SEVEN: CHAPTER Introduction to the Nervous System Can divide the nervous system into two parts: o Central Nervous System (CNS) Brain and spinal cord o Peripheral Nervous System (PNS) All of the nerves and receptors outside of the CNS Functional units: neurons o o Three primary parts to a neuron Cell body Part of the neuron that has the nucleus and most of the organelles o Ribosomes, golgi apparatus, rough ER o Protein synthesis will occur in the cell body o Lipid synthesis will occur in the cell body Dendrites Transmit information towards the cell body o Conducted in the form of graded potentials most of the time Axon Carries information away from the cell body in the form of action potentials Axon hillock or initial segment: the site of action potential formation Axon terminal: site of neurotransmitter release o Neuron Classification By Function Afferent or Sensory Neurons: carry information (action potentials) toward the CNS o Are unipolar (see Structure) o Efferent Neurons or Motor: carry information away from the CNS o Multipolar (see structure) Interneurons or Association Neurons: carry information within the CNS o Typically also Multipolar o Found in the CNS ONLY By Structure Unipolar or Pseudounipolar neurons: cell body, one axon but no true dendrites o Only one process comes off the axon technically Multipolar Neurons: cell body, axon, many dendrites Bipolar Neurons: can only really be found in retina of the eye o IGNORE for this class!! o Look at one afferent neuron How many efferent neurons will there be for every afferent neuron? Approximately 10 efferent neurons for 1 afferent neuron How many interneurons would there be for those? 200,000 interneurons for 10 efferent and 1 afferent neuron o o Important Definitions (Table 7.1) o Central Nervous System (CNS): brain and spinal cord
o Peripheral Nervous System (PNS): nerves, ganglia, and nerve plexuses (outside of the CNS) o Association neuron (interneuron): multipolar neuron located entirely within the CNS o Sensory Neuron (afferent neuron): neuron that transmits impulses from a sensory receptor into the CNS o Motor neuron (efferent neuron): neuron that transmits impulses from the CNS to an effector organ, for example, a muscle o Nerve: cablelike collection of many axons in the PNS; may be "mixed" (contain both sensory and motor fibers) No nerves in the CNS, called TRACTS in the CNS o Somatic motor nerve: nerve that stimulates contraction of skeletal muscles o Autonomic motor nerve: nerve that stimulates contraction (of inhibits contraction) of smooth muscle and cardiac muscle and that stimulates glandular secretion o Tracts or Fiber Tracts: collections of axons that interconnect regions of the CNS o Ganglion: grouping of neuron cell bodies located outside the CNS (within the PNS) Nucleus: grouping of neuron cell bodies within the CNS Glial Cells o Cells which surround neurons and take the place of connective tissue in the nervous system o Found in the CNS and PNS o Make up about 90% of CNS o Physically and metabolically support the neurons o o Glial Cells in the PNS Satellite Cells Found in ganglia (in the PNS) Surround and support the cell body physically, metabolically, provide nutrients etc Schwann Cells Myelin sheaths in the PNS Wrap themselves around the axon to provide protection, insulation, and increase the rate of action potential conduction o Wraps so tightly all components of cytoplasm get pushed to the outside o End up with layers of cell membrane which are primarily lipid Insulates the axon Schwann Cells don't cover entire axon; there are gaps between the Schwann cells o Node of Ranvier: open spots between Schwann Cells o Have high concentrations of voltage gated Na+/K+ channels Myelinated areas: low number of voltage gated Na+/K+ channels o Saltatory Conduction; from saltare: to leap o Myelinated axons conduct action potential much faster probably most important function of the Schwann cells o We need a fast conduction velocity so when we put our finger on something hot, the information can travel to our brain fast enough to send an action potential back to your muscles to move your hand before it gets burnt Demylenation o The action potential might not propagate if this happens o In demylenation we see the action potential doesn't travel down the axon
