The Nervous System

1. The Nervous System

2. Different Nervous Systems Among Animals • Hydra- has a net like nervous system because it is less structured and has cavities that expand and contract so a net is advantageous. • Sea star- has a central nerve ring that has radial nerves branching off down each of the limbs. This nervous system accommodates its body structure and is the reason that it can regenerate from a limb and a center portion. • Planarian- planarians have two parallel nerve cords with transverse nerves attached to them. These transverse nerves exist to accommodate the muscles that contract the body for movement. • Insect – an insect has a brain with a ventral nerve cord that extends down its back with segmental ganglia at each segment of its body. • Squid- the squid has a complex brain with nerve cords down the length of its body which is split at two ganglia into multiple smaller nerve cords. • Vertebrate- vertebrates have a complex brain dorsal nerve cord called a spinal cord. Off of the spinal cord there are ganglion that receive information from the sensory nerves.

3. Information Processing • sensory input- during sensory input peripheral nerves send electro-chemical signals to the Central Nervous System where the signal is interpreted and analyzed by the interneurons. • Integration- the process that takes place in the interneurons that consists of analysis and interpretation of chemical messages. • Motor output- the sending of signals from the central nervous system to motor neurons to perform an action.

4. Neurons • Types of neurons • Sensory neurons- receive and transmit data from the environment through electro-chemical signals. • Interneurons- responsible for integration of chemical signals. • Motor neurons- control effector cells (muscle and endocrine cells).

5. Parts of a neuron • Dendrites- highly branched extensions that receive signals from other cells. • Axons- typically longer projections that transmit signals. • Axon hillcock- where the axons meet the cell body • Myelin sheath- a protective layer on the axons. • Synaptic terminal- the end of an axon which has two branches and connects to the next cell’s dendrites. • Synapse- the sight of communication in nerve cells. • Presynaptic cell- transmiter. • Postsynaptic cell- receiver.

6. Membrane potential • Membrane potential is responsible for the electrical charges passed along nerve cells. • A resting potential is representative of the concentration gradient of Na and K ions across the cell membrane. • At rest the Na+ concentration is about 10 mM and K+ concentrations 1/30 mM inside the cell. (mM= mili molar) • Resting potential is between -60 and-80 mV • Resting potential is maintained through the use of a sodium potassium pump that helps keep the cell at the correct concentration.

7. Graded potential • Graded potential is a change in membrane potential due to the opening of a gated ion channel. • A graded potential has a magnitude that is relative to the strength of the stimulus that triggered it.

8. Action potential • An all or nothing reaction in which all of the gated ion channels are either opened or closed in response to a stimulus. • Used to send electrical signals long distances (ie. From the toes to the spinal cord)

9. Gated Ion Channels • Ungated- ion channels that are always left open. • Gated- open or close in response to three different stimuli • Stretch-gated ion channels- found in cells that sense stretch and open the membrane is mechanically deformed • Ligand-gated ion channels- found at synapses and open or close when a certain neurotransmitter binds to the channel. • Voltage-gated ion channels- found in axons and in some dendrites. Open or close when the membrane potential changes.

10. Depolarization • Some stimulus opens the activation gates of some Na+ ion channels. • The Na+ influx triggers other ion channels to open causing the membrane potential to move toward the threshold. • This triggers an action potential. • The refractory period is the period of time after an action potential when the cell cannot become depolarized.

11. Action potential propogation • NA+ influx during the rising phase creates an electrical current that depolarizes the neighboring region. This depolarization in the neighboring region is large enough to reach the threshold causing the action potential to be re-initiated there. This process is repeated many times as the action potential travels the length of the axon.

12. Factors that affect transmission speed • One factor that affects speed is the diameter of the axon. The larger the diameter the faster the signal will travel. The myelin sheath is also responsible for increasing conduction speed. Myelin is an insulator so it prevents the electric current from dissipating through the extracellular fluid during depolarization and repolarization.

13. Synapses • Electrical- allow electrical current to flow directly from cell to cell. • Chemical- release neurotransmitters that open ligand gated ion channels and cause an action potential. • Saltatory conduction- the depolarization from node to node on an axon.

14. Parts of a Synapse • Synaptic vessels- carry neurtransmitters to the synaptic cleft • Synaptic cleft- where the signal is communicated. • The ligand gated ion channels- allow NA+ and K+ to traverse the membrane after coming in contact with the appropriate neurotransmitter.

15. EPSPs and IPSPs • EPSPs- depolarizations that bring the postsynaptic potentials to the midpoint between Ek+ and ENa+ and therefore toward the threshold. A result of Na+ and K+ channels opening. • IPSPs- hyperpolarizations that are caused by K+ selective openings bring the potential farther from the threshold.

16. Temporal summation is the addition of two EPSPs when multiple occure across the same synapse. Spatial is when two synapses impart an EPSP on the same post synaptic cell. • The axon hillcock focuses the electric current created at the synapse.