Neurohistology PDF

Main functions of the nervous system Nervous system + Endocrine system = Internal HOMEOSTASIS ○ The nervous system controls the 11 body systems ○ Main difference is the fast and response is short in nervous system compared to the endocrine system whereas hormones...

Main functions of the nervous system Nervous system + Endocrine system = Internal HOMEOSTASIS ○ The nervous system controls the 11 body systems ○ Main difference is the fast and response is short in nervous system compared to the endocrine system whereas hormones are being released to blood supply and takes a little bit longer Integrate and control all body systems activities Sensing and responding to changes in normal physiology Organization of the Nervous System Neurons and neuroglia (most abundant in nervous system - 80% of its weight) Nervous structures ○ Brain + Spinal cord ○ 12 pairs of peripheral cranial nerves (arising from the brain) ○ 31 pairs of spinal nerves (originate spinal cord) Accessory structures ○ Ganglia (PNS sensory info) and (Autonomic ANS) ○ Periphery sensory receptors Carotid sinus- B Body- C Sensory Information Sensory receptors (external or internal) detects changes This info. is sent to sensory neurons (afferent) ○ Unipolar neurons (mainly) Target of the sensory information is the CNS (Brain or SC) Special senses = vision, hearing, taste, smell, equilibrium Somatic senses = touch, temperature, pain, itch, proprioception Somatic stimuli = muscle length and tension, proprioception Visceral stimuli = BP, distension of gastrointestinal tract, blood glucose concentration Sensory Receptors  Krause- cold thermoreceptor Ruffini- hot thermoreceptor Tactile- light touch and deep touch (pressure) ○ Sensory receptor include Meissner’s corpuscle, Merkel’s discs, Pacinian corpuscles Proprioception- position sense, movement sense, force ○ Sensory receptor are muscle spindle Nocioception- pain, temperature, tactile (crude) Interneurons Integrative function ○ Receive sensory inputs ○ Integrate this input to CNS (SC and brain cortex through collaterals) ○ Receive the motor output (somatic) Motor output Motor function ○ Motor or efferent neurons carry motor commands from the brain to the spinal cord and then to peripheral nerves that innervate body effectors (glands, muscles, etc.) 31 peripheral nerves arise from SC and there are classified as MIXED NERVES and 30 vertebra ○ Mixed nerves are related to motor function and autonomic nervous system Somatic nervous system- voluntary Comprimes of sensory and motor neurons in circuits Sensory ○ Carry information from receptors for vision, hearing, taste, and smell ○ Carry information of somatic receptors of head, body wall, and limbs Motor ○ Innervate skeletal muscles under voluntary control Effector organs ○ Skeletal muscle and its effect is (+) stimulators and smooth muscle (e.g, in gut, glands and cardiac muscle) and its effect is (+/- stimulatory or inhibitory) Preganglionic neuron (always cholinergic) and postganglionic is mostly adrenergic ○ Main exception is the parasympathetic sweat glands =  Pregang. and postganglionic are cholinergic Autonomic nervous system (ANS)- involuntary Comprised of sensory and motor neurons in circuits Sensory ○ Convey information of the body viscera Motor ○ Innervated effectors (by involuntary control) Cardiac muscle Smooth muscle Glands ANS- Under voluntary control 3 subdivisions ○ Sympathetic (adrenal medulla) ○ Parasympathetic Both innervate body effectors except skeletal muscles ○ Enteric division Regulates the digestive tract Neuron Responsible for most of the functions of the nervous system Parts of the neuron: ○ Cell body (Soma) Nuclei (DNA) Nissi bodies- clusters of RER Golgi Lysosomes Mitochondria Cytoskeleton (neurofibrils and microtubules) ○ Dendrites (input reception) ○ Axon Axon Hillock Axoplasm Transport Kinesin (anterograde- soma to terminal route) and Dynein (retrograde- terminal to soma route) Both are attached by microtubules Cytoskeletal proteins End terminal (telodendria) Structural classification of neurons Multipolar neurons (more related to sensory neurons, CN nerves and spinal nerves) ○ Have several dendrites and one axon  ○ Common in the CNS ○ Bipolar neurons (more related to special senses) ○ Have one dendrite and one axon ○ Retina, inner ear, and olfactory area ○ ○ Inner nuclear layer contains bipolar neurons Unipolar neurons ( ○ Have one process that arises form the cell body and then branches into two axon-like processes ○ Sensory neurons  ○ Neuroglia Collectible of cells that support, nourish and protect neurons, and helps to maintain neural homeostasis Are smaller and more numerous than neurons (~50% of the total volume of nervous tissue, if it’s dry it is 80%) Do NOT conduct impulses 6 types ○ CNS Astrocytes Maintain blood-brain barrier; provide structural support; regulate ion, nutrient, and dissolved-gas