Three Major Organs:

                 Brain                Spinal Cord                    Nerves

Organization:

I) The Central Nervous System (CNS): Brain and Spinal Cord

II) Peripheral Nervous System (PNS): The Cranial and Spinal Nerves 

            A) Sensory (afferent division)

            B) Motor (efferent division)

                        1) Somatic Nervous System (Voluntary)

                        2) Autonomic Nervous System (Involuntary)

                                    a) Sympathetic Division

                                    b) Parasympathetic Division

Histology:

I) Two cell types form nerve tissue:

        A) The Neuron

                1) transmits electrical signals called impulses

                2) live a long time---a lifetime

                3) amitotic (cannot divide)

                4) high metabolic rate; need more glucose and oxygen than other

                    cells

                5) live only for a few minutes without oxygen

                6) At rest the brain  (3.1 lb) uses as much oxygen as 61.6 lb of

                     skeletal muscle

                7) Basic structure of a neuron (Figure 11.4) include the cell

                    body (soma), axons, dendrites, telodendrites,the synapse        

        B) The Neuroglia (glial cells) supporting cells of the nervous system, they outnumber neurons 10 to 1; 50% of the brain consists of glial cells; found primarily in the CNS.  There are four types found in the CNS and two types found in the PNS:

CNS:

-------Astrocytes: The largest and most numerous; secrete chemicals for the blood brain barrier; form the structural framework for the CNS; repairs damaged nerve tissue; star shaped; wrap around neurons and capillaries so they function in the exchange of oxygen, carbon dioxide and glucose; mops up leaked K+ ; recycles neurotransmitters.-----

-------Microglia: The smallest rarest neuroglia which contain thorny processes that touch and monitor the health of neurons; migrate toward injured cells and turn into macrophages as they phagocytize neruonal debris and microorganisms (they have an immunological function; cells of the immune system are denied access to the CNS).----

-------Ependymal Cells: They form the ependyma, which lines the cnetral canal and ventricles of the brain; range in shape from squamous to columnar and many are ciliated; ciliated forms circulate the cerebrospinal fluid (CSF); formed into the choroid plexus in the ventricles of the brain which produces CSF------

-------Oligodendrites: Cells containing cytoplasmic extensions form the myelin sheaths around axons in the CNS (forming nodes and internodes). Myelin is  fatty material surrounding axons.

PNS:

-----Schwann cells surround and form myelin sheaths around the larger nerve fibers  in the  PNS;----

-----Satellite cells surround neuron cell bodies within ganglia and their function is unknown-----

Structural Classification of Neurons

1) Multipolar neurons are the most common(99%) and is the major nueron in the CNS; Three or more processes extend from the cell body; all dendrites except for a single axon

2) Bipolar neurons contain two processes that extend from the cell body; one is a fused dendrite and the other is an axon; found only in the sense organs (receptor cells in the retina, and nasal passages)

3) Unipolar neurons contain a single axon emerging from the cell body; begin as bipolar neurons in the embryo; axon consists of a central (proximal) process and a peripheral (distal) process receiving impulses from a sensory receptor.  Found only in the PNS in the dorsal root ganglia of the spinal cord, and the sensory ganglia of cranial nerves.

Structural Variations of Neurons(table 11.1)

1) Sensory (afferent) neurons; carry impulses toward the CNS; unipolar in that the cell bodies are found in the ganglia close to the spinal cord

2) Motor (efferent) neurons; carry impulses away from the CNS; multipolar cell bodies are located primarily in the CNS (cell bodies form the grey matter of the brain and spinal cord; axons form the white matter).

3) Interneurons (association neurons); most are multipolar; 99% of all neurons in the body are interneurons; transfer impulses between sensory and motor neurons in the CNS

Neurophysiology:

I) Membrane Ion Channels:

a) Leak channels- channels that are made of integral membrane proteins and are always open, involved in passive transport (diffusion and facilitated diffusion which requires no energy-page 73 figure 3.6)

b) Gated channels- channels that are made of integral membrane proteins and have a molecular "gate" which is usually one or more protein molecules that can change shape to open or close the channel in response to various signals.  Changing the shape of proteins require energy therefore gated channels are an example of active transport (page 76 figure 3.9). Chemically gated channels open when the appropriate neurotransmitter binds, and voltage gated channels open and close in response to changes in membrane potential.  The sodium potassium exchange pump is an example of a gated channel and is the one which is involved in our study of neurophysiology.

II) The Resting Potential:

a) There exists a potential difference (potential difference is a measure of potential energy generated by a separation of charges and is measured in voltage or millivolts; the higher the voltage the greater the difference in charge) between the two sides of a cell membrane of a neuron. It is more appropriately called  a membrane potential since the charges are separated by a membrane.  The potential difference of an undisturbed cell is equal to .070 volts or -70 milivolts. The - sign means that the inside of the membrane is negatively charged with respect to the outside.  This is called the resting membrane potential and again is equal to -70 millivolts.

b) Factors responsible for the membrane potential involve the types of ions that are naturally found inside and outside of our cells.  Sodium (Na+) is most concentrated outside of the cell membrane in the extracellular fluid while potassium (K+) is more highly concentrated on the inside of the cell membrane along with negatively charged proteins  in the intracellular fluid. Postassium (K+) tends to diffuse outward through potassium leak channels driven by the concentration gradient of K+ ions, and sodium ions (Na+) tend to diffuse into the membrane from the outside. The membrane however is much more permeable to K+ rather than Na+ so K+ diffuses out faster than Na+. The result is that the cell experiences a net loss of + charges from the inside to the outside of the membrane resulting in an excess of negative charges on the inside primarily due to the negatively charged proteins (page 398, figure 11.8).   Resting potential remains stable over a long period of time due to the sodium potassium exchange pump. It would appear that the K+ and Na+ would eventually be of equal concentrations on each side of the cell membrane due to diffusion, but the cell maintains the resting potential at -70 millivolts by the sodium potassium exchange pump in that when sodium enters the potassium is exchanged.

