The Nervous System - Anatomy & Physiology PDF

The Nervous System Roland Oswald A. Banta, RMT, MD General Physician-Primary Care - Philippine Pharmaceutical Directory (PPD) Clinic Program Head - College of Medical Technology, WCC-QC The Nervous System “The human nervous system, by far the most complex system in the...

The Nervous System Roland Oswald A. Banta, RMT, MD General Physician-Primary Care - Philippine Pharmaceutical Directory (PPD) Clinic Program Head - College of Medical Technology, WCC-QC The Nervous System “The human nervous system, by far the most complex system in the body, is formed by a network of many billion nerve cells (neurons), all assisted by many more supporting cells called glial cells.” Two Major Divisions: Central nervous system (CNS), consisting of the brain and spinal cord Peripheral nervous system (PNS), composed of the cranial, spinal, and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves) and ganglia that are small aggregates of nerve cells outside the CNS The Neuron → functional unit of the nervous system “Neurilemma” → the thin sheath around a nerve axon, acts as a “cell membrane” The Cell Body Contains the nucleus Serves as the “trophic center” for the entire neuron In contact with nerve endings ○ Most of the cell bodies make up the gray matter (darkly stained) and is found peripherally in the brain and deeper in the spinal cord Nissl bodies - large masses or regions with concentrated RER and polyribosomes; chromatophilic substances Axons Transmits impulses away the cell body Components: ○ Axon hillock: pyramid-shaped region of cell body where the axon originates ○ Axoplasm: contains the contents, acts as the cytoplasm ○ Terminal bouton (Presynaptic Axon Terminal): Dilated ends of small axonal branch which contains abundant mitochondria and synaptic vesicles where neurotransmitters are released ○ Neurilemmal sheath: Made of Schwann cells (PNS) or Oligodendrocytes (CNS) ○ Myelin: Compacted layers of cell membrane internal to neurilemmal sheath Axonal Transport Anterograde Transport Retrograde Transport (Forward) (Backward) Movement from the cell body Movement from the periphery through axonal microtubules through axonal microtubules (via Kinesin) to the synaptic (via Dynein) to the cell body terminals Dendrites Principal signal reception and processing sites Short, small processes emerging and branching off the soma Characteristics: ○ Extensive arborization or branching ○ Presence of dendritic spine Most of the synapses occur here Functions for neural plasticity: The capacity of the nervous system to modify itself in response to experience and injury Glial Cells → neurons support for survival; 10x abundant than neurons; existing six types  Ependymal cells functions for the circulation of the CSF, as well as its production.  Microglia → monocyte- derived, antigen-presenting cells of the CNS (CNS Macrophage) Microglia are MOTILE CELLS, used for immune surveillance of CNS tissues; causes phagocytosing damage in times of danger and stress.  Satellite glial cells functions to insulate, nourish, and regulate the microenvironment of the neuronal cell bodies. SATELLITE CELLS → SUPPORTING CELLS Central Nervous System The Central Nervous System Major structures comprising the CNS including the cerebrum, cerebellum, and spinal cord The CNS is completely covered by connective tissue layers (meninges) CNS tissue contains very little collagen making it relatively soft and easily damaged by injuries Many regions show organized areas of white matter and gray matter Gray matter - made up of cell bodies, making it darker in color White matter - made up of myelinated axons, which makes it lighter in color Cerebral Cortex Neurons of the cerebral cortex can be divided into: ○ Pyramidal cells: pyramid-shaped cell bodies that vary in size and arise from the 5th or ganglion in layer of the neocortex forming most of the efferent pathway. Betz Cells - largest pyramidal cells in the motor cortex ○ Stellate Cells (Granule Cells): Small neurons with short vertical axon and several short branching dendrites, giving the cell body a star-shape. ○ Cells of Martinotti: Small polygonal cells usually found in the most superficial layer ○ Fusiform Cells: Spindle-shaped cells oriented at right angles to the surface of the cerebral cortex and found in deeper layers. ○ Horizontal Cells of Cajal: Small and spindle-shaped but oriented parallel to the surface and considered as the least common type. Cerebellum It coordinates muscular activity and maintains posture and equilibrium. Thick Outer Molecular Layer: consists of neuropil and scattered neuronal cell bodies. Thin Middle Layer: consists only of very large neurons called Purkinje cells Thick Inner Granular Layer: contains various very small, densely packed neurons and little neuropil Spinal Cord White matter is peripheral and the gray matter forms a deeper, “H” shaped/butterfly-like mass Anterior / Ventral Horn → contains the cell bodies of the large alpha lower motor neurons Posterior/Dorsal Horn → contains the cell bodies of small second- order sensory neurons Blood Brain Barrier Functional barrier that allows much tighter control over the passage of substances moving from blood into the CNS tissue Protects neurons and glia from exogenous substances (toxins, etc.), and helps maintain the composition and balance of ions in the interstitial fluid. Blood Brain Barrier’s Composition Capillary endothelium → main structural component since the junctions between endothelial cells are sealed (zonula occludens) Basement membrane → it surrounds the capillary endothelium Limiting layer of perivascular astrocytic feet → it envelopes the basement membrane Choroid Plexus Highly vascular tissue → projecting into the large ventricles of the brain. Found in the roofs of the third and fourth ventricles and in parts of the two lateral ventricular walls, all regions in which the ependymal lining directly contacts the pia mater. Each villus of the choroid plexus contains a thin layer of well- vascularized pia mater covered by cuboids of ependymal cells. The function of the choroid plexus is to remove water from the blood and release it as the CSF. Arachnoid Villi → provides the main pathway for CSF absorption back into the venous circulation. Peripheral Nervous System The Components of the PNS Nerves Ganglia Nerve Endings Nerve Fibers Congruent to CNS tracts Contains axons enclosed within the sheaths of glial cells ○ Sheathed by Schwann cells or neurolemmocytes Nerve Organization Nerve fibers are grouped into bundles to form nerves Nerves have a whitish, glistening appearance due to their myelin and collagen content Parts: ○ Endoneurium → surrounds an axon ○ Perineurium → surrounds a group of axons (forming fascicles); makes up the blood-nerve barrier ○ Epineurium → surrounds a group of fascicles (forming peripheral nerves) Ganglia Ovoid structures containing neuronal cell bodies and glial satellite cells Relay stations to transmit nerve impulses Sensory ganglia ○ Neurons of these ganglia are Pseudounipolar ○ Sensory ganglia receive ardent impulses that go to the CNS ○ No perineurium (No blood-nerve barrier function) Autonomic Ganglia ○ Neurons of these ganglia are multipolar neurons ○ Nerves affect the activity of involuntary activities ○ Maintains homeostasis References Mescher, Anthony L. (2024). Junqueira's Basic Histology: Text & Atlas, 17th International ed. New York:McGraw Hill. Esteban and Gonzales' Textbook of Histology 6th edition. Eduardo G. Gonzales. C & E Publishing, Inc. 2022.