spinal cord
Motor Component (Efferent)
- Two-neuron system: preganglionic (myelinated) and postganglionic (unmyelinated)
- Most prominent: Vagus nerve
Autonomic Nervous System Divisions
- Sympathetic: "Fight or Flight" response
- Parasympathetic: conserves/restores energy
Sympathetic Nervous System
- Organization: paravertebral ganglia (two longitudinal chains) and prevertebral ganglia (nerve tissue nodes anterior to the spinal column)
- Primary neurotransmitters: epinephrine and norepinephrine
- Preganglionic neurons: travel in the sympathetic trunk and synapse with postganglionic neurons at various levels
Parasympathetic Nervous System
- Organization: neuroanatomically distinct from the sympathetic nervous system, with ganglia located near or embedded in target organs
- Primary neurotransmitters: acetylcholine
- Affects cardiac conduction
- Does not innervate uterus, sweat glands, skeletal muscles, or adrenal gland
Anatomical Differences between Sympathetic and Parasympathetic Divisions
- Origin: sympathetic (thoracolumbar region) vs. parasympathetic (craniosacral region)
- Location of ganglia: sympathetic (close to spinal cord) vs. parasympathetic (close to target organs)
- Length of postganglionic fibers: sympathetic (long) vs. parasympathetic (short)
- Postganglionic branching: sympathetic (lots) vs. parasympathetic (little)
Adrenal Medulla
- Endocrine gland that behaves like a sympathetic ganglion
- Sympathetic nerve fibers from sympathetic trunk → Adrenal Medulla → Epi/Norepi released directly into bloodstream
Autonomic Nervous System - Neurotransmitters
- Sympathetic preganglionic fibers: acetylcholine (cholinergic receptor)
- Sympathetic postganglionic fibers: norepinephrine (adrenergic receptor)
- Parasympathetic preganglionic and postganglionic fibers: acetylcholine (cholinergic receptor)
Adrenergic Receptors
- Class of G-protein coupled receptors that are the target for catecholamines (epinephrine, norepinephrine)
- α-adrenergic receptors: α1 (excitation/stimulation), α2 (relaxation/inhibition)
- β-adrenergic receptors: β1 (increases heart rate and contractility), β2 (relaxes muscles in bronchi, bladder, and facilitates other β receptors)
Clinical Correlations
- Vasopressors/Inotropes: give patients with shock "exogenous sympathetic nervous system support" with Norepinephrine, Epinephrine, Dopamine, and inotropic drugs like Dobutamine, Milrinone
- Smoking: nicotine acts as a nicotinic receptor agonist, involved in reward pathways with dopamine
- Quitting smoking: use a nicotinic receptor antagonist, such as Varenicline (Chantix)
- Atropine: muscarinic receptor antagonist, used to treat bradycardia, dilate pupils, and as an antidote for organophosphate poisoning
The Spinal Cord
- Lies within the vertebral column and is covered by the meninges
- Originates in the medulla oblongata and carries two types of information:
- Efferent motor neuron outputs from the brain
- Afferent sensory neural inputs to the brain from the body
- In the adult, the spinal cord extends to the lower border of L1, with the conus medullaris at L1/L2 and the cauda equina at S5
Spinal Cord: Gray Matter Horns
- Dorsal Horn:
- Axons/interneurons of sensory (afferent) neurons
- Sensory fibers travel through the dorsal roots to the posterior horn of the gray matter
- Ventral Horn:
- Cell bodies for motor (efferent) pathways leaving the spinal cord
- Motor fibers travel out through the ventral roots from the anterior horn of the gray matter
Upper and Lower Motor Neurons
- Upper Motor Neurons:
- Cell bodies live in the motor cortex of the brain with axons reaching down to a certain vertebral level in the corticospinal tract
- Crosses from one side of the body to the other at the level of the brain stem
- Use glutamate as their neurotransmitter
- Lower Motor Neurons:
- Cell bodies live in the ventral horn of the spinal cord with axons reaching out into the peripheral nervous system (PNS)
- Use acetylcholine as their neurotransmitter at the neuromuscular junction
Spinal Reflexes
- Definition: Involuntary, near-instant response to a stimulus that does not involve input from the brain
- Types of reflex arcs:
- Autonomic (visceral organs)
- Somatic (muscles)
- Stretch reflexes (maintaining posture, deep tendon reflex, Golgi tendon reflex)
- Crossed extensor reflexes (compensating for a stimulus on one side of the body)
- Flexor/withdrawal reflexes (response to noxious stimulus)
Stretch Reflex Arc
- Four processes:
- Monosynaptic response (information only goes through one synapse)
