NERVOUS-SYSTEM-Part-2-LEC-by-BOLANONMD-sEpt-2024.pdf

THE NERVOUS SYSTEM PART II By: Dr. Mary Hazel Bolanon Organization of the Nervous System  Central Nervous System  Brain  Spinal Cord  Peripheral Nervous System  Cranial nerves  Spinal nerves  Specialized receptors Organization of the Nervous System...

THE NERVOUS SYSTEM PART II By: Dr. Mary Hazel Bolanon Organization of the Nervous System  Central Nervous System  Brain  Spinal Cord  Peripheral Nervous System  Cranial nerves  Spinal nerves  Specialized receptors Organization of the Nervous System Cont.  Organization of the Nervous System Peripheral Nervous System (PNS) Divisions: Divided into Somatic Nervous System (SNS) and Autonomic Nervous System (ANS) based on functional differences. Somatic Nervous System (SNS): Responsible for conscious perception and voluntary motor responses. Controls skeletal muscle contraction, though not always voluntary (e.g., reflexes). Some responses, like reflexes, occur without conscious decision (e.g., startled by a friend). Motor responses can become automatic over time through habit learning or procedural memory.  Organization of the Nervous System Autonomic Nervous System (ANS): Responsible for involuntary control of the body to maintain homeostasis. Sensory input can come from both external and internal stimuli. Motor output extends to smooth muscle, cardiac muscle, and glandular tissue. Example: Sweating is controlled by the ANS for temperature regulation (homeostatic) and also in response to emotions (non-homeostatic).  Organization of the Nervous System Enteric Nervous System (ENS): Controls smooth muscle and glandular tissue in the digestive system. Functions independently of the CNS but overlaps with the ANS in digestive regulation. Part of the PNS but shares functions with the autonomic system in regulating digestion. Organization of the Nervous System Cont. Functions of The Nervous System  Sensory Input  Integration  Motor Output  Homeostasis Credit: https://open.oregonstate.education Functions of The Nervous System  Sensory Input: First major function of the nervous system: sensation. Sensory functions detect changes in homeostasis or specific environmental events (stimuli). Commonly known senses: taste, smell, touch, sight, and hearing. Taste and smell: Respond to chemical substances. Touch: Physical/mechanical stimuli. Sight: Light stimuli. Hearing: Perception of sound, a type of physical stimulus. Sensory input can also come from internal stimuli, like organ stretch or ion concentration in the blood. Functions of The Nervous System  Motor Output: Produces a response based on perceived stimuli. Includes movement of all three types of muscles: Skeletal muscle for voluntary movement (e.g., moving the skeleton). Cardiac muscle (e.g., increased heart rate during exercise). Smooth muscle for involuntary actions (e.g., moving food through the digestive tract). Controls glands, such as sweat glands for temperature regulation. Responses are divided into: Voluntary (somatic nervous system, e.g., skeletal muscle contractions). Involuntary (autonomic nervous system, e.g., smooth/cardiac muscle regulation, gland activation). Functions of The Nervous System  Integration: Process of processing and comparing stimuli with other stimuli, memories, or current state. Integration determines the appropriate response. Example: A baseball batter deciding to swing or not based on the ball's trajectory, count, or team's lead. Histology of Neural Tissue  Neurons  Glial cells (Credit: Regents of University of Michigan Medical School © 2012)  Histology of Neural Tissue Nervous Tissue Composition:Composed of two types of cells: Neurons: Primary cells associated with the nervous system. Responsible for computation and communication. Electrically active and release chemical signals to target cells. Glial cells (glia): Play a supporting role for neurons. Ongoing research explores their expanded role in signaling. Neurons are essential for nervous system function, but rely on glial support to operate properly. Neural Tissue Characteristics of Neurons Excitable Longevity High Metabolic Rate Sometimes Large Amitotic May Regenerate Parts of a Neuron  Cell Body, or Soma  Perikaryon  Nissl Bodies  Cell Processes  Dendrites  Axon  Parts of a Neuron Parts of a Neuron:Cell body (soma): Contains the nucleus and most major organelles. Processes (extensions of cell membrane): Axon: Neurons have one axon that emerges from the cell body. Axon can branch to communicate with many target cells. Propagates