Neurons, Synapses and Signaling A peek inside the nervous system

Peripheral vs Central Nervous System

Neural Networks

Neurons Nerve - bundle of axons

Supporting Cells Glial Cells

Functions of Supporting Cells Schwann Cells - surround axons of PNS, form myelin sheaths Oligodendrocytes - form myelin sheaths around axons in CNS, white matter of CNS Microglia - phagocytose pathogens and cellular debris in CNS Astrocytes - cover capillaries, create blood-brain barrier Ependymal cells - epithelial lining of ventricles (brain cavities) and central canal of the spinal cord

Neurons are excitable neurons and muscle cells are excitable transmit signals 20-100 meters/second cell membrane generates impulses or action potentials

Resting Potential voltage - measures the electrical charge difference between 2 points (potential energy), produce currents ions carry electric current (Na+, K+, Ca2+, Cl-) membrane potential - different ion concentrations across a membrane ->voltage inactive neuron -60 to -70 mV due to many negatively charged proteins the inside of the cell is more negatively charged compared to the outside maintained by K+ leak channels

Na+-K+ Pump (ATPase) plasma membrane impermeable to ions channels and ion transporters, glycoproteins, all animal cells uses ATP to pump 3 Na+ outside and pumps 2 K+ inside against the gradient creates concentration gradients generates resting potential [Na+] higher outside the cell while [K+ ] is higher inside the cell

K+ Leak Channels leaves behind an overall negative charge always open, K+ diffuses out due to concentration gradient leaves behind an overall negative charge negative charges want to pull K+ back in the tug-and-pull created is the resting potential maintains the resting potential

Gated Channels voltage-gated - open or close due to changes in membrane potential stretch-gated - open or closed to due to tension applied to the cell membrane ligand-gated - ligand binding sites, open and closed based on the binding of the matching ligand (neurotransmitter)

Action Potential Brief, rapid changes in membrane potential Originates in the axon hillock, after -50mV or greater membrane potential reached Voltage gated Na+ channels open, depolarizing the membrane Na rushes into the axon, briefly creating a positive charge within the axon, from -65mV to +40 mV Each region stimulates the next region of the axon K+ channels open restoring the membrane potential Depolarization - charge on inside of membrane becomes less negative relative to the outside Hyperpolarization - charge on inside becomes more negative Positive feedback

Synapses signal transmission via cell-to-cell contact points synaptic cleft is 20-30 nm (human hair 20,000 nm) chemical synapse - neurotransmitters electrical synapse - gap junctions that spread the action potential

Chemical Synapse

Neuromuscular Junction

Neurotransmitters Acetylcholine -muscle stimulation, memory, learning; nicotine binds to receptor, sarin (nerve gas) blocks enzymatic breakdown of, botulinum toxin inhibits release of Glutamine - long term memory GABA - amino acid in brain, inhibitory synapse; Valium reduces anxiety by binding to a GABA receptor Biogenic amines - dopamine, serotonin: LSD binds to receptors leads to hallucinations; Parkinson’s due to lack of dopamine; Prozac inhibits reuptake of serotonin Neuropeptides - endorphins- relieve pain, produce euphoria; opiates bind to same receptors

Multiple Sclerosis Loss of myelin in the CNS - demyelinating immune mediated response damage to nerve fibers most common autoimmune disorder

Lou Gehrig’s Amyotrophic lateral sclerosis 5-10% inherited, autosomal dominant Death of neurons that control voluntary muscles Neurotransmitter Glutamate may be involved potentially many causes

Parkinson’s