Nervous System Part 1 PDF

Nervous System Dr. Luann Cullen Functions of the Nervous System Receive information from receptors to monitor changes in external AND internal environments Process and integrate all incoming information Elicit appropriate responses of cells and tissues to maintain homeostasis and beh...

Nervous System Dr. Luann Cullen Functions of the Nervous System Receive information from receptors to monitor changes in external AND internal environments Process and integrate all incoming information Elicit appropriate responses of cells and tissues to maintain homeostasis and behavior  Anatomical organization Central Nervous System (CNS) – Brain – Spinal cord Peripheral Nervous System (PNS) – All other nerves and ganglia CENTRAL AND PERIPHERAL NERVOUS SYSTEM The structures of the PNS are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the CNS (nuclei and tracts) are not obvious from this overall perspective and are best examined in prepared tissue under the microscope. The central nervous system (CNS) GRAY MATTER AND WHITE MATTER A brain removed during an autopsy with a section removed shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain and contains many different nuclei. White matter contains tracts.  Gray and White Matter Gray matter – Neuron cell bodies, dendrites, synapses. – Cortex (surface layer) of cerebrum & cerebellum – Nuclei: masses of functionally related cell bodies. White matter – Axons & nerve tracts that are myelinated (Tract = bundle of axons in CNS) – Inner layers of cerebrum & cerebellum – Outer layers of spinal cord  CROSS-SECTION OF SPINAL CORD The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. Note that the organization of gray and white matter is reversed compared to the brain.  TRACTS AND NUCLEI IN THE CNS In the CNS, Nuclei (groups of cell bodies) are interconnected by tracts (axons) Analogous to ganglia and ……………… ……………………….nerves of the PNS  DIFFERENT DEFINITIONS OF NUCLEUS (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the CNS is a localized center of function with the cell bodies of several neurons, shown here circled in red. The PNS (peripheral nervous system) Structure of a nerve (PNS) Connective tissue coverings Epineurium Perineurium Endoneurium Fascicles Blood vessels Axons of neurons Schwaan cells (myelinating) Afferent fibers (approaching the CNS) Efferent fibers (exiting the CNS) Structure of a ganglion (PNS) Cluster of neuronal cell bodies Cells with similar function Axons project to same structures Satellite cells homeostatic support for neurons  SOMATIC, AUTONOMIC, AND ENTERIC STRUCTURES OF THE NERVOUS SYSTEM The CNS includes only the brain and spinal cord Somatic structures include the spinal nerves (both motor and sensory), as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract. Organization of the nervous system  Functional Organization of the PNS Sensory Division (Afferent signaling) – Incoming signals to CNS from sensory receptors in the body – Somatic Sensory division – from receptors in skin, muscle, bones, joints, & specialized sensory organs (eye, ears, nose) – Visceral Sensory division – from internal visceral organs (heart, lungs, stomach, bladder) Divisions of the visceral motor division (the autonomic nervous system) Sympathetic division – “fight or flight” responses (increase heart rate, breathing rate, inhibit digestion) Parasympathetic division – calming effects (slows heart rate, breathing rate, stimulates digestion) THE SENSORY INPUT Receptor structures in the body sense environmental stimuli. For example, temperature, touch, pain and stretch. The special senses involve receptors for light, sound, odor NEURON CLASSIFICATION BY SHAPE Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites. OTHER NEURON CLASSIFICATIONS Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong. PARTS OF A NEURON The major parts of the neuron are labeled on a multipolar neuron from the CNS. THE PROCESS OF MYELINATION Myelinating glia wrap several layers of cell membrane around the membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve. In the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. THE MOTOR RESPONSE On the basis of the sensory input and the integration in the CNS, a motor response is formulated and executed. RECEPTOR NEURONS CLASSIFICATION BY CELL TYPE Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. (c) Photoreceptors in the eyes, such as rod cells, are examples of specialized receptor cells. Receptor cells generate a receptor potential that triggers an action potential which carries a signal to the CNS where the sensory information is processed. GLIAL CELLS OF THE CNS Astrocytes-homeostasis Microglia-immunity Oligodendrocytes-myelin Glial Cells of the CNS The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways. GLIAL CELLS OF THE PNS Ganglionic neuronal cell bodies are covered with satellite cells Axons within a nerve are covered by Schwann cells. THE CELL MEMBRANE OF A NEURON CONTAINS MANY TRANSMEMBRANE PROTEINS The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of receptor proteins that serve as ion channels. WHEN THEIR GATES ARE OPENED, ION CHANNELS CAN CHANGE THE MEMBRANE VOLTAGE OF A CELL A recording electrode is inserted into a cell, and a reference electrode outside the cell. By comparing the charge measured by these two electrodes, the membrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside. Most cells have a negative voltage near -70 mV.  