- Reciprocal innervation (opposing muscle group receives efferent message)
- Synergistic muscle recruitment (accessory muscle group receives efferent message)
- Ascending information (afferent fibers travel to the brain)
Clinical Correlation – Stretch Reflex
- Operates as a protective mechanism to prevent strain/tear of muscles and tendons
- When the muscle is stretched/engaged, an automatic impulse to contract helps to protect it from being pulled forcefully or stretched beyond normal ROM
- Clinically, we can get information about the spinal cord, peripheral nerves, and muscle tone/strength by checking reflexes
Spinal Cord Organization
- Dermatomes and myotomes:
- Sensory (dermatomes)
- Motor (myotomes)
- Ascending and descending tracts:
- Ascending tracts (afferent/sensory): spinothalamic, dorsal column
- Descending tracts (efferent/motor): corticospinal
- Afferent pathways (ascending):
- Spinothalamic (vague touch, pain, temperature)
- Dorsal columns (basic perception of touch, proprioception, vibration)
- Efferent pathways (descending):
- Corticospinal tracts (efferent/motor control below the head, contain upper motor neurons)
Spinal Cord Injuries
- Complete vs incomplete spinal cord injury:
- Complete spinal cord transection: interrupts all three tracts with complete loss of motor/sensory function below the level of injury
- Incomplete spinal cord injury: variable degree of dysfunction due to only part of the spinal cord being affected
- Types of spinal cord injuries:
- Cord concussion (transient dysfunction with or without vertebral damage)
- Cord contusion (bruising of the spinal cord leading to bleeding, edema, and tissue death)
- Cord compression (pressure on the spinal cord from an external source)
- Cord transection (tear within the spinal cord itself, compromising blood supply and CSF)
- Hemorrhage and infarct:
- Hemorrhage: typically due to trauma or vascular malformation, leading to edema and tissue destruction
- Infarct: blockage of vertebral arteries due to atherosclerosis, inflammation, or blood clots, leading to tissue death
Evolution of Spinal Cord Injury
- Primary cord injury: the actual trauma/compression/transaction
- Secondary cord injury: cascade of events leading to swelling and tissue destruction, starting within minutes and lasting for weeks
Incomplete Spinal Cord Injuries (SCI)
- Central cord syndrome:
- Most common incomplete spinal cord injury
- Most common cause: hyperextension injury
- Clinical presentation: motor impairment greater in the arms than the legs, variable sensory loss below the level of injury, sacral sparing
- Anterior cord syndrome:
- Most common cause: hyperflexion
- Clinical presentation: variable loss of motor/sensory function below the injury, but posterior/dorsal column is intact
- Brown-Sequard syndrome:
- Injury damaging one side of the cord
- Clinical presentation: motor/dorsal column sensory loss on the same side of injury, and spinothalamic sensory loss on the opposite side
Other Spinal Cord Diseases
- Subacute combined degeneration:
- Cause: B12 deficiency
- Seen more often in elderly, malnourished patients
- Demyelination of dorsal/posterior columns and lateral corticospinal tracts
- Clinical presentation: sensory deficits, weakness, ataxia, and gait disturbances
- Autonomic dysreflexia (hyperreflexia):
- Massive, sudden sympathetic discharge that can be life-threatening
- Seen in spinal cord lesions above T6, with a higher risk at higher levels of injury
- Clinical manifestations: severe hypertension, bradycardia, headache, blurred vision, flushed skin, and sweating
Autoimmune Reaction and Autoimmune Diseases
- Autoimmune reaction causes inflammation in areas of the body where it is not needed, leading to pain, swelling, and organ damage.
- Autoimmune diseases originate from an initiating event in a genetically predisposed individual, leading to an autoimmune mechanism that affects specific target tissues or cells.
Systemic Lupus Erythematosus (SLE)
- More common in women (10:1) and in the 20-40 years old age group.
- A chronic, multisystem, inflammatory disease with frequent remissions and exacerbations.
- Pathophysiology involves production of a large variety of autoantibodies, including ANA, anti-DS DNA, anti-Smith, and APLA.
- Autoantibodies are typically against nucleic acids, histones, ribonucleoproteins, and other nuclear materials.
- Immune complexes (DNA + antibody) cause inflammatory lesions in various organs, including the kidneys, brain, heart, spleen, lung, gastrointestinal tract, skin, and peritoneum.