nerve impulses to other cells. Dendrites: Receive information from other neurons at synapses. Highly branched to allow communication with the cell body. Information flow in a neuron: Flows from dendrites → cell body → axon. gives the neuron polarity (information flows in one direction). Additional Parts of a Neuron  Axolemma  Axoplasm  Axon Hillock  Initial Segment  Telodendria Source: https://commons.wikimedia.org/wiki/File:Blausen_0657_MultipolarNeuron.png Additional Parts of a Neuron Axon Hillock: Special region where the axon emerges from the cell body. Tapering of the cell body toward the axon fiber. Axoplasm (a solution of limited components) begins here. Also referred to as the initial segment of the axon.  Additional Parts of a Neuron Myelin and Axon Segments: Many axons are wrapped in myelin (made from glial cells), which acts as an insulating substance. Myelin is similar to insulation on electrical wires, but with gaps known as nodes of Ranvier. The gaps in myelin are important for the transmission of electrical signals. The length of the axon between these gaps is called an axon segment. Additional Parts of a Neuron Axon Terminal: At the end of the axon, there are several branches extending toward the target cell. Each branch ends in a synaptic end bulb, which connects with the target cell at the synapse. Axon Terminal and Synapse  Axon (or synaptic) Terminal  Synaptic vesicles  Synapse or synaptic cleft  Pre-synaptic neuron  Post-synaptic neuron Myelin Sheath  Process of myelination occurs as the glial cell wraps tighter and tighter around the axon.  The inner layers of the glial cell form the insulating myelin sheath while the outer layer is called the neurolemma.  Gaps between adjacent glial cells form the nodes of Ranvier Myelin Sheath Axon Insulation: Glial cells provide insulation for axons in the nervous system. Oligodendrocytes insulate axons in the CNS. Schwann cells insulate axons in the PNS. Myelin Sheath Myelination Process: myelination process is mostly the same Myelin = lipid-rich sheath that surrounds the axon Function of Myelin: Myelin facilitates the transmission of electrical signals along the axon. The lipids in myelin are the phospholipids of the glial cell membrane. Myelin Sheath Continued Credit: Regents of University of Michigan Medical School © 2012 White Matter Versus Gray Matter Credit: modification of work by “Suseno”/Wikimedia Commons  White Matter Versus Gray Matter Function of Myelin: Myelin protects and electrically insulates neurons from one another. Myelinated fibers conduct impulses rapidly and form the white matter of nervous tissue. Unmyelinated fibers conduct impulses slowly and form the gray matter of nervous tissue.  White Matter Versus Gray Matter Nodes of Ranvier: Gaps within the myelin sheath are called nodes of Ranvier, which aid in impulse transmission. The length of the axon between each gap is referred to as an axon segment. Multiple Sclerosis (MS)  Multiple Sclerosis (MS) is an autoimmune disease in which the body's immune system mistakenly attacks myelin. Antibodies produced by lymphocytes (a type of white blood cell) target myelin for destruction -> leads to inflammation and damage to the myelin in the central nervous system As the myelin is destroyed, scarring (sclerosis) occurs, particularly in the white matter of the brain and spinal cord. presence of multiple scars in affected areas. Symptoms : somatic and autonomic deficits, affecting muscle control and the function of organs like the bladder. Guillain-Barre Syndrome  Guillain-Barre Syndrome GBS is a demyelinating disease of the peripheral nervous system. Cause: autoimmune reaction -> inflammation in peripheral nerves Common symptoms include sensory disturbances and motor deficits Autonomic dysfunction can also occur, causing changes in heart rhythm or a drop in blood pressure, particularly when standing, which may lead to dizziness. Classification of Neurons Unipolar Neurons =have only one process emerging from the cell. True unipolar cells are found only in invertebrate animals. Human unipolar cells are more accurately called “pseudo-unipolar” cells. Exclusively sensory neurons. Dendrites receive sensory information, sometimes directly from the stimulus. Cell bodies are always located in ganglia.  Classification of Neurons Bipolar Cells Have two processes extending from each end of the cell body (one axon, one dendrite). Not very common. Found mainly in: Olfactory epithelium (smell stimuli) Retina  Classification of Neurons Multipolar Neurons Include all neurons that are not unipolar or bipolar. Have one axon and two or more dendrites (usually many more).  