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Bone Tissue and Nervous System: Structure, Function and Clinical Correlations

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Osseous Tissue

Functions:

  • Support
  • Movement
  • Protection: housing the spinal cord and organs
  • Storage of ions (99% of calcium)

Constituents of Osseous Tissue

  • A - Bone matrix
  • B - Cells
  • C - Endosteum (microscopically)
  • D - Periosteum

A - Bone Matrix

  • Mineralized (about 65%): inorganic crystals of hydroxyapatite (calcium and phosphorus), also containing carbohydrates, citrates, etc.
  • Non-mineralized (about 35%): organic components, including type I collagen and a non-collagenous protein group (osteoinduction), e.g., bone morphogenetic proteins (BMPs)

B - Cells

  • Undifferentiated osteogenic cells: found in the endosteum and periosteum
  • Osteoblasts: located at the periphery of the bone tissue. Function: synthesize the bone matrix.
  • Osteocytes: located in the interior of the bone matrix. Cells with processes (prolongations). Function: maintenance of the tissue.
  • Osteoclasts: located at the periphery of the bone. Origin: bone marrow cells. Morphology: multinucleated cells. Function: bone destruction. The matrix is degraded by organic enzymes (lysosomes) and by inorganic acidification of the medium.

C - Endosteum

  • Internal lining of the bone
  • Single layer of osteogenic cells and osteoblasts

D - Periosteum

  • External lining of the bone
  • Dense connective tissue
  • Mainly fibroblasts and undifferentiated osteogenic cells
  • Source of bone cells
  • Nutrition: supplied by two blood vessels
  • Growth: subperiosteal osteogenesis

Calcium Homeostasis

  • Note: 99% of stored calcium is in bone.
  • Normocalcemia: ideal blood calcium levels.
  • Osteoclasts absorb or release calcium from bone as needed.
  • Hypercalcemia: high blood calcium, caused by excess calcium release from bones.
  • Hypocalcemia: lack of blood calcium.
  • When hypocalcemia occurs, the parathyroid glands secrete PTH (parathyroid hormone). PTH stimulates osteoblasts to signal osteoclasts; when osteoclasts reach the matrix, they reabsorb it and release calcium into the blood.

Clinical Correlations

  • Osteoporosis: during bone remodeling, osteoclasts reabsorb bone and osteoblasts rebuild it. Rebuilding requires estrogen. Estrogen deficiency impairs bone neoformation, and over time this leads to significant bone loss called osteoporosis.
  • Growth hormone acts on bone tissue to promote its development. Excess in children causes gigantism; deficiency in children causes dwarfism; excess in adults causes acromegaly.
  • Primary hyperparathyroidism: a tumor of the parathyroid glands, causing high blood calcium levels and demineralized bone.

Anatomical Classification of Bone Tissue

  • Compact bone: highly organized bone tissue. Components: Volkmann canals and Haversian canals. Osteocytes receive nutrients through the Haversian canals and pass them between one another. Two Haversian canals may be connected by a Volkmann canal.
  • Spongy (trabecular) bone: not highly organized; a sponge-like network of trabeculae that allows nutrient passage.

Histogenesis

  • Primary bone tissue: the first bone tissue to form. Found in the fetus, after bone trauma, and in areas of ossification.
  • Secondary bone tissue: has already undergone bone remodeling. It may be spongy or compact.

Types of Ossification

  • Intramembranous: connective tissue becomes bone. Bone does not form from a cartilage model; it arises directly from connective tissue. When the periosteum forms from bone, this is intramembranous ossification.
  • Endochondral: connective tissue becomes cartilage, which then becomes bone.

Zones of bone development: resting zone, proliferative zone, hypertrophic zone, calcification zone, and ossification zone.

Nervous System

Nervous tissue is a fundamental tissue of the human body.

Constituents of nervous tissue: neurons and neuroglia (glial cells).

Constituents of the Neuron

  • Dendrites: receive stimuli
  • Perikarya (cell body): process the stimulus
  • Axon: conducts the information
  • Myelin sheath: insulating covering around the axon (absent in short axons)

Neurons generally do not undergo cell division.

Constituents of Neuroglia

  • Astrocytes: transport nutrients from blood vessels to neurons
  • Oligodendrocytes (CNS): form myelin in the CNS; act as satellite cells
  • Microglia: defense, through phagocytosis
  • Schwann cells (PNS): form myelin in the PNS
  • Ependymal cells: line the central canal of the spinal cord; the choroid plexus secretes cerebrospinal fluid (CSF)

Types of Neurons

  • Pseudounipolar (embryonic): the impulse does not pass through the nucleus region
  • Bipolar: the impulse passes through the cell body; about 2%
  • Multipolar: receives stimuli through all dendrites and the impulse passes through the cell body; about 98%

Generation and Conduction of the Nerve Impulse

Neurons are polarized at rest, with a resting membrane potential arising from differences in ion concentration inside and outside the cell.

  • Potassium (K+) is much more concentrated inside the cell than outside.
  • Sodium (Na+) and chloride (Cl-) are much more concentrated outside the cell than inside.

Phases of the Neuron

  • Resting: positive charge concentrated outside and negative charge inside. Each time 100 nA enter, K+ leaves.
  • Depolarization: reversal of charge and propagation of the impulse. Only sodium channels remain open.
  • Repolarization: sodium and potassium channels close and the membrane returns to normal.

Transmission of the Nerve Impulse (Synapse)

  • Electrical synapse: neurons are directly connected and the impulse passes directly.
  • Chemical synapse: neurons are separated by a gap.

In a chemical synapse, depolarization opens calcium (Ca2+) channels. This causes vesicles containing neurotransmitters to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. The neurotransmitters bind receptors on the postsynaptic membrane.

Neurotransmitters include norepinephrine, serotonin, adrenaline, endorphins, and acetylcholine.

Anesthetics block sodium channels, preventing depolarization and blocking transmission of the impulse.

Neuroglial Cells (Glia)

  • Protoplasmic astrocytes: found in gray matter
  • Fibrous astrocytes: found in white matter
  • Functions: support, repair (scar formation), nutrition

Nerve Fibers

  • Unmyelinated fibers: conduct information slowly
  • Myelinated fibers: conduct information quickly

Anatomical Classification of the Nervous System

  • CNS: brain (cerebrum, cerebellum, brainstem) and spinal cord
  • PNS: nerves (cranial and spinal) and ganglia

Ganglia (gânglios) are aggregations of neuron cell bodies (perikarya) in the PNS. Nerves are bundles of axons in the PNS.

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