Resting Membrane Potential The Na+/K+ ATPase generates a voltage across the membrane of all cells. -70 mV for a resting (unstimulated) neuron Outside (mM) 5 Na+ 3 Na+ pumped out 135 K+ 2K+ pumped in 1 ATP hydrolyzed Inside (mM) 5 K+ 135 Na+ The resting membrane potential K+ K+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ Na+ 2 ADP + Pi K+ Na+ Na+ + 3 ATP Vm Na + Na/K K+ Na Na+ ATPase Na+ Ca 2+ Na+ LEAKAGE CHANNELS K+ and Na+ ions also need to equilibrate across the membrane using channels that don’t have any gating (leak channels). This prevents the Na/K ATPase from building up gradients that are too big. MECHANICALLY GATED CHANNELS When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Ions diffuse across the membrane along their concentration gradients. The charge (voltage) across the membrane changes accordingly. VOLTAGE-GATED CHANNELS Voltage-gated channels open when the resting membrane voltage rises to a threshold voltage. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion. LIGAND-GATED CHANNELS When the neurotransmitter, (acetylcholine, glutamate, GABA, etc.), binds to a specific location on the extracellular surface of the channel protein, the gate opens to allow select ions through. The ions, in this case, are sodium, calcium, and potassium.  NEUROTRANSMITTER RECEPTOR TYPES (a) An ionotropic receptor is a channel that opens when the neurotransmitter binds to it. (b) A metabotropic receptor is a complex that causes metabolic changes in the cell when the neurotransmitter binds to it.  Classes of neurotransmitters Acetylcholine Monoamines (biogenic amines) – Modified amino acids – Epinephrine (adrenaline) – Norepinephrine (noradrenaline) catecholamines – Dopamine – Histamine – Serotonin (5-HT = 5-hyrdoxytryptamine) Amino acids – Glycine – Glutamate – Aspartate – GABA (γ-aminobutyric acid) Neuropeptides – 2-24 amino acid chains – β-endorphin – Substance P (pain) THE SYNAPSE The synapse is a connection between a neuron and its target cell (which is not necessarily a neuron). The presynaptic cell contains the synaptic bouton where Ca2+ enters to cause vesicle fusion and neurotransmitter release. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor on the postsynaptic cell. The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal uptake, or glial uptake. GRADED POTENTIALS Graded potentials are caused by opening of ion channels that are activated in the cell membrane. Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. Excitatory post synaptic potential Inhibitory post synaptic potential Excitable tissues generate action potentials In an action potential, the membrane potential changes polarity (becomes positive) and the change spreads along the membrane (like a wave). Spreading is due to opening of voltage gated Na+ channels as the neighboring membrane potential reaches the gating threshold for opening. GRAPH OF ACTION POTENTIAL Voltage is measured at a single spot on the membrane as an action potential passes by. The action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to its resting potential.  STAGES OF AN ACTION POTENTIAL (1) At rest, the membrane voltage is -70 mV. (2) The membrane begins to depolarize when an external stimulus is applied. The potential crosses the threshold for voltage gated Na+ channels to open (3) The membrane voltage begins a spontaneous and rapid rise toward +30 mV (Na+ channels open). (4) The membrane voltage starts to return to a negative value (voltage gated K+ channels open). (5) Repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) The voltage returns to the resting value shortly after hyperpolarization (as K+ channels slowly close). Saltatory conduction Node of Ranvier  Summation The combined effect of ALL postsynaptic potentials resulting from multiple stimuli converging on a neuron’s dendritic tree Note: By intense we mean high frequency Axon hillock is the trigger zone for forming a post-synaptic action potential (lots of VGSCs) Note: lower frequency stimulation by multiple neurons caused post synaptic cell to fire POSTSYNAPTIC POTENTIAL SUMMATION The result of summation of postsynaptic potentials is the overall change in the membrane potential. At point A, several different excitatory postsynaptic potentials add up to a large depolarization. At point B, a mix of excitatory and inhibitory postsynaptic potentials result in a different end result for the membrane potential. QUESTIONS?