- Most common clinical manifestations include:
- Arthralgias or arthritis (90%)
- Vasculitis and rash (malar and discoid)
- Renal disease (40% to 50%)
- Hematologic abnormalities (50% with anemia being the most common complication)
- Cardiovascular diseases (30% to 50%)
- Diagnosing SLE is challenging due to the intermittent nature of symptoms affecting almost every body system.
Arthropathies
- Joint diseases can be classified into:
- Non-inflammatory arthropathy (Osteoarthritis)
- Inflammatory Arthropathies (Rheumatoid Arthritis, Gout, Ankylosing Spondylitis)
Osteoarthritis
- Most common joint disease and leading cause of disability in middle-aged and older populations in the US.
- Most often affects the knees, hip, neck, hands, and lower back.
- Characterized by:
- Local areas of loss and damage of articular cartilage
- Bone spurs (osteophytes)
- Subchondral bone changes
- Variable degrees of mild synovitis
- Thickening of the joint capsule
- Susceptibility to osteoarthritis is influenced by:
- Joint vulnerability (systemic factors: age, female, genetics, nutritional factors)
- Intrinsic factors (previous damage, muscle weakness, malalignment, proprioception deficiency)
- Joint load factors (obesity, physical activities prone to injury)
- Pathogenesis involves degeneration, loss, and disordered repair of articular cartilage, leading to:
- Increased remodeling of articular cartilage
- Loss of the smooth, frictionless joint
- Cartilage becomes thin and may be absent over some areas
- Loss of proteoglycans from articular cartilage
- Clinical manifestations include:
- Joint pain and stiffness
- Stiffness in the morning and usually brief
Gout
- Clinical manifestations:
- Increase in serum urate concentration
- Recurrent attacks of monoarticular arthritis
- Severe pain, swelling, tenderness, erythema
- 50% of initial attacks are in the great toe metatarsophalangeal joint
- Tophi: Deposits of monosodium urate in and around the joints
- Renal disease involving glomerular, tubular, and interstitial tissues and blood vessels
- Formation of renal stones
- Three clinical stages:
- Asymptomatic Hyperuricemia
- Acute Gouty Arthritis
- Tophaceous Gout
The Human Skeletal System
- Composed of the body's bones and associated ligaments, tendons, and cartilages
- Functions: support, protection, movement, reservoir for minerals/adipose tissue, and hematopoiesis
Bone Classification
- 4 types of bones: long, short, flat, and irregular
- Long bones: longer than they are wide, found in limbs except patella and wrist/ankle bones, consists of shaft and 2 expanded ends
- Short bones: roughly cube-shaped, found in wrist and ankle bones
- Flat bones: thin, flattened, and curved, found in scapulae, sternum, ribs, and most skull bones
- Irregular bones: have weird shapes, fit none of the previous classes, found in vertebrae, hip bones, sphenoid, and ethmoid bones
Bone Components
- Bones are organs composed of multiple tissue types
- Components: osseous tissue, fibrous connective tissue, cartilage, vascular tissue, lymphatic tissue, adipose tissue, and nervous tissue
Bone Functional Cells
- Osteoblasts: bone-building cells, found in periosteum and endosteum, synthesize and secrete collagen and bone matrix, initiate calcification process
- Osteocytes (90-95% of bone cells): mature bone cells, former osteoblasts trapped in matrix, maintain bone tissue, signal osteoblasts and osteoclasts
- Osteoclasts: huge cells created by monocyte fusion, digest bone matrix, part of bone growth and development, concentrated in endosteum
Bone Structure
- Bone matrix: consists of 1/3 organic and 2/3 inorganic components
- Organic component: proteins secreted by osteoblasts, mainly collagen, provides bone resilience and resistance to stretching
- Inorganic component: mainly calcium phosphate and hydroxide, interacts to form hydroxyapatite, contains magnesium, fluoride, and sodium
Bone Marrow
- Bone marrow: soft tissue occupying medullary cavity, spaces among trabeculae, and haversian canals
- 2 main types: red and yellow
- Red bone marrow: hematopoietic tissue, produces blood cells, found in axial skeleton, pectoral girdle, pelvic girdle, and proximal heads of humerus and femur
- Yellow bone marrow: inactive, no longer produces blood, but can transform back to red marrow in severe anemia
Layers of Bone
- Endosteum: delicate connective tissue membrane, covers internal bone surfaces, lines canals and trabeculae, contains osteoblasts and osteoclasts
- Periosteum: double-layered membrane, covers external bone surface except joint surfaces, contains osteoprogenitor cells, blood vessels, nerves, and lymphatics
Compact Bone
- Consists of multiple cylindrical structural units (osteons or haversian systems)
