1. Chapter 48 ~ Nervous System

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3. downloads\animationsRaven test.htm

4. Nervous System Overview
Sensory Input
Integration
Motor Output-signal conducted from processing center to effector cells
Signals Conducted by Nerves-extensions of nerve cells
Nervous System Composition:
Neurons and Glia (supporting cells)
Neurons communicate information via electrical and chemical signals

5. Both Divisions of the Nervous System Involved
1. Central nervous system (CNS)~ brain and spinal cord; Integration
2. Peripheral nervous system (PNS)~ sensory (input) and motor neurons (output)
Effector cells~ muscle or gland cells
Nerves~ bundles of neurons wrapped in connective tissue

6. Neuron structure Neuron- structural and functional unit
Cell body- nucelus and organelles
Dendrites- signals to cell body. Short, numerous
Axons- away from cell body. Long,
Myelin sheath- supporting, insulating layer produced by Schwann Cells
Schwann cells-PNS support cells; surround axons
Axon hillock-Hillock-axon extends from here
Synaptic terminals~ neurotransmitter releaser
Synapse- gap / neuron junction

7. 3 Classes of neurons
1. Sensory neuron: receive & convey from sensory environment information to spinal cord
2.Interneurons: information integration; located in CNS. Synapse only with other neurons.
3. Motor neurons: convey impulses from CNS to effector cell. (muscle or gland)

8. Neurons Grouped into Nerve Circuit
The Reflex Arc
Simplest :
Knee-Jerk Reflex (Patellar Reflex)
Stretch receptor
simple response; sensory to spinal cord to motor neurons—knee contracts

9. Neural Signaling
Signal transduction depends on voltages across neuron plasma membranes.
Membrane Potential: voltage differences across the plasma membrane).
Net negative charge of about -70mV
Ions
Intracellular ( -) ; K+ principal cation
Large organic ions- anions
Extracellular (less negative) Na+- principal cation
Cl- main anion.
Ion channels- ungated, gated; all selective
K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane

10. Creating & Maintaing the Membrane Potential
Na + - K + Pumps --pump against their conc. gradients
ATP
K+ pumped back in
Na+ pumped back out

11. Changes in membrane potential key to neural transmission
Only neurons and muscle cells can change their membrane potentials in response to stimuli
Excitable Cells
Sensory neurons-environmental stimuli
Interneurons stimuli transmitted via other neurons
Resting Potential: M.P. of excitable cell at rest.
Change due to flow of ions as gated ion channels open.
stimuli cause ion channels to open
Stimuli that open K+ channels HYPERPOLARIZE the neuron
Stimuli that open NA+ channels DEPOLARIZE the neuron

12. Graded Potentials –these voltage changes
1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative
2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

13. Clusters allow coordination of activities by all parts of Nervous System
Nucleus=cluster of nerve cell bodies in the brain
Ganglion (ganglia)= cluster of nerve cell bodies in the PNS
Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection

14. The Types of Glia Functions: Reinforce, protect
1. Astrocytes-encircle capillaries of brain—blood-brain barrier
2. Oligodendrocytes—form mylen sheaths—insulate CNS
3. Schwann Cells—form the myelin sheaths around axons in PNS

15. Mylenation Electrical insulation—lipid is poor conductor
Increasing speed of nerve impulse propagation
Multiple Sclerosis: myelin sheaths deteriorated-los of coordination

16. Neural signaling
Gated ion channels (open/close response to stimuli): photoreceptors; vibrations in air (sound receptors); selective
1. chemically-gated (neurotransmitters)
2. voltage-gated (membrane potential changes)

17. Normal Membrane Potential
Resting Potential: Resting Neuron mV
Cytoplasm is negatively charged relative to cell interior
Resting potential~ the membrane potential of the unexcited nerve.
A change in voltage MAY result in an electrical impulse.

18. The rapid change in membrane potential in an excitable cell
When the Threshold potential is reached, usually sl. More positive (-50 to -55 mV)….
The action potential is triggered….
The rapid change in membrane potential in an excitable cell
b/c stimulus triggered the selective opening and closing of voltage-gated ion channels

19. Action Potential- All Or None change in the Membrane Potential Phases
1. Resting stage •both channels closed
2-Depolarization: •a stimulus opens some Na+ channel gates
Na+ influx reverses membrane polarity.
Threshold reached. (cell interior sl. positive)
Action potential generated .
3-Repolarization •Na+ channels close. K+ channels open; K+ leaves
cell returns to resting potential—then ..
4-Undershoot •K+ channels still open-temporarily HYPERPOLAR.

20. The Action Potential
Followed by a Refractory period~ insensitive to stimulus.
Amplitude not affected by stimuli Intensity

21. Action Potentials are self-propagating
Action Potential regenerated along axon membrane
begins at Axon Hillock
“Travel” of the action potential is self-propagating
One direction only.
Nodes of Ranvier-action potential jumps from one node to the next
Gaps, ion sensitive channels concentrated here, extracellular fluid contact here
Forward direction only

22. Action potential speed:
1) Axon diameter (larger = faster; 100m/sec)
2) Saltatory Conduction:
Mylenation
Nodes of Ranvier (concentration of ion channels in gaps of the myelin).
A.P. “jumps” from node to node. 120m/sec

23. Chemical or Electrical Communication between cells occurs at synapses
Synapse-tiny gap
Presynaptic cell: transmitting cell
Postsynaptic cell: receiving cell
1) Electrical Synapses-via gap junctions; no delay or less in signal strength; less common; fish tail-swim away quickly from predator
2) Chemical Synapses: synaptic cleft separates pre and post-synaptic cells.
Not electrically coupled

24. Synaptic communication
Synaptic cleft: separation gap
Synaptic vesicles: neurotransmitter releasers
When an Action Potential arrives at synaptic terminal of presynaptic cell
Causes Ca++ influx;
Synaptic vesicles fuse with
presynaptic membrane and release….
Neurotransmitter
Neurotransmitters quickly degraded

25. Neurotransmitter may do one of the following
1. Excite the membrane by depolarization Or
2. Inhibit the postsynaptic cells by hyperpolarization

26. Types of Neurotransmitters
Acetylcholine (most common)
may be excitatory or inhibitatory
skeletal muscle
Biogenic amines (derived from amino acids) •norepinephrine , epinephrine •dopamine •serotonin (from tryptophan)
Amino acids
GABA—most abundant inhibitory transmitter in brain
Neuropeptides (short chains of amino acids) •endorphin-natural analgesics for the brain

27. Gaseous Signals of the Nervous System
NO (nitric oxide)—blood vessel dilation.
Acetylcholine stimulates blood vessel walls to release NO; neighboring smooth muscles relax & dilate heart’s blood vessels.
Nitroglycerine is converted to NO—similar response

28. Nervous system organization tends to corrolate with body symmetry

29. Vertebrate PNS Cranial nerves (brain origin)
Spinal nerves (spine origin)
Sensory division
Motor division •somatic system voluntary, conscious control •autonomic system √parasympathetic conservation of energy √sympathetic increase energy consumption

30. The Vertebrate Brain
Forebrain •cerebrum~memory, learning, emotion •cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres •thalamus; hypothalamus
Midbrain •inferior (auditory) and superior (visual) colliculi
Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions