Chapter 5 Nerve Cells l Chris Rorden University

Chapter 5 Nerve Cells l Chris Rorden University of South Carolina Norman J. Arnold School of Public Health Department of Communication Sciences and Disorders University of South Carolina 1

Nerve cells l Around 100 billion neurons in the brain initially – Adult stage 15 billion Means of communication in the nervous system l Excitatory and inhibitory in nature l 2

Nerve cells l Neurons – – l Sensorimotor activities Higher mental functions Neuroglia cells – Support and protect neurons 3

Neurons l Three parts 1. 2. 3. Soma: body with nucleus Dendrite: receives inputs Axon: sends information – – Signal is transmitted electrochemically down shaft of axon. Connects to target cells (neurons, muscles, glands) at synapse. Can either inhibit or excite target cells. Can be up to x 10, 000 length of soma 4

Soma l Protoplasm – – Nucleus - DNA Cytoplasm l l Protein filled liquid Microscopic Organelles – – – neurofibril mitochondria – powerhouse of the cell lysosomes – digest worn out organelles Ribosomes – Create proteins l Nissl bodies Golgi complex – Package Proteins Endoplasmic reticulum - system of tubes for transport of materials within cytoplasm 5

Metabolic activity l Firing requires oxygen and glucose – – Constant supply is required (via capillaries of vascular system). If supply is disrupted: l Meaningful neural activity will cease immediately l Neurons will die within hours if supply is not restored 6

Intracellular transport l Proteins generated and degraded in the soma. – l Proteins must be moved to axons/synapse Proteins transported via Microtubules in Axon – – Anterograde Transport – toward synapse (new proteins, transported by protein kinesin) Retrograde Transport – toward cell body (recycling, transported by protein dynein) Suicide Transport (apoptosis, form of retrograde transport) Microtubules act as tracks for moving proteins 7

Nucleus Control Center l Contains DNA l Contains Nucleolus l – – Engaged in Protein Synthesis Contains RNA - Code for Enzyme for Synthesis of Specific Enzymes Needed 8

Nuclear structures and functions l l l Neurofibrils: Channels for Communication in Cell. These filaments repel each other, so their development enlarges the diameter of the axon and dendrite Mitochondria: Contain Enzymes for Metabolism: creates ATP, used for chemical energy. Free Ribosomes: Synthesize Proteins Lysosomes: Intracellular Digestion (break down waste) Nissl Bodies: Amino Acids to Proteins Golgi Complex: Protein Secretion and its Transportation (post office of the cell) 9

Neural communication l Dendrites: – – l Afferent: influenced by other neurons/receptors. Branching and short Axons – – – Efferent: influence other neurons/receptors. Dependent on electrical signal at axon hillock End in axon terminals with synaptic knobs that release neurotransmitters. 10

Myelin Sheath l Myelin Sheath – – Multilayer Lipid (fatty) Material Insulates Nerve Fiber l Analogy: – plastic insulation on wire in house Regulation l Speed: Signals can jump between nodes l Efficiency: Escape of Electrical Energy – Segmented: nodes of Ranvier with internodes 11

Saltatory conduction Normally, chemical signal is relatively slow down an axon. l However, in myelinated segments the signal can jump between nodes. l – Facilitates Speed of Fiber (120 msec/m) 12

Glial cells for myelin l In CNS: – – l Oligodendroglia: single cell can support Myelin sheaths for up to 50 axons Multiple Sclerosis In PNS: – – Schwann cells: each covers only about 1 mm, so many cells per axon. Can Regenerate l l Injury to Motor Pathways Amputation 13

Synapse l l Connection Between Neurons Three Parts – Knob (Axon; presynaptic) l l Prozac – a selective serotonin (5 HT) reuptake inhibitor l Interferes with 5 HT presynaptic reuptake pump l Increases concentration of 5 HT in synaptic cleft – Synaptic Cleft: gap between one neuron and the next. l – Contain Vesicles Filled with Neurotransmitters Released when Necessary Chemically Stimulate Receiving Nerve Cell Body 2/100, 000 mm Wide Receptive Sites (Receiving Cell Axon or Dendrite; postsynaptic) 14

Action Potentials l l l When a neuron fires, it releases neurotransmitters from terminals into synaptic cleft. Chemical in Neurotransmitter Stimulates or inhibits Postsynaptic Cell (Dendrite, Axon or Cell Body) If the postsynaptic cell receives enough stimulation, this neuron will fire. – – Firing is an all-or-none response After firing, neuron must wait a while before it can fire again. 15

Types of neuron l Classified By Number of Processes – – – Multipolar – many dendrites, one axon (brain) Bipolar – retina, smell, facial nerve Unipolar – dorsal spine 16

Axonal Length Types l Golgi Type I – – l Axons range from feet to inches. E. G. Motor cortex Golgi Type II – – Axons are very short E. G. interneurons 17

MCQ l A. B. C. D. How do cooks keep chicken from tasting dry? Soften meat with a hammer (tenderizing) Soak in water before cooking (hydrating) Soak in salty water before cooking (brining) Cook in boiling water (boiling) 18

Brining l Brine has more salt than inside meat: salt diffuses into cell. – l Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The increased salinity of the cell fluid causes the cell to absorb water from the brine via osmosis. – Osmosis is the diffusion of a solvent (frequently water) through a semi-permeable membrane. 19

Gradients l l l 3 Sodium ions (Na+) are pumped out of the cell 2 Potassium ions (K+) are pumped into the neuron This creates gradients – – – l Cell is negatively charged relative two outside Cell has less sodium than outside Cell has more potassium than outside The cell membrane maintains gradients 20

Gradient l l If a sodium channel is opened, sodium will rush in (due to diffusion). Cell will depolarize. If a potassium channel is opened, K+ will rush in. Na+ Na+ K+ K+ K+ Na+ Na+ 21

l l Excitatory Connections Na channels Open Na+ rush in Cell becomes depolarized l l Inhibitory Connections K channels Open K+ rush out Cell becomes hyperpolarized 22

When does a cell fire? At any moment, the voltage of a cell will be influenced by many excitatory and inhibitory synapses. l Neuron will fire if sum of all inputs crosses threshold. l 23