The Nervous System: CNS and PNS - CNS: consists of the brain and the spinal cord; located in the dorsal body cavity surrounded by meninges- PNS: consists of all the neural structures outside of the CNS including the cranial nerves, spinal nerves and sensory receptorsThe nervous system is composed mainly of nervous tissue, connective tissue and blood vessels1. Define the following: Nervous tissue is composed of 2 types of cells: supporting cells (glial cells) and neurons (nervous cells) Nissl body: specialized endoplasmic reticulum; Function: protein synthesis Neurofilaments: intermediate filaments in the neuron; Function: provide tensile strength Ganglia: a cluster of neuron cell bodies in the PNS Nuclei: a cluster of neuron cell bodies in the CNS Tract: a bundle of axons in the CNS Nerve: a bundle of axons in the PNS Axonal terminals: the bulbous ends of terminal branches of an axon Neurilemma: The nucleus and cytoplasm of Schwann cells outside of the myelin sheath Nodes of Ranvier: gaps between adjacent myelin sheath Axolemma: plasma membrane of an axon Endoneurium: the delicate connective tissue each axon is wrapped in. A bundle of endoneurium-covered axons is called a fascicle.1- In an unmyelinated axon (axon not covered by myelin sheath) the endoneurium surrounds the axon exterior to the axolemma. - In a myelinated axon (axon is covered by myelin sheath) the endoneurium is exterior to the myelin sheath. The myelin sheath is external to the axolemma. o Perineurium: the coarse CT that wraps around fascicles Epineurium: the tough CT that wraps bundle of Perineurium-covered fascicles (this bundle forms the nerve or tract) 2. Which part of a neuron is known as the: - There are three parts of a neuron the cell body, dendrites and axon. a. Biosynthetic center: the biosynthetic center is part of the cell body (2 parts the soma and perikaryon) it contains all cytoplasmic organelles except for centrioles – contains well-developed rough ER and nissl body or chromatophilic substance, contains intermediate filamentscalled neurofibrils. Protein synthesis occurs in the cell body, which is why it is called the “biosynthetic center”b. Receptive center: - Is also known as the dendrites, a neuron may have one or more dendrites. There is no uniform diameter, dendrites appear tapered. - Dendrites receive signals or messages coming to the neuron and develop electrical signals called graded potentials that are conveyed toward the cell body. - The axon: a single process extending from the cell body – each neuron has only one axon- Generates and transmit action potentials – conducting region of a neuron -Branches at the end to form terminal branches which end in bulbous ends called axon terminals = synaptic knobs = boutons 3. Name and describe the function of all 6 types of supporting cells- Supporting cells in the CNS (integrates information it receives, contains brain and spinal cord) Astrocytes: Most abundant Numerous extensions that wrap around neurons Regulate brain function Form blood-brain barrier (BBB) which keeps toxins from getting to neurons in the brain - BBB – acts as a selective barrier. Lipid soluble substances (non-polar) can cross (i.e. drugs)- In Parkinson’s disease, dopaminergic neurons are deployed so dopamine is not released. In management of Parkinson’s, dopamine can’t be given because it won’t cross the BBB (a non-polar precursor dopamine is given instead) Microglia: specialized macrophages (engulf and digest pathogens and cell debris). Since the CNS is cut off from the immune system, microglia are the only immune cells to fight pathogens. 2 Ependymal cells: are involved in filtration of blood to produce CSF. Their cilia (ciliated columnar cells) line the ventricles (cavities in the brain) beat to circulate CSF Oligodendrocytes: use their extensions to wrap around segments of axons to form myelin sheaths (neurilemma is absent)- Supporting cells of the PNS (consists of nerves and ganglia outside of the brain and spinal cord, the main function of the PNS is to connect the CNS to the limbs and organs, it is divided into the somatic nervous system and the autonomic nervous system) Satellite cells: function is unknown, but believed to maintain chemical environment of cell bodies Schwann cells (neurolemmocytes): wraps around a segment of the axon in the PNS to produce myelin sheaths4. Explain why myelinated axons in the CNS do not regenerate when severed. - The microglia poorly clean up area of damage – debridement is not complete- No neurilemma to guide growth of severed axon- Presence of growth-inhibiting proteins in the CNS inhibit regeneration of a severed axon. Severed axons in the PNS can regenerate because…- When the axon is severed in the PNS, cells of the immune system clean up the damaged area ofcells debris, a process known as debridement, which sets the stage for regeneration- The neurilemma of the Schwann cell forms a REGENERATION TUBE that guides regeneration of the severed axon5. Explain why impulses are conducted faster in a larger myelinated axon than in a smaller unmyelinated axon. Diameter of the axon: larger axons transmit impulses at a faster rate than smaller axons becausethe larger axon have larger diameter and therefore presents with less resistance impulse transmission; the resistance in the smaller axons is higher which impedes impulse transmissionDegree of myelination: myelinated axons transmit impulses at a faster rate than unmyelinated axons.- Myelinated axons use salutatory conduction where action potentials are generated only at the nodes of Ranvier hence, the impulse “jumps” from node to node down the axon- Unmyelinated axons use continuous conduction where action potential developed stepwise across the entire axolemma 6. Describe the a. Depolarization phase of an action potential: When a neuron is stimulated, Na+ channels on the plasma membrane open, allowing Na+ to enter the cell down its concentration gradient Na+ entry is referred to as Na+ influx (sodium influx) As Na+ enters, the membrane potential becomes less and less negative until a threshold potential is reached When a critical membrane potential called the threshold potential is reached (typically around -30 mV), a positive feedback mechanism occurs. As more Na+ enters the cell, more Na+ channels open, resulting in an explosive entry of Na+. This drives the
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