III) Changes in the Membrane Potential:

a) Changes will occur when a stimulus either alters a membranes permeability to N+ or K+, or alters the activity of the exchange pump

b) Examples of stimuli include: exposure to specific chemicals, mechanical pressure, changes in temperature, or shifts in extracellular ion exchange.

c) Stimuli will open gated ion channels (gated ion channels are closed during the resting potential).  Opening of gated channels will accelerate Na+ transport and this causes a change in membrane potential.

d) As Na+ increases on the inside the membrane potential will apporach 0 mv. A shift in this direction is called membrane depolarization (equal number of + and - ions.

e) A stimulus that opens K+ gated channels will result in increasing membrane potential from -70 to -80 and is called hyperpolarization.

IV) Graded and Action Potentials

a) Graded potentials affect only a limited portion of the cell membrane and occurs in all cells. Graded potentials can stimulate or inhibit glandular secretions and is produced by a graded portion by a neuron. The graded potential affects too small an area and the impulse diminishes rapidly with distance and is not great enough to affect a skeletal muscle or a neuron. Postsynaptic potentials are other examples and will be considered later.

b) Action potentials  can occur in skeletal muscle fibers and axons. 

-----In neurons it begins at the axon hillock and travels the length of an axon toward a synapse---

-----Action potentials are created by opening and closing of the Na+ and K+ channels---

---Graded potentials act like pressure on the trigger of a gun. A gun fires after a minimum pressure has been applied to the trigger. When the trigger is pulled either gradually or suddenly the bullet fires at the same speed and range regardless of the forces that are applied to the trigger. This is called the all or none principle.

---In the axon, the graded potential is the pressure on the trigger and the action potential is the firing of the gun---

----An action potential will not appear unless the membrane depolarizes sufficiently to a level called the threshold.

c) The phases of the Action Potential (page 402 figure 11.12)

---Begins with a graded potential from -70 mv to -60 mv and ending with a return  to the resting potential of -70 mv---

Step 1: Sodium leak channels open only at threshold (-60 mv) and depolarization occurs.  As these channels open, sodium ions rush into the cytoplasm.  The sudden influx of positive charges causes the rapid depolarization of the membrane. Membrane potential becomes less negative--

Step 2: The sodium channels close. In less than a millisecond, the inner surface contains more positive ions than negative ones; the membrane potential has changed from -60 mv (a relative excess of negative charges inside) to +30 ( a relative excess of positive charges on the inside). At this membrane potential, sodium channels close.

Step 3: Potassium channels open. The sudden change in the membrane potential triggers the opening of potassium ion channels.  Driven by their concentration gradient and repelled by the surplus of positive charges in the area, potassium ions rush out of the cell.  The loss of positive charges shift the membrane potential back toward resting levels.  This return of the membrane potential toward resting levels is called repolarization.

Step 4: The membrane permeability returns to normal. As the the membrane repolarizes to  -70 mv, the potassium channels begin closing.  As the potassium channels close, the membrane potential returns to normal resting levels. Note that during the undershoot or hyperpolarization  both gates of the sodium channels have closed but potassium channels remain open and membrane potential has increased beyond -70mv. The sodium-potassium pump however quickly restores the resting potential within another millisecond or two.

---From threshold to the end of repolarization is  what is called the refractory period, during which the membrane cannot respond normally to further stimulation. The refractory period limits the number of action potentials (maximum number is 500 to 1000 per second----

****Neurotoxins like tetradoxin from the puffer fish and other neurotoxins from poisonous snakes of the cobra family block the Na+ channels resulting in paralysis of respiratory muscles

d) Conduction of an action potential

---Action potential affects the entire membrane surface rather than a small section of membrane like a graded potential--

---At a given site at the peak of the action potential (+30mv) the inside of the membrane contains an excess of + ions---

---These + ions will immediately  spread along the inner surface of the membrane; the positive charges being attracted toward the negative charges producing a local current on the outside of the membrane.  This local current depolarizes adjacent portions because it is a stimulus.  The sodium channels open on an adjacent portion of the membrane and a new set of Na+ and K+ move in and out producing another action potential.  This continues in a chain reaction along the entire length of the axon and is called continuous conduction---

----Unmyelinated axons will conduct impulses at a rate of 1 meter/sec (2mph)---

----Myelinated axons conduct impulses about 7x faster.  Action potentials occur at each node of Ranvier in which the impulse skips the internode and impulses are conducted must faster.  This is called saltatory conduction (satare-to leap).

***Progressive destruction of myelin sheaths accompanied by axon damage and scarring of neural tissue that can occur for example in multiple sclerosis, can result in a gradual loss of sensation and motor control which leaves affected regions numb and paralyzed. Common symptoms also include partial loss of vision, problems with speech, balance, and general motor coordination