1. The Neuron and the Nervous System
SGN 8

2. Overview of nervous system
3 overlapping functions
Reception of signal
Interpretation of signal
Reaction based on signal
This is true on organismal and cellular level

3. Nerve signal pathway through organism
Stimulus  sensory receptors (PNS) sensory neurons (PNS)  interneurons (CNS)  motor neurons (PNS)  effector cells (glands, tissue or muscles)

4. The neuron and associated cells
A neuron consists of cell body, signal receiving dendrites and signal transmitting axon

5. Neurons are involved in nerve pathways (as described above)
Sensory neuron  Interneuron  Motor neuron
A complete circuit channels the impulse through brain, but a reflex arc only involves interneurons of the spinal cord

6. Glia cells – supporting cells of central nervous system
Much more numerous in CNS than neurons; numerous functions, from guiding nerve cell migration in embryo to forming a barrier between brain cells and rest of body (blood-brain barrier)
Oligodendrocytes and Schwann cells (types of glia cells) form myelin sheaths (lipid material) around axons of neurons, which insulates and helps speed of nerve impulses (multiple sclerosis, Tay Sachs disease both involve problems with glia cells and brain lipids)

7. The nature of nerve signals All cells have an electrochemical gradient across plasma membrane (membrane potential), with inside of cell typically more negative than outside of cell (called a resting potential)

8. Sodium - potassium ion pump actively establishes gradient
3Na+ out, 2K+ in important in establishing relatively negative interior
Negative interior also produced because open K+ channels that allow K+ to diffuse out

9. Nerve cells are specialized to convey an electrical impulse brought about by the
sudden dissipation of the membrane potential (Na+ rushing in, K+ rushing out) in
cascading fashion down the membrane

10. Only neurons and muscle cells have several types of specialized ion
transporters that allow them to do this (excitable cells)
Lots of open K+ channels (vs
few open Na+ channels)
Gated ion channels (Na+ and K+)
Ligand-gated
Voltage-gated

11. Ligand -gated ion channels
For Na+ and K+
Typically stimulated by neurotransmitter

12. Voltage-gated ion channels Stimulated by depolarization (shift in membrane potential) in adjacent membrane

13. Transmission of a nerve impulse Beginning with reception of the signal in the dendrite and continuing to the passage of the impulse to the next neuron

14. The dendrite’s postsynaptic membrane ligand-gated ion channels are stimulated by neurotransmitters, causing them open, which depolarizes the adjacent membrane (inside becomes less negative)

15. Adjacent voltage gated ion channels are stimulated to open, which reverses potential, further stimulating additional VGIC down the line

16. These are weak signals, however, and fade out unless reinforced by a rendezvous of other signals, received by other synapses of the same neuron, at the axon hillock If the depolarization at the hillock is strong enough to push the change in charge past the threshold, a depolarization wave is generated, called an action potential, which travels down the axon to the synapse
Potential at the axon hillock

17. Impulse moves forward and not backward because there is brief phase after stimulation (refractory phase) wherein membrane is insensitive to stimulation

18. When the action potential reaches the presynaptic membrane of the axon bulb it induces voltage gated calcium channels in the cell membrane to open, allowing a diffusion of calcium into the cell from the extracellular fluid, where it is concentrated This increase in calcium concentration (calcium as a second messenger) induces fusion of vesicles with the membrane and release of neurotransmitters , by exocytosis, into the synaptic cleft

19. Neurotransmitters stimulate ligand gated ion channels in the receiving neuron and the process continues ; neurotransmitters can also stimulate muscle cells, gland cells, or be released into the blood stream (neurosecretory cells)

21. Speed of nerve impulse along axon and along nerve pathway
Speed of signal increases as diameter of axon increases, as number of synapses on a neuron increases, and in other ways

22. Myelination increases impulse speed
Saltatory conduction – axons are myelinated but segments of membrane are left uncovered; in these segments voltage-gated ion channels are concentrated; signal jumps from uncovered segment to uncovered segment, speeding up impulse

23. A synapse is a unique cell junction between one neuron’s axon terminal and another neuron’s dendrite or cell body, muscle cell or gland cell, or lumen of blood vessel
Most synapses are chemical synapses; electrical signals cannot jump the synaptic cleft (some electrical signals can pass directly through gap junctions)
Neurotransmitters are quickly removed from synapse, causing signal to be transient
Synapses help to assure that signal will be transmitted in only one way, and also allows for regulation of nerve impulse

24. Numerous other factors regulate and complicate transmission of nerve impulse, for example
A single neuron might form thousands of receptive synapses with other cells, some excitatory and some inhibitory (neurotransmitters released inhibit opening of ion gates or hyperpolarize membrane)
Summation of numerous stimulatory events (in time or space; often excitatory and inhibitory at same time) allows axon hillock to reach threshold (or not), generating action potential
After refractory period signal might continue, be inhibited or be desensitized (hyperpolarization)
 Same neurotransmitter can produce different effects on different cells, based on receptors in postsynaptic membrane
Some neurotransmitters bind directly to chemically gated ion channels and open them directly, causing rapid diffusion of ions and rapid response
Some neurotransmitter receptors initiate complicated signal transduction pathways, so response is delayed

25. Examples of neurotransmitters Acetylcholine
Used as a neuron to neuron signal within both the peripheral and central nervous system; produced by motor neurons to stimulate skeletal muscle

26. Biogenic amines – one or two ring structures with nitrogens
Epinephrine (adrenaline), dopamine, serotonin, etc
Typically used within the CNS

27. Modified amino acids
GABA
Other substances such as small peptides and even gases such as NO

28. Evolution and Diversity of Nervous Systems
Simplest plan involves only nerve nets or nerve nets with coordinating masses of nerve cells called ganglia (jellyfish)

29. Many simple invertebrates (worms, for example) show primitive cephalization (existence of brain like mass of cells at anterior end with some degree of control of body), and two or one nerve cords running through body with localized control by numerous ganglia

30. Many complex invertebrates (insects, arachnids) have more developed brains with greater control of body and single nerve cord, but still ganglia associated with body segments

31. Vertebrates have developed brain with centralized control in brain and
Vertebrates have developed brain with centralized control in brain and dorsal spinal cord showing lessened localized control

32. Evidence suggests that animals evolved in order presented above, and so did nervous system, with change from autonomous control throughout the body to increased control in head region Within several groups (for example, arthropods, mollusks) there is great diversity

33. Vertebrate nervous system
Peripheral NS - sensory vs motor neurons
Central NS - interneurons
Somatic versus autonomic system (PNS)
SNS – carries signals to skeletal muscles; voluntary or reflex action
ANS– carries signals that regulate internal environment; involves smooth and cardiac muscle; generally involuntary, and divided into…
Sympathetic – arousal and energy generation; induces activity
Parasympathetic – calming and return to emphasis of self maintenance