Chapter 5 Nerve Cells

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

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

3. Nerve cells • Neurons • Sensorimotor activities • Higher mental functions • Neuroglia cells • Support and protect neurons

4. Neurons • Three parts • 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 x10,000 length of soma

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

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

7. Intracellular transport • Proteins generated and degraded in the soma. • 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

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

9. Nuclear structures and functions • Neurofibrils: Channels for Communicationin 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)

10. Neural communication • Dendrites: • 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.

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

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

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

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

15. Action Potentials • 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.

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

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

18. MCQ • 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)

19. Brining • Brine has more salt than inside meat: salt diffuses into cell. • 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.

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

21. Gradient • 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+ Na+ Na+ Na+ Na+ K+ Na+ K+ Na+ K+ K+ K+ Na+ Na+ Na+

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

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