o Myelin forms late in fetal development Its formation increases rapidly through the first year of life This can be seen in a child's clumsiness at first then they get much more coordinated o Glial Cells in the CNS Myelin is formed in the CNS by oligodendrocytes Cell itself doesn't wrap itself around the axon, it has processes which wrap around the axons Advantage of this system is that each oligodendrocyte covers a lot of axons versus the Schwann cells which only cover a small part of an axon Disadvantage is that if the oligodendrocyte would die, you'll lose a lot of myelin Multiple sclerosis (MS) o Affects oligodendrocytes o Referred to as a demylenation disease o Begin to lose muscle function and eventually can become paralyzed There nothing is wrong with the muscles but the action potentials cannot travel to the muscles because of the loss of myelin Microglial cells Function: to wander CNS and phagocytize debris and pathogens Phagocytosis: eat stuff Equivalent of white blood cells in the CNS Ependymal Cells Line the ventricles of the CNS: central canal, spinal cord, brain etc Function: produce cerebrospinal fluid (CSF) Astrocytes We have a lot of these Named after their star like appearance with foot processes Function: o Take up K+ from the extracellular fluid Picture from looseleaf o Take up neurotransmitters released from axon terminals o Take up glucose from the blood o Necessary for correct formation of synapses o Regulate neurogenesis of stem cells into glial cells and neurons Stem cells: undifferentiated cells which can differentiate into many different things Neural stem cells: can become any neuron or different nervous cells Embryonic stem cells: can become anything o Induce the formation of the blood-brain-barrier Capillaries in brain aren't very permeable
In the liver, for example, there are gaps or holes between the cells in the capillaries where materials can pass from the blood to the interstitial fluid There are no gaps between the cells in the capillaries in the brain This means anything will have to go through the cells Anything that cannot pass through the lipid bilayer easily needs a channel or carrier o Small, nonpolar things can cross through the lipid bilayer easily Protects CNS from a lot of environmental toxins Astrocytes play an essential role in causing the blood-brain-barrier to form Add another layer to the cells as well o Release chemicals that stimulate or inhibit the activity of neurons. Play a role in what neurons are active in your brain Synapses o Functional connection between a neuron and a second cell Second cell may be another neuron or another type of cell: muscle cell, gland cell, etc o Two general types of synapses Electric Synapses Usually referred to as "Gap Junctions" Allow action potentials to travel directly from one cell to the next Not very common in the nervous system Chemical Synapse Involve neurotransmitters Presynaptic Cell: cell that comes before the synapse Postsynaptic Cell: cell that comes after the synapse Synaptic Cleft: gap between the axon terminal (presynaptic) and the postsynaptic membrane Action potential will start at the cell body in the axon hillock, travels down the axon, when it reaches the axon terminal, neurotransmitters (NT) are released into the synaptic cleft, NT will bind to receptors on the postsynaptic membrane; this will increase or decrease the probability of the postsynaptic cell forming an action potential How it works: o Action potential passes down the axon to the axon terminal o Causes the opening of voltage gated Ca+2 channels Ca+2 will enter the axon terminal o Calcium binds to and activates a protein called Calmodulin Calmodulin is ubiquitous: found everywhere o Active calmodulin activates protein Kinase Kinases always phosphorylate: add a phosphate, it will add a phosphate to a protein Protein kinases phosphorylate proteins
o Active protein kinase phosphorylates synapsin which activates the synapsin o Synapsin causes exocytosis of vesicles containing neurotransmitter into the synaptic cleft o Neurotransmitters Excitatory Increase probability that postsynaptic membrane will form an action potential o Neurotransmitter will bind to a receptor this leads to the opening of chemically gated ion channels o Channel that allows passage of cations (+ charged ions) Na+/K+ channel Equilibrium potential: charge that stops an ion from diffusing down its concentration gradient Potassium is much closer to its equilibrium potential If membrane is more permeable to both ions, sodium will go down the concentration gradient and in toward the negative charge, meaning it will diffuse much more than potassium Depolarization which leads to a graded potential SEPT 14 PICTURE #1 Excitatory postsynaptic potential (EPSP) Graded potentials are not all or nothing and don't have a refractory period SUM: when you add things together Summation: when graded potentials are added together o Inhibitory Neurotransmitters Decrease the probability that the postsynaptic membrane will form an action potential This will cause opening of chemically gated K+ channels. Inhibitory Postsynaptic Potential (IPSP) Makes it more negative: hyperpolarized IPSPs can sum with other IPSPs and EPSPs If you add IPSPs and EPSPs, they add together to get RMP (resting membrane potential) Or it can cause the opening of chemically gated Cl- channels SEPT 14 #2 picture o Neurotransmitters can: Open chemically gated ion channels Open chem. Gated Cl- channels Open chem. Gated K+ channels o o Graded potentials can be of various sizes Summation: Temporal Summation: o Release of neurotransmitter from one presynaptic cell at different times
Spatial Summation: o Release of neurotransmitter on the postsynaptic cell at different places/locations o