SG 17

Here is the completed study guide:

1. The two structural divisions of the nervous system are the
   a. Central Nervous System (CNS) which is made up of the brain and spinal cord.
   b. Peripheral Nervous System (PNS) which is made up of spinal nerves and cranial nerves.

2. The functional divisions of the nervous system are the
   a. Motor Division which stimulates the muscles.
   b. Sensory Division which detects sensory stimuli in our environment.

Sensory Divisions

1. The Visceral Sensory division detects sensation in the internal organs such as stretch and pain.

2. The Somatic Sensory division detects sensation on the skin such as touch and temperature.

Motor Divisions

1. The Somatic Motor division controls skeletal muscles.

2. The Autonomic division controls smooth muscles, cardiac muscles and glands.
   a. This division is also called the Autonomic Nervous System.

3. Divisions of the ANS
   a. The Sympathetic nervous system is responsible for mobilizing the body during activities.
   i. This division produces changes such as increased heart rate, dilation of pupils, increased rate of breathing.
   b. The Parasympathetic nervous system is responsible for maintenance activities at rest.
   i. This division produces changes such as decreased heart rate, increased stomach activity, increased urine production.

Cells of the Nervous System

Neuron

1. Structure of a neuron
   a. The Soma of a neuron contains the nucleus.
   b. Dendrites are branches that are attached to the soma.
   c. The Axon extends away from the soma.
   d. Synaptic vesicles are found in the axon terminal; these vesicles contain neurotransmitters.
   e. Most axons are covered by segments of fat tissue called myelin sheath. These segments are interrupted by Nodes of Ranvier.

2. Structural classes of neurons
   a. Unipolar neurons have one process that extends from the soma.
   i. This process has dendritic and axon process.
   ii. This neuron is used to convey general sensory information.
   b. Bipolar neurons have two processes that extend from the soma.
   i. One process is the axon and the other is the dendrite.
   ii. This neuron is used to convey special sensory information.
   c. Multipolar neurons have multiple dendrites and single axon.
   i. This neuron is used to convey motor information.

3. Functional classes of neurons
   a. Sensory (Afferent) neurons carry information into the CNS such as pain. These neurons are also called afferent.
   b. Motor (Efferent) neurons carry commands out of CNS to muscles. These neurons are also called efferent.
   c. Interneurons neurons act within the CNS to create local networks of neurons. These neurons are also called association neurons.

Glial Cells

4. Oligodendrocytes produce myelin in the CNS.

5. Schwann cells produce myelin in the PNS.

6. Microglia act as local immune cells in the CNS.

7. Astrocytes repair neurons.

8. Astrocytes control chemical environment of the CNS.

9. Astrocytes help to create the blood brain barrier.

10. Ependymal cells line cavities in the CNS.

11. Satellite cells surround and protect somas in the PNS.

Axonal Transport

1. Materials are moved from the soma to the axon terminal by using anterograde axonal transport.
   a. The fast version of this transport is used to move materials such as organelles.
   b. The slow version is used for enzymes.

2. Substances are shipped from the axon terminal to the soma by using retrograde axonal transport. This form of transport is done using the fast way. This is used to shuttle materials such as growth factors.

3. Pathogens utilize retrograde axonal transport to infect patients.

4. The average speed of fast axonal transport is $\text{50-400 mm/day}$.

5. The average speed of slow axonal transport is $\text{0.5-10 mm/day}$ .

Membrane Potential

1. At rest there are more negatively charged ions inside the cell.

2. The difference in charges across the cell membrane is called the membrane potential.
   a. In the unexcited state, this is called the Resting Membrane Potential.
   b. The value of the RMP is $-70 mV$.

3. Changes in Membrane Potential
   a. Depolarization is a change that makes the membrane potential more positive.
   i. This may be caused by the influx of cations such as Na+.
   b. Repolarization is a change that returns the membrane potential from a more positive value towards the RMP.
   i. This may be caused by the efflux of cations such as K+.
   c. Hyperpolarization is a change that makes the membrane potential more negative.
   i. This may be caused by the efflux of cations such as K+.

4. Ion Channels
   a. Ions can enter and leave the cell by passing through ion channels.
   b. Ligand-gated ion channels open or close when a chemical binds to it.
   c. Voltage-gated ion channels open or close when the membrane potential changes.
   d. Ligand-gated channels are most active on the dendrites and soma of the neuron.
   e. Voltage-gated channels are most active on the axon of the neuron.
   f. Voltage-gated sodium channel
   i. opens at $\text{-55 mV}$.
   ii. becomes inactivated at $\text{+30 mV}$ .
   iii. closes at $\text{-70 mV}$ .
   iv. ion can pass through the channel when it’s in the open state.
   v. The channel can open from the closed state.

5. Local Potentials
   a. Local potentials occur on dendrites and soma of neurons.
   b. Local potentials can have different sizes (T).
   c. Local potentials get bigger as they spread away from the site of stimulus (F).
   d. All local potentials change the membrane potential to a more positive value (F).
   e. Local potentials are required to generate action potentials (T).
   f. Local potentials can propagate down the axon (F).

