The Cells Of The Nervous System

The human brain contains approximately 100 billion neurons that work together to produce unified experience and coordinate, organized behavior. The nervous system is composed of:

1. Neurons, which receive and transmit information to other cells, and

2. Glia cells (neuroglia, "glue"), which play a supportive role in brain functioning. One tenth of size, but ten times as many à same volume as neurons

Neurons’ basic components

1. Cell membrane (plasma membrane): Regulates what goes in and out. Composed of two layers of fat molecules (bilipid); this membrane allows some particles (water, oxygen, co2) to flow both into and out of the neuron while restricting most chemicals. (As rule: fat soluble molecules can pass through membrane.) Contains (protein-) channels specific for some particles

2. Cell nucleus: A membrane-enclosed structure which contains all of the DNA (genetic material) of the cell; the DNA is organized into units called chromosomes.

3. Mitochondrion: Provides cell with energy. Requires oxygen to function.

4. Ribosomes: Site of protein synthesis in the cell.

5. Endoplasmic reticulum: Transports newly synthesized proteins to other locations. Ribosomes may be attached.

The Structure of a Neuron:

1. Neuron components:

a. Dendrites: (can have many) Branches which extend from the cell body; dendrites receive information (e.g., sensory) from other neurons and send that information to the rest of the neuron.

Dendritic spines: Short outgrowths found on some dendritic branches, increase surface area, receive info thought to be involved in learning and memory

b. Cell body (soma): Contains the nucleus, ribosomes and mitochondria.

c. Axon: (only one, but can branch) A long, thin fiber (usually longer than dendrites) that is the information sending part of the neuron, sending an electrical impulse toward other neurons, glands, or muscles. Myelin sheath: Insulating covering found on some vertebrate axons.

d. Presynaptic terminal (bouton or end bulb): Swelling at the tip of the axon.

Part of the neuron that releases chemicals that cross the junction between one neuron and the next.

2. The most distinctive structural feature of neurons is their shape.

3. Most neurons contain four major components: dendrites, cell body, axon, and

presynaptic terminal. Compare the structure of these components in the following

two types of neurons:

A motor neuron (Conduct impulses to muscles and glands from the spinal

cord) and

A sensory neuron (receptor neurons): Sensitive to certain kinds of stimulation

(e.g., light, touch, etc.).

A local neuron: Small neuron with no axon or a very short axon.

4. Other terms associated with neurons:

a. Afferent axons: Brings information to a structure.

b. Efferent axons: Sends information away from a structure.

c. Intrinsic neurons (interneurons): Entirely located within a single structure of

the nervous system.

5. Variations among neurons

a. Neurons vary enormously in size, shape, and function.

b. A neurons function is closely related to its shape.(ex: wide dendritic tree

à integrating info from many sources)

c. A neuron's shape is plastic (changeable) as new experiences can modify the shape of a neuron.

E. Glia ("glue"): A 10:1 ratio of glia cells to neurons exists in the brain. Glia one tenth of neuron size à about equal volume

Numerous functions/shapes:

1. Astrocytes: star-shaped, connects to group of functionally related neurons. Absorb chemicals released by axons and later returns those chemicals back to the axon to help synchronize the activity of neurons; also, astrocytes remove waste products particularly those created after neurons die.

2. Oligodendrocytes: In brain and spinal cord. Build myelin sheaths around certain neurons in the brain and spinal cord. à thought to be responsible for preventing regeneration of central neurons. Why does growth in CNS need to be restricted??

3. Schwann cells: In periphery (nerves): Build myelin sheaths around certain neurons in the periphery of the body. Guide axon re-growth after nerve injury

4. Radial glia: Type of astrocyte. Guides the migration of neurons and the growth of axons and dendrites during embryonic development.

F. The Blood-Brain Barrier: The mechanism that keeps most chemicals out of the vertebrate brain. Paul Ehrlich (1800s): Injected dye into blood stream, everything but brain and spinal cord turned color , first thought brain had no affinity for dye, but found that brain has barrier that somehow "filters" blood flow

1. The blood-brain barrier is needed because

the brain lacks the type of immune system present in the rest of the body. (immune system destroys infected cells, but brain cells do not regenerate).

some chems have great effect on brain (ex MSG)

right ionic environment must be maintained (ex Na, in table salt) for neuron fct

2. Structure: endothelial cells forming the walls of the capillaries are tightly joined blocking most molecules from passing. In the rest of the body the endothelial cells are separated by large gaps. Glial cells make contact with endothelial cells in brain, thought to play role in forming barrier.

3. Can pass:

- Small uncharged molecules (e.g., oxygen and carbon dioxide) and molecules that can dissolve in the fats of the endothelial cell membrane can cross passively (without using energy) through the blood-brain barrier

- glucose: active transport system, pump (a protein-mediated process that uses energy)

G. Nourishment of Vertebrate Neurons

1. Almost all neurons depend on glucose (a simple sugar) for their nutrition. Need high O2 supply to use glucose. Glucose crosses BBB, other sugars don’t. When BBB is weak neurons also use other sources (ex infants). Intense stimulation, low glucoseà brain uses lactate.

