Chapter 12: Nervous System Overview

(Nervous tissue) Chapter 12 Nervous System Overview · Anatomical Divisions: ↳ central nervous system (CNS) ↳ peripheral nervous system (PNS) · Functional Divisions: ↳ afferent → to the CNS ↳ efferent → to effectors -somatic nervous system (SNS) -autonomic nervous system (ANS) Nervous Tissue · 2 types of cells: 1. Neurons (nerve cells) ↳ basic functional unit of the nervous system ↳ responsible for the transfer and processing of information 2. Neuroglia ↳ glial cells ↳ supporting cells that separate and protect neurons Structure of a Neuron Know the parts of the neuron and what they do Glial Cells of CNS: Glial Cells of the PNS: · oligodendrocytes: forms myelin sheath around axons bodies in ganglia, regulate interstitial · ependymal cells: simple cuboidal epithelium of vesicles, · schwann cells: form a myelin sheath · astrocytes: maintains blood brain barrier (BBB) · satellite cells: surround neuron cell · microglia: phagocytic neuroglia fluid around neurons produces CSF around axons in the PNS Structural Classification of Neurons: · anaxonic neurons ↳ found in brain and special sense organs · bipolar neurons ↳ rare; found in special sense organs · Unipolar neurons ↳ most sensory neurons of PNS are unipolar; longest axons can extend from toes to spinal cord · multipolar neurons ↳ most common neuron in CNS ↳ neurolemma Functional Classification of Neurons: · sensory neurons ↳ afferent fibers (from PNS to CNS) ↳ includes somatic sensory neurons and visceral sensory neurons ↳ sensory receptors: - interoreceptors: monitor internal systems, sensations of stretch, pressure & pain - exteroceptors: info about outside environment (touch, temp, pressure) - proprioreceptors: monitors position and movement · motor neurons ↳ efferent fibers ↳ includes somatic motor neurons and visceral motor neurons ↳ preganglionic vs. postganglionic fibers · interneurons ↳ most abundant ↳ exist between sensory and motor neurons ↳ analyze sensory input and coordinate motor output ↳ excitatory or inhibitory Synapses · specialized site where the neuron communicates with another cell ↳ presynaptic cell to post synaptic cell · 2 types of synapse: ① electrical synapse: cardiac and smooth muscle ② chemical synapse: nervous system communication by neurotransmitters Membrane potential · plasma membranes have a membrane potential or are polarized electrically · resting membrane potential (cell is unstimulated, -70 mv) · 2 kinds of potential charge: 1. Graded potential ↳ temporary localized change in resting potential ↳ caused by a stimulus ↳ small signal, short range 2. Action potential ↳ electrical impulse ↳ produced by graded potential ↳ large signal, long range Action Potential · a brief, rapid, and large (100mv) change in membrane potential · threshold "all-or-none principle" · propagated through entire membrane in nondecremental fashion Steps in Action Potential: ① depolarization to threshold ② activation of voltage-gated sodium ion channels (rapid depolarization) ③ inactivation of voltage-gated sodium ion channels and activation of potassium ion channels (beginning of repolarization) ④ closing of voltage-gated potassium channels (brief hyperpolarization) and a return to resting potential Propagation of Action Potential · action potentials are propagated from the axon hillock (trigger zone) to the axon terminals · Two types of propagation: ① Continuous propagation ↳ conduction in unmyelinated fibers ↳ action potential spreads down every portion of the membrane ② Saltatory propagation ↳ rapid conduction in myelinated fibers ↳ impulse jumps quickly over myelinated sections Electrical Synapses · presynaptic and postsynaptic membranes linked by gap junctions · ions pass through pores · action potentials are propagated quickly · rare: found in retina, cerebral cortex, and ciliary ganglia Chemical Synapses · most common synapse between neurons, only type with neurons and other cells · cells separated by synaptic cleft · types of chemical synapses: ↳ neuromuscular junction → between neuron and skeletal muscle ↳ neuroglandular junction → between neuron and gland cells · neurotransmittters Neurotransmitters · chemical compounds released by one neuron to affect the membrane potential of another · contained within synaptic vesicles · released into synaptic cleft to affect receptors on post synaptic membrane · broken down by enzymes and reabsorbed Neurotransmitter Release: ① action potential passes along axons and over synaptic knob/terminal ② synaptic terminals become more permeable to calcium ions, which diffuse inward ③ synaptic vesicles fuse to terminal membranes ④ synaptic vesicles release neurotransmitters into the synaptic cleft ⑤ synaptic vesicles re-enter axon's cytoplasm to pick up more neurotransmitters Classification of Neurotransmitters · excitatory neurotransmitters: cause depolarization and promote the generation of action potentials · inhibitory neurotransmitters: cause hyperpolarization and suppress the generation of action potentials · the effect of a neurotransmitter depends on the properties of the receptor, not on the nature of neurotransmitter Major Classes of Neurotransmitters: 1. Biogenic Amines 2. Amino Acids 3. Neuropeptides 4. Dissolved Gases Biogenic Amines · Norepinephrine (NE) ↳ released by adrenergic synapse Amino Acids · Glutamate ↳ excitatory ↳ excitatory and depolarizing effect ↳ in brain and portions of ANS · Dopamine ↳ CNS neurotransmitter ↳ excitatory or inhibitory ↳ parkinsons disease and cocaine use ↳ memory and learning · Aspartate ↳ excitatory ↳ voluntary motor control over skeletal muscle · Glycine ↳ inhibitory · Serotonin ↳ CNS neurotransmitter ↳ affects attention and emotional states ↳ processing motor/sensory info · Gamma-aminobutyric acid (GABA) · Epinephrine · Histamine ↳ inhibitory ↳ opens chloride channels Neuropeptides · small peptide chains synthesized and released by axon terminal · neuromodulator: compounds that alter the release or change the response of neurotransmitters on the postsynaptic neuron · endogenous opiods: ↳ bind to same receptors as morphine or opium ↳ pain relief by inhibiting Substance P Opiods include: - enkephalins - endorphins - dynorphins Dissolved Gases · lipid soluble gases · Nitric Oxide (NO) ↳ from axon terminals in regions of the brain and in smooth muscle of blood vessel walls · Carbon Monoxide (CO) ↳ from specialized axon terminals in brain Post-Synaptic Potential (PSP) · Postsynaptic potentials: graded potentials that develop in the postsynaptic membrane in response to neurotransmitter · Excitatory postsynaptic potential (EPSP) · Inhibitory postsynaptic potential (IPSP) EPSP: IPSP: excitatory response inhibitory response · synapse responds to neurotransmitter with ↳ causes depolarization ↳ promotes generation of action potentials · excitatory neurotransmitters - acetylcholine - glutamate · synapse responds to neurotransmitter with ↳ causes hyperpolarization ↳ suppresses the generation of action potentials · inhibitory neurotransmitters - GABA - glycine Neurotransmitters and neuromodulators can have direct or indirect effects on receptor functioning