concentrations; absorb and recycle neurotransmitters; form glial scar tissue after injury Similar to police; protects the CNS from bacteria, viruses, parasites Oligodendrocytes Myelinate CNS axons; provide structural framework Similar to Schwann cells in the PNS but the main difference is that oligodendrocytes can surround more than one axon to myelinate all at the same time versus Schwann cells can only myelinate one Schwann cell at a time Microglia Remove cell debris, wastes, and pathogens by phagocytosis Ependymal cells Line ventricles (brain) and central canal (spinal cord); assist in producing, circulating, and monitoring cerebrospinal fluid The ependymal cells located in the brain are considered a family of the Choroid Plexus, analogous to the cells in CSF and are mostly located close to the ventricular system (3rd ventricle) ○ PNS Schwann cells  Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury Satellite cells Surround neuron cell bodies in ganglia; regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia Myelin Synthesizing Cells Multilayered structure that covers and insulates myelinated axons Composed by ○ Lipid (70-85% of total volume) (40% cholesterol, 40% phospholipids, and 20% glycolipids) ○ Protein (15-30% of total volume) covering that wrap around myelinated axons Synthesized cells ○ Oligodendrocytes (CNS) ○ Schwann Cells (PNS) Schwann cell is only capable of myelinating a single axon, but oligodendrocytes can myelinate multiple axons The gap between the myelinated and unmyelinated axons moves action potential through the whole axon ○ ○ Nodes of Ranvier: unmyelinated gaps in myelinated axons Myelinated axon in PNS ○ An axon wrapped w/ a fatty insulating sheath formed from Schwann cells Unmyelinated axon in PNS ○ Axons that are not covered w/ an insulating sheath Gray and white matter  White matter ○ White due to the myelinated axons Gray matter ○ Darker is composed of unmyelinated axons, cell bodies and neuroglia Opposite in direction and brain, the cell bodies are located in the cortex, white matter is more inside of the brain tissue ○ The spinal cord is in the opposite direction Neuronal communication Action potential needs to comply the ALL-OR-NONE PRINCIPLE (needed to convert a -10 to -15 mV so we need to reach from -70, a -55 in order to move from graded potential to action potential) ○ Graded potential can elicit an action potential but needs to reach a threshold (which is the minimum change needed to move from graded to action potential and the value is -10 mV) ○ What is the resting membrane potential? -70 mV ○ What elements are most important to maintain resting membrane potential? Na+ and K+ channels, Leak Na+ and K+ channels, Na+-K+ pump ○ ○ Maximum depolarization stage value? +30 mV ○ What is the equilibrium potential value (Keq) for Na+ and K+? Na+: +66 mV K+: -90 mV ○ What type of channel is open in the depolarization V-gated Na+ moves from -70 to +30 and +30 is closer to +66 so the depolarization value tries to reach the equilibrium potential ○ Neurons never reach +66, if it does it explodes and dies ○ In the repolarizing phase, the voltage K+ is open and we are moving from +30 to -70 and a little more in the hyperpolarizing phase are moving towards the Keq of -90. In the hyperpolarization, we may have -80 but never -90. ○ What phase of an action potential is more close to the equilibrium potential of an ion? Hyperpolarization and repolarization; closer to K+ value ○ Neuron is more permeable to Na+ or K+? And why?  More permeable to K+ than Na+ because the hyperpolarizing potential gets closer to Kequilibrium of K+ Continuous conduction: all the axonal projection needs to be depolarized Saltatory conduction: ONLY the NODES OF RANVIER are depolarized in a neuron ○ Saltatory is the fastest and the depolarized is shorter compared to the continuous conduction (takes more time) Chemical: physical space (synaptic cleft) in which the neurotransmitters are release and bind to receptors in postsynaptic cells Electrical: gap junctions and connexins connects pre- and postsynaptic cells PNS regenerative capabilities PNS neurons have a greater capacity for repair and regenerate ○ In an injury, the distal end dies but the proximal end stimulates a growth cone receives the inputs of fibroblasts and regenerates. Then after it establishes new connections with the effector cells. CNS have no regenerative capacity ○ If CNS suffers from a lesion, it is unable to recover to have the same function as the previous because the neuroglia related to the CNS releases a huge amount of inhibitory proteins and these inhibitory proteins cannot help the CNS in terms of restoration of neural pathways. Some of these proteins are called NOGO, MAG, Ephrins. The majority are related to the myelination proteins that do not allow the CNS to regenerate and are repulsive. ○ Stimulatory proteins comes from fibroblasts cells (FGFs