Classification of Neurons Anaxonic Neurons Very small. Processes cannot be distinctly identified as axons or dendrites under standard histological magnification (400X to 1000X). Multiple processes function as axons depending on conditions. Despite indistinguishable processes, these neurons are considered multipolar. Classification of Neurons Continued (3) Glial Cells of the CNS Glial Cells (Neuroglia) Also known as glia. supporting cells in nervous tissue. Help neurons complete their function for communication Etymology and History “glia” comes from the Greek word for “glue.” by German pathologist Rudolph Virchow in 1856 -> as a “kind of glue” in which nervous elements are planted Summary of Glial Cells Astrocytes  Star-shaped glia that make up half of all neural volume  Control the chemical environment around neurons  Degrade and recycle neurotransmitters  Form the blood- brain barrier (BBB) Microglia  Ovoid-shaped glial cells with highly branched, narrow cell processes  Act as macrophages that engulf microbes and dead neural cells  Generally provide protection for the brain and spinal cord Oligodendrocytes  There are a few processes that extend from the cell body  Each cell process reaches out and surrounds an axon to insulate it in myelin  One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons Ependymal Cells  Line the ventricles of the brain and the spinal cord  Ependymal cells assist in producing, monitoring, and circulating CSF  They use their cilia to circulate the CSF from the brain and down into the spinal cord Glial Cells of the PNS  Satellite Cells  Schwann cells  Nodes of Ranvier  Neurilemma Neurophysiology Neurophysiology Continued (3) Neurophysiology Continued (4) Neurophysiology Continued (5) The transmission of a nerve impulse is very similar to the process of a muscle contraction that we discussed in chapter nine and can be more or less summed up in the following steps. ❑ Resting membrane potential ❑ Depolarization and production of a graded potential ❑ Conversion of the graded potential to an action potential ❑ Repolarization and temporary hyperpolarization of the polarized membrane. Action Potential Resting Membrane Potential When the cell is at rest, the ligand-gated and voltage-gated ion channels are closed. The concentration of Na+ outside the cell is 10 times greater than the concentration inside. Also, the concentration of K+ inside the cell is greater than outside. The inside of the cell is has a relatively negative charge (-70mV) compared to the outside of the cell. Depolarization If a stimulus is received (such as a neurotransmitter, distortion of the membrane, etc.), the Na+ gated ion channels open and Na+ begins to flood to the interior of the cell. This causes the charge within the cell to move from -70mV toward zero. This change in charge is called depolarization and it begins to sweep across the cell membrane as a graded potential. Propagation of Action Potential If threshold (-55mV) is reached, the graded potential is converted to an action potential and all of the voltage-gated ion channels along the axon open. This allows for a massive influx of Na+ into to the cell so the charge continues to move in a positive direction (+30mV). Repolarization and Hyperpolarization Once the stimulus is removed, the Na+ ion channels begin to close but the K+ ion channels remain open. This causes the K+ to flow out of the cell and internal charge begins to move back into a negative direction, called repolarization. Because the K+ channels remain open even once the -70mV is reached, the membrane becomes hyperpolarized (-90mV). Synapses  Synapses serve as the site where the first neuron (called the pre-synaptic neuron) is communicating with a second neuron (called the post-synaptic neuron).  There are two types of synapses:  Chemical synapses – use neurotransmitters  Electrical synapses – use gap junctions Chemical Synapse Electrical Synapse Source: https://commons.wikimedia.org/wiki/File:Gap_cell_junction-en.svg Classes of Neurotransmitters  Neurotransmitters are chemical ligands released at a synapse may have an excitatory or inhibitory effect.  The effect on the axon’s initial segment reflects a summation of the stimuli arriving at any moment.  The frequency of action potentials generated is an indication of the degree of sustained depolarization at the axon hillock. Classes of Neurotransmitters Continued