6. Action Potentials
   a. Action potentials are initiated at the axon hillock of the neuron.
   b. The phases of the action potential are Depolarization followed by repolarization followed by Hyperpolarization and finally ending with a return to RMP.
   c. Phases of the action potential
   i. Threshold
   1. The membrane potential depolarizes towards a threshold value of $\text{-55 mV}$ .
   2. This is caused by influx of cations through (ligand/voltage) gated channels.
   3. The changes in membrane potential that are caused by the movement of ions is called local potential (T).
   ii. Depolarization phase
   1. At threshold, Na+ ions enter through VG channels to make the membrane potential more positive.
   2. This phase lasts from $\text{-55 mV}$ to $\text{+30 mV}$ .
   3. At $\text{+30 mV}$ , VG sodium channels inactivate to stop the flow of sodium ions into the cell.
   iii. Repolarization phase
   1. This phase is caused by the efflux of K+ ions through VG ion channels.
   2. This phase begins at $\text{+30 mV}$ and ends at $\text{-70 mV}$ (or below).
   iv. Hyperpolarization phase
   1. This phase is caused by the efflux of K+ ions through VG ion channels.
   2. During this phase, excess K leaves the cell as the voltage-gated potassium gates close slowly.
   v. Return to RMP
   1. After the K gates close, the membrane potential is returned to $\text{-70 mV}$ by the Na+/K+ pump as well as by leak channels.
   d. Properties of an Action Potential
   i. The size of the action potential depends on the size of stimulus (F).
   ii. All stimuli generate an action potential (F).
   iii. The size of the action potential decreases as it travels towards the axon terminal (F).
   iv. The action potential can be stopped before it reaches the axon terminal (F).
   v. The action happens along the entire length of the axon at the same time (F).
   vi. The Refractory period
   1. During absolute refractory period, the membrane is incapable of generating another action potential.
   2. During relative refractory period, a greater than normal stimulus is required to generate another action potential.
   3. The first half of the absolute refractory period is due to inactivated Na+ channels.
   4. The second half of the absolute refractory period is due to open K+ channels.
   5. The refractory period prevents the action potential from traveling towards the soma.
   6. Absolute refractory period lasts from $\text{-55 mV}$ to $\text{-70 mV}$ (return to RMP after repol).
   7. The relative refractory period lasts from $\text{-70 mV}$ to $\text{-90 mV}$ (hyperpolarization minimum).
   vii. The action potential lasts for less than 5 ms.
   e. Propagation of the action potential
   i. In unmyelinated axons, the action potential is regenerated sequentially at every possible region of the axonal membrane (T).
   ii. In myelinated axons, the action potential is generated in the Nodes of Ranvier.
   1. This is called saltatory conduction.
   iii. Action potentials are sequentially generated down the axon by the influx of Na+ ions from the previous action potential.
   iv. Action potentials propagate faster in myelinated neurons.

Synapses

1. Types of Synapses
   a. A synapse is a junction between neurons or between neurons and effector cells.
   b. The two classes of synapses are chemical and electrical synapses.
   c. In electrical synapses, both cells are connected by channel protein which allows ions to spread directly from one cell to another.

2. Structure of a chemical synapse
   a. In a synapse,
   i. the presynaptic neuron releases the neurotransmitter.
   ii. the postsynaptic neuron contains ligand-gated channels that respond to neurotransmitters.
   iii. Both neurons are separated by a synaptic cleft.
   b. Axodendritic synapses occur between dendrites and axon.
   c. Axosomatic synapses occur between soma and axons.
   d. Axoaxonic synapses occur between axon and axon.

3. Events at a Chemical Synapse
   a. The arrival of the action potential at the axon terminal triggers the influx of Ca2+ ions through voltage gated channels in the presynaptic membrane.
   b. Entry of Ca2+ triggers the release of neurotransmitters which cross the cleft to bind to ligand gated channels on the postsynaptic neuron.
   i. The channels are also called receptor channels.
   c. When the ion channels open, ions pass through to generate postsynaptic potentials which may eventually trigger an action potential.
   d. The synapse becomes inactive when the neurotransmitter is reuptaken or degraded.

4. Postsynaptic potentials
   a. Entry of sodium into the postsynaptic cell will generate an EPSP.
   b. Exit of potassium from the postsynaptic cell will generate an IPSP.
   c. A local potential that moves the RMP towards a more negative value is called an IPSP.
   d. A local potential that moves the RMP towards the threshold value is called an EPSP.
   e. All EPSPs generate an action potential (F).

5. Summation
   a. Several neurons may form synapses with the same neuron (T).
   b. A neuron can be stimulated and inhibited by different neurons at the same time (T).
   c. Adding up the small effects of multiple EPSPs over time until your reach threshold is called temporal summation.
   d. To achieve threshold by adding up the effects of multiple postsynaptic potentials that are occurring at the same time is called spatial summation.
   e. Both temporal and spatial summation can occur at the same time (T).

6. Actions of Some Neurotransmitters
   a. A neurotransmitter has the same effect on all cells (F).
   b. The effect of a neurotransmitter on a cell depends on the receptor type.
   c. Acetylcholine is the used in neuromuscular junctions.
   d. GABA is the most common inhibitory neurotransmitter in the brain.
   e. Acetylcholine is used by all preganglionic fibers in the ANS.
   f. Glutamate is the most common stimulatory neurotransmitter in the brain.
   g. Norepinephrine is an appetite suppressant.
   h. Dopamine is involved in addiction.
   i. Glycine is the most common inhibitory neurotransmitter in the spinal cord.
   j. Glutamate is the most common stimulatory neurotransmitter in the spinal cord.
   k. Norepinephrine is released by sympathetic postganglionic neurons in the ANS.
   l. Serotonin is enhanced by anti-depression medications.
   m. Substance P is released by pain fibers.

Neural Coding

1. The nature of the stimulus (sound/taste/pain) is determined by the identity of the neuron firing.

2. The intensity of the stimulus is determined by the firing rate/frequency.

3. Sound and taste use different synaptic pathways (T).

4. Sound and taste synaptic pathways terminate in different brain regions (T).