- Components: haversian canal, concentric lamellae, lacunae, osteocytes, canaliculi, veins, arteries, nerves, and lymphatics
- Function: weight-bearing pillars, arranged parallel to long axis of long bone, withstand twisting forces
Spongy Bone
- Less organized than compact bone, no Haversian systems
- Trabeculae: align along stress positions, creating cross-bracing for compact bone, contain irregularly arranged lamellae and osteocytes
Bone Development and Remodeling
- Ossification: process of bone tissue formation
- Bone development: embryonic formation of bony skeleton, bone growth, and bone remodeling
- Intramembranous ossification: development of bone from fibrous membrane, occurs in skull cap, facial bones, clavicles, pelvis, and scapulae
- Endochondral ossification: replacement of hyaline cartilage with bone, most bones develop via this model
Endochondral Ossification
- Begins with a cartilaginous model of the bone
- Large chondrocytes grow, creating big spaces in the middle of the bone
- Death of chondrocytes leaves large spaces, which create a path for blood vessels to grow in
- Blood vessels bring osteoblasts, which create ossification centers
- Compact bone develops around the sides of the shaft
- Bone replaces cartilage near the epiphysis
- Secondary ossification centers are created within each epiphysis, forming the epiphyseal growth plate
Growth in Bone Length
- Epiphyseal cartilage (near epiphysis) of the epiphyseal plate divides to create more cartilage
- Diaphyseal cartilage (near diaphysis) of the epiphyseal plate is transformed into bone, increasing shaft length
Growth in Bone Thickness
- Osteoblasts beneath the periosteum secrete bone matrix on the external surface of the bone, making it thicker.
- Osteoclasts on the endosteum break down bone and widen the medullary cavity, resulting in an increase in shaft diameter.
- At puberty, growth in bone length is increased dramatically by the combined activities of growth hormone, thyroid hormone, and the sex hormones.
Bone Remodeling
- Bone is a dynamic tissue that grows or remodels in response to the forces or demands placed upon it (Wolff's law).
- Bone remodeling allows the skeleton to respond to mechanical loading, repair and prevent microdamage, and release growth factors and minerals stored in bone matrix to the circulation.
- The process involves osteoclastic cells mediating the first phase of remodeling, followed by osteoblasts forming new bone that replaces the resorbed bone.
- The sequence takes 4 to 6 months.
Nutritional and Hormonal Effects on Bone
- Bone density is governed by the amount of available calcium, phosphorus, vitamin D, and parathyroid hormone.
- Normal bone growth and maintenance require sufficient dietary intake of calcium and phosphate salts.
- Vitamins C, A, K, and B12 are necessary for bone growth.
- Calcium is essential for bone growth, with 99% found in bone and teeth, and the remaining 1% in blood and extracellular fluid.
Clinical Conditions
- Osteomalacia: a term that encompasses many disorders in which osteoid is produced but inadequately mineralized, causing "soft bones".
- Rickets: the children's form of osteomalacia, characterized by bowed legs, and deformities of the pelvis, ribs, and skull.
- Osteoporosis: a group of diseases in which bone resorption occurs at a faster rate than bone deposition, leading to porous bones and increased risk of fractures.
- Primary hyperparathyroidism: excessive secretion of parathyroid hormone, leading to elevated calcium levels and symptoms such as bone pain, fatigue, and GI complaints.
Hormonal Effects on Bone
- Insulin stimulates bone formation, while glucocorticoids stimulate osteoclast activity.
- Parathyroid hormone and calcitonin antagonistically maintain blood calcium levels at homeostatic levels.
- Parathyroid hormone increases calcium levels in response to low blood calcium, while calcitonin lowers calcium levels when they are too high.
- Growth hormone and thyroid hormone stimulate bone growth, especially during pre-adolescence.
- Sex hormones, particularly estrogen, stimulate bone growth and closure of the epiphyseal plate during adolescence.
Fracture Repair
- Fractures can occur due to extreme loads, sudden impacts, or stresses from unusual directions.
- Fracture types include open, closed, comminuted, greenstick, spiral, and impacted.
- Fracture repair involves four stages: inflammation, granulation tissue formation, bony callus formation, and remodeling.
- Avascular necrosis can occur if the vascular supply to the bone is interrupted and not restored, leading to bone death.
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This quiz covers the stages of nervous system development from embryonic ectoderm, including neural tube formation, neuronal proliferation, and myelination. Test your knowledge of this complex process!
