4- Neurophysiology- Pt 2.docx

- **Nervous System: General Info** - Cells in the nervous system can be categorized as "neurons" or "neuroglia". - **Saltatory conduction** describes actions potentials jumping from node to node down the axon. - Saltatory conduction in myelinated fibers all...

- **Nervous System: General Info** - Cells in the nervous system can be categorized as "neurons" or "neuroglia". - **Saltatory conduction** describes actions potentials jumping from node to node down the axon. - Saltatory conduction in myelinated fibers allows for more rapid depolarization in comparison to unmyelinated fibers. - The myelin sheath is the reason why there is **white matter** on the brain. - **Gray matter** is a composition of a high population of soma within that area of the brain. - The cerebral cortex is an example of an area composed of grey matter. - **Wallerian degeneration** involves axonal degeneration distal to a lesion. - Regeneration is guided by Schwann cells. - Regeneration is only efficient in the PNS but not in the CNS. - **Neurons: General Info** - Neurons are nerve cells that are specialized in processing information. - Neurons are the major functional unit of the nervous system. - Neurons do not divide once they reach maturity, so any injury leading to neuronal death will permanently change the structure and function to the affected area. - Neurons communicate vis synapses. - **Synapses** are specialized contact areas with other neurons, muscle fibers, or glands. - **Neuron: Composition** - Neurons are composed of axons, dendrites, axon hillock, soma, presynaptic terminal, myelin sheath, and Nodes of Ranvier. - **Dendrites** are an information receiving area of the cell membrane. - The **soma (AKA: cell body or perikaryon)** contains the organelles. - The **axon** is an information carrying extension of the cell membrane. - Action potentials travel down the axon at a speed of 0.5-120 meters per second. - Larger axons are myelinated while smaller axons (under 1 micrometer in diameter) are not myelinated. - These larger axons have longer internodes and faster conduction velocities. - Conduction is proportional to diameter (in square feet) for unmyelinated fibers. - The **axon hillock (AKA: trigger zone)** is where the axon originates along with the action potential origination. - The **presynaptic terminal** is located at the end of the axon and is used for information transmission. - The **myelin sheath** is a greatly modified plasma membrane which is wrapped around the axon in a spiral fashion. - The myelin sheath enhances the speed of information transmission by acting as an electrical insulator and allowing saltatory conduction of impulses. - Myelin facilitates conduction while conserving energy and space. - The myelin sheath originates from and is a part of Schwann cells (in the PNS or peripheral nervous system) and Oligodendrocytes (in the CNS or central nervous system). - The myelin sheath is formed by the cytoplasm of Schwann cells forming a ring inside and outside of the sheath. - The **Nodes of Ranvier** are the gaps present within the myelin sheath. - **Neuron: Classification** - Neurons can be classified based on **structure**. - **Multipolar** - Multipolar neurons have 1 axon and many dendrites. - Multipolar neurons are the most common structure type. - The length and arrangement of multipolar neurons varies. - **Bipolar** - Bipolar neurons have 1 axon and 1 dendrite, ultimately containing 2 processes. - **Pseudo-unipolar (AKA: unipolar)** - Pseudo-unipolar neurons have 1 single stem process which bifurcates to form 2 processes, where 1 process goes to the CNS and the other process goes to the PNS. - True unipolar neurons are common in insects. - Neurons can be classified based on **function**. - **Sensory (afferent) neuron** - Sensory (afferent) neurons send information from the receptors in sensory organs towards the CNS (brain and spinal cord). - Most sensory (afferent) neurons are pseudo-unipolar. - **Motor (efferent) neuron** - Motor (efferent) neurons send information to effector organs (muscles and glands) from the CNS. - All motor (efferent) neurons are multipolar neurons. - **Interneuron (AKA: association neuron)** - Interneurons connect motor and sensory neurons together. - Interneurons are commonly found in the CNS. - Interneurons are usually multipolar or bipolar neurons. - **Neuroglia (Glial) Cells: General Info** - Glial cells lack axons and dendrites, causing them to be smaller than neurons. - Glial cells fill all the spaces in the nervous system that are not occupied by neurons or blood vessels. - Glial cells are capable of dividing and are more numerus than neurons within nervous tissue. - Glial cells do NOT produce action potentials and do NOT participate directly in synaptic interactions and electrical signaling. - Glial cells are **located** in the: - **CNS**: Microglial cells, astrocytes, oligodendrocytes, and ependymal cells - **PNS**: Schwann cells - **Neuroglia (Glial) Cell: Function** - Glial cell functions include: - Participating in myelin sheath production for the axon - Modulating growth and development of damaged neurons - Buffering extracellular concentrations of potassium and neurotransmitters - Participating in synapse (neuron contacts) formation - participating in some immune responses of the nervous system - **Glial Cells of the CNS** - **Microglial Cells** - Microglial cells are the brain's immune cells where they act as macrophages and release NO. - Microglial cells act as macrophages by participating in phagocytosis to clear debris. - Microglial cells release NO (nitric oxide) to prevent viral replication. - Microglial cells also protect the brain against injury and infection. - Microglial cells aid brain development by helping to destroy unnecessary synapses. - **Astrocytes** - Astrocytes are star-shaped cells with numerous long processes. - Astrocytes make up 50% of the glial cell population within the CNS. - Astrocytes **provide structural and metabolic support to neurons** by: - Forming outer and inner glial limiting membranes of the CNS - Releasing neurotrophic factors that are important for neuronal survival - Helping elongate axons and dendrites - Participating in repair processes following tissue injuries - Astrocytes **maintain neurons working environment** by: - Controlling level of neurotransmitters around the synapse - Controlling the concentration of important ions (such as K, Na, Ca, Cl, and HCO3) via the astrocyte ion channels and exchange ions with neighboring cells - Providing metabolic support via its close contact with capillary endothelium which is important for glucose transport, regulation of extracellular movement, glutamate metabolism, and maintenance of the blood-brain barrier. - Astrocytes modulate how neurons communicate by releasing molecules that influence neuronal activity. - **Oligodendrocytes** - Oligodendrocytes provide support to axons and neurons in the CNS. - Oligodendrocytes have numerous processes that extend to adjacent axons to form myelin. - **Ependymal Cells** - Ependymal cells cover the brain ventricles, central canal of the CNS, and the choroid plexus in the 4^th^ ventricle of the medulla oblongata. - Ependymal cells are involved in creating CSF (cerebrospinal fluid). - **Glial Cells of the PNS** - Schwann Cells (AKA: neurolemmocytes) - Schwann cells are arranged side by side along the axon where each cell forms internodes of the myelin sheath (at varying lengths from 25-1000micrometers). - Schwann cells provide support to the axons in the PNS, similarly to oligodendrocytes. - Schwann cell provide support to the axons in the PNS by producing myelin in axons that are over 1micrometer (in diameter). - Schwann cells guide regeneration of damaged axons in the PNS.