Normally glucose supply no problem, but: A thiamine (vitamin B1) needed to use glucose. Deficiency (ex alcoholics)à neuron death, Korsakoffs syndrome (disorder; severe memory impairment).
 

II. The Nerve Impulse

A. The Resting Potential

1. Figure: Measure voltage in / outside cell: inside negative

2. Concentration gradient (difference in distribution of ions between the inside and the outside of the membrane): sodium (Na+) 10 times more concentrated outside the membrane than inside and potassium (K+) 20 times more concentrated inside than outside.

Because the body has far more sodium ions than potassium ions, the concentration of sodium ions outside is greater then the potassium ions inside making the outside more positively charged than the inside.

Forces behind RP:

3. The neuron membrane has selective permeability, which allows some molecules to pass freely (e.g., water, carbon dioxide, oxygen, etc.). Large, charged molecules can’t pass. Na, K, Cl have selective gates/channels that open and close

4. RP: K+ and (chloride) C1- gates (channels) open along the membrane, Na+ gates remain closed à Na stays out

5. Sodium-potassium pump: Protein mechanism found along the neuron membrane which transports 3 Na+ ions out , 2 K+ ions in; active transport mechanism (requires energy )

6. Electrical gradient (difference in positive and negative charges across the membrane): K+ wants in to follow el gradient, but wants out due to conc gradient – 2 forces almost balanced, small net drive out,. Na+ wants in due to both el and conc gradient, but channels are closed and pump keeps it out.

7. Why RP – it takes energy?? The advantage of the resting potential is to allow the neuron to respond quickly to a stimulus: sets up great force for influx of Na à open channels cause fast influx, electrical change. Like stretched rubber band.

B. The Action Potential

1. Hyperpolarization (increased polarization): Occurs when the negative charge inside the axon increases (e.g., -70mV becomes -80mV)

2. Depolarization (decreasing polarization towards zero): Occurs when the negative charge inside the axon decreases (e.g., -70mV becomes -55mV).

3. Threshold of excitation (threshold): The level that a depolarization must reach for an action potential to occur.

4. Action potential: A rapid depolarization and slight reversal of the usual membrane polarization. Occurs when depolarization meets or goes beyond the threshold of excitation.

5. When the potential across an axon membrane reaches threshold, voltage-activated (membrane channels whose permeability depends on the voltage difference across the membrane) Na+ gates open and allow sodium ions to enter; this causes the membrane potential to depolarize past zero to a reversed polarity (e.g., -70mV becomes +50mV at highest amplitude of the action potential).

6. At AP peak: voltage-activated Na+ gates close, but K+ ions flow outside of the membrane due to conc gradient (no longer opposed by el gradient ), and increased membrane permeability for K

7. A temporary hyperpolarization occurs before the membrane returns to its normal resting potential (this is due to K+ gates opening wider than usual, allowing K+ to continue to exit past the resting potential).

8. After the action potential, the neuron has more Na+ and fewer K+ ions for a short period (this is soon adjusted by the sodium-potassium pumps to the neuron's original concentration gradient).

9. Local anesthetic drugs (e.g., Novocain, Xylocaine, etc.) hinder the occurrence of action potentials by blocking voltage-activated Na+ gates (preventing Na+ from entering a membrane).

10. General anesthetics (e.g., ether and chloroform) cause K+ gates to open wider, allowing K+ to flow outside of a neuron very quickly.

11. Action potentials only occur in axons as cell bodies and dendrites do not have voltage-dependent channels.

12. All-or-none law: The size, amplitude, and velocity of an action potential is independent of the intensity of the stimulus that initiated it. If threshold is met or exceeded an action potential of a specific magnitude will occur, if threshold is not met, an action potential will not occur.

13. Refractory period: A period of immediately after an action potential occurs when the neuron will resist the production of another action potential.

a. Absolute refractory period: Na+ gates are incapable of opening; hence, an action potential cannot occur regardless of the amount of stimulation.

b. Relative refractory period: Na+ gates are capable of opening, but K+ channels remain open; a stronger than normal stimulus (i.e., exceeding threshold) will initiate an action potential.

Propagation (movement)of the Action Potential
1. The action potential begins at the axon hillock (a swelling located where an axon exits the cell body).
2. The action potential is regenerated due to Na* ions moving down the axon, depolarizing adjacent areas of the membrane.
3. Propagation of the action potential: Transmission (movement)of an action potential down an axon. The action potential moves down the axon by regenerating itself at successive points on the axon.
4. The refractory periods prevent the action potentials from moving in the opposite direction (i.e., toward the axon hillock).

Myelin Sheath and Saltatory Conduction
1. Myelinareal axons: Axons covered with a myelin sheath. The myelin sheath is found only in vertebrates and is composed mostly of fats.
2. Nodes of Ranvier: Short unmyelinated sections on a myelinated axon.
3. Saltatory conduction: The "jumping" of the action potential from node to node.
4. Multiple sclerosis: A disease characterized by the loss of myelin along axons; the loss of the myelin sheath prevents the propagation of action potentials down the axon.

Signaling without Action Potentials
1. Local neurons: Small neurons with no axons which do not produce action potentials but instead produce:
2. Graded potentials: Potentials that vary in magnitude and do not follow the all-or- none law. Graded potentials get smaller as they travel.