Nervous System - Introduction PDF

# Nervous System There are two regulatory systems: * The nervous system: rapid activities e.g. muscle contraction. * The endocrinal system: slow activities (duration) e.g. metabolism. ## Organization of the nervous system ### Histological * Glial cells: supportive cells do not generate action p...

# Nervous System There are two regulatory systems: * The nervous system: rapid activities e.g. muscle contraction. * The endocrinal system: slow activities (duration) e.g. metabolism. ## Organization of the nervous system ### Histological * Glial cells: supportive cells do not generate action potential. 10:50 times the number of neurons. * Microglial cells: scavenger (phagocytic) cells. * Oligodendrites: production & maintenance of myelin sheath. * Astrocytes: blood brian barrier (BBB). * Neurons: structural units of CNS → Action potential. A diagram shows a neuron, with the parts labeled: * Soma (cell body) * Dendrites * Axon * Myelin sheath * Terminal button ### Anatomical: 2 divisions 1. Central nervous system: * Brain: * Cerebrum: cerebral hemispheres, basal ganglia. * Diencephalon: thalamus & hypothalamus. * Brain stem: mid brain, pons & medulla. * Cerebellum. * Spinal cord: 31 segments, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral & 1 coccygeal. A diagram shows part of the brain and spinal cord labeled: * Cerebrum * Corpus callosum * Thalamus * Hypothalamus * Brain stem * Mid brain * Pons * Medulla * Cerebellum * Spinal cord * Central Canal * White matter * Grey matter ### Functional: 1. Sensory division: receptor → afferent → tracts → center. 2. Motor division: center → upper & lower motor neurons → organ. 3. Integrated centers: * Spinal cord: immediate automatic activities e.g. walking, micturition, flexor withdrawal reflex. * Lower brian level: brainstem & diencephalon for regulation of respiration, CVS, GIT....etc. * Higher brain level: Cortex, for fine sensations, motor activities, language, memory, thinking, sleep & wakefulness. ## Synaptic transmission Transmission of an impulse (action potential) from one neuron to another. It may be: 1. Electric transmission: * Very rare in humans e.g. vestibular nuclei. * Neurons are connected by low-resistance gap-channel pathway. 2. Chemical: release of a chemical transmitter from presynaptic neuron to act on postsynaptic neuron. A diagram show the terminal end of two neurons where the action potential is transmitted from one to the other, labeled: * Cell nucleus * Cell body * Dendrites * Axon * Synapse * Axon terminals ## Synapse: The site of junction between two neurons. Dynamic structure. **Types of synapses:** 1. Axo-dendritic: the most numerous, but the least excitable. 2. Axo-somatic. 3. Axo-axonic: the least numerous but the most excitable (low firing level at the axon hillock) A diagram shows the three types of synapses, with the arrows indicating the direction the action potential is transmitted: * **Axodendritic synapse**: Axon of one cell attaches to the dendrites of another * **Axosomatic synapse**: Axon of one cell attaches to the soma of another * **Axoaxonic synapse**: Axon of one cell attaches to the axon of another ## Functions of synapses: ## Functional anatomy of the synapse: 1. Presynaptic terminal (knob): * Large number of mitochondria to provide ATP needed for synthesis and exocytosis of the neurotransmitter. * Synaptic vesicles: 3 types: * **Small clear vesicles:** rapidly acting transmitter e.g. acetylcholine, glycine, glutamate and GABA. * **Small granular vesicles:** catecholamines. * **Large granular vesicles:** slowly acting neuropeptides. A diagram shows the synapse labeled: * Axon * Synaptic vesicles * Neurotransmitter * Receptor * Dendrites ## N.B. Clear vesicles contain in their walls 2 proteins: * **Synopsin:** fixes the vesicles to the cytoplasmic skeleton. * **Stenin:** unites with another protein at the clear zone called neurin → rupture of vesicles & release of the neurotransmitter. **N.B. The membrane of the terminal knob contains large numbers of voltage gated calcium channels.** **N.B.** * v-SNARE protein: on the membrane of the vesicles. * t-SNARE protein: on the presynaptic membrane. ## 2) Synaptic cleft: * 30-50 nm in width. * Contains extracellular fluid. ## 3) Postsynaptic membrane: * Contains receptors specific for the released neurotransmitter which are 2 types: * **Ionotropic receptors:** * Binding protein to unite with the neurotransmitter. * Ligand-gated channels (Na, K or Cl). * **Metabotropic receptors:** * G protein-coupled receptors. A diagram shows the synaptic cleft labeled: * Axon * Synaptic vesicles * Neurotransmitter * Receptor * Dendrites ## Mechanism of synaptic transmission 1. **Release of the chemical transmitter.** * Action potential in the presynaptic nerve opens voltage gated Ca⁺ channels. * Ca⁺ via Ca⁺ calmodulin kinase II phosphorylates synopsin to loosen the attachment of the vesicles to the cytoskeleton. * Vesicles move to the clear zone where stenin of the vesicles unites with neurin of the clear zone → rupture of vesicles → release of the neurotransmitter in the synaptic cleft. ## 2) The chemical neurotransmitter crosses the synaptic cleft. ## 3) Union of the chemcial neurotransmitter with its receptors. * This changes the permeability of the post synaptic membrane to one or more ions. ## 4) Changes in ion fluxes. * Lead to change in RMP to become less negative (+) or more negative (-). ## 5) Removal of the neurotransmitter & termination of the response by: * **Passive diffusion away from the synaptic cleft.** * **Inactivation by specific enzymes at the post synaptic membrane.** * **Active reuptake by the axon terminal (stored or destroyed).** * **Removal by glial cells.** A diagram shows the synaptic cleft labeled: * Presynaptic neuron * Terminal knob with neurotransmitter * Synaptic cleft * Postsynaptic neuron * Ligand-gated channels * Intracellular Ca⁺ * Postsynaptic potential