Chapter 12 Nervous Tissue Central Nervous System CNS brain and spinal cord Interneurons association neurons entirely in CNS receive signals process store retrieve information make decisions Peripheral Nervous System PNS nerves and ganglia Sensory afferent division carries signals from receptors to CNS Somatic sensory from skin muscles bones joints visceral sensory from visceral of the thoracic and abdominal cavities Motor efferent division carries signals from the CNS to glans and muscles cells responses Somatic motor to skeletal muscles voluntary somatic reflexes Visceral motor autonomic nervous system no voluntary control Neurons nerve cells Properties Highest degree of excitability respond to stimuli Conductivity electrical signals quickly conducted to other cells Secretion release of neurotransmitters stimulate next cell Soma neurosoma cell body single nucleus Nissl bodies rough ER compartmentalized by microtubules and neurofibrils Dendrites primary site for receiving signals from other neurons Axon hillock gives rise to axon nerve fiber specialized for rapid conduction of nerve signals axoplasm axolemma Terminal arborization with synaptic knobs with synaptic vesicles Structure Variation Multipolar one axon multiple dendrites brain spinal cord Bipolar one axon one dendrite olfactory some in retina inner ear Unipolar single process leading from the soma T branches sensory signals to spinal cord Anaxonic multiple dendrites no axon brain retina Axonal transport Anterograde transport away from soma down the axon kinesin Fast moves organelles ions enzymes small molecules etc Slow moves enzymes etc renews worn out components supplies for regeneration Retrograte toward the soma up the axon dynein Fast returns used synaptic vesicles etc to the some Informs of the conditions at the terminals Some pathogens exploit this process to invade nervous system Neuroglia Supportive cells Central Nervous System Oligodendrocytes insulate the nerve fiber from ECF with myelin sheath faster conduction Ependymal line internal cavities in brain and spinal cord Produce and circulate CSF Microglia small macrophages Also aid in synaptic remodeling Astrocytes form supportive framework blood brain barrier Monitor blood vessels blood flow Convert blood glucose to lactate Secrete nerve growth factors Monitor levels of neurotransmitters and K released into tissue fluid by absorbing them When neurons are damaged form hardened scar astrocytosis sclerosis Peripheral Nervous System Schwan cells neurilemmocytes wrap around nerve fiber myelin sheath Assist in regenerations of damaged fibers Satellite cells surround neurosomas in ganglia insulation chemical environment regulation Myelin sheath insulating layer around a nerve fiber makes it faster nodes of Ranvier gaps between the segments of myelin sheath initial segment the short section between axon hillock and first glial cell trigger zone axon hillock initial segment internodes myelin covered segments Regeneration of nerve fiber PNS 1 Fiber distal to the injury degenerates 2 Soma swells ER breaks up nucleus moves off center some neurons die at this stage 3 Axon stump sprouts multiple growth processes 4 Regeneration tube formed near the injury by Schwann cells basal lamina neurilemma 5 One growth process finds the way into the tube the other processes retract 6 The regeneration tube guides the growing sprout back to its original target cell 7 When contact is established soma returns to its original appearance Electrophysiology of Neurons Resting membrane potential electrolytes are unequally distributed diffusion of ions down the concentration gradient through the membrane selective permeability of the membrane most permeable to K ions electrical attraction of anions and cations to each other RMP is maintained by sodium potassium pump 70 of ATP use of nervous system Local potential Neuron stimulated usually at dendrite Na channels open depolarization local potential travels to trigger zone Graded vary in magnitude according to the strength of the stimulus Decremental get weaker as they spread Reversible if stimulation ceases membrane returns to RMP Can be excitatory or inhibitory Action potentials more dramatic change produced by voltage gated ion channels Occurs if local potential is strong enough 55mV threshold if reaches trigger zone Threshold 55mV Na channels open K is more slow further depolarizes positive feedback 0mV Na channels start to inactivate 35mV polarity is reversed by this time K channels are fully open K ions are repelled out repolarizes slightly more K flows out hyperpolarization All or none law non graded Nondecremental Irreversible Refractory period absolute until Na channels close and relative until K channels close Unmyelinated fibers continuous conduction Myelinated fibers saltatory conduction action potentials occur at nodes of Ranvier Presynaptic neuron releases neurotransmitter postsynaptic neuron responds to it Axodendritic most common axoaxonic most often are inhibitory axosomatic Signal conduction Synapses Neurotransmitters Acetylcholine Excites skeletal muscle Inhibits cardiac muscle Amino acids Glutamate Aspartate spine 75 excitatory transmission in the brain learning memory Mostly ionotropic Glycine GABA Inhibits neurons in brain spinal cord most common and retina The most common inhibitory neurotransmitter in the brain alcohol enhances it Monoamines Norepinephrine Epinephrine Brain spinal cord etc dreaming walking mood Excite inhibit smooth muscles and glands Mostly metabotropic Neuropeptides Enkephalins Inhibit substance P Endorphins Suppress pain Cessation of the signal 1 Presynaptic nerve fiber stops adding new neurotransmitters 2 Diffusion neurotransmitters leave synapses in CNS astrocytes absorb and return to neurons 3 Reuptake by synaptic knob and break down 4 Degradation in synaptic cleft and reabsorption of components Neuromodulators increase release of neurotransmitters adjust sensitivity of postsynaptic neurons alter the rate of reuptake or break down Nitric oxide neuropeptides etc Neural Integration the ability of neurons to process information store recall it make decisions chemical synapses are the decision making devices of the system Excitatory postsynaptic potential EPSP neuron more likely to fire Na flow in Inhibitory postsynaptic potential ISPS less likely Cl flow in or K flow out Summation process of adding up postsynaptic potentials and responding to their net effect Temporal and spatial Facilitation a process