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

Around 100 billion neurons in the brain initially 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

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

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

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

Firing requires oxygen and glucose 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

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

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

Nuclear structures and functions 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)

Neural communication Dendrites: Axons 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.

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

Normally, chemical signal is relatively slow down an axon. 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)

Glial cells for myelin In CNS: In PNS: 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

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

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.

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

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

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)

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.

The cell membrane maintains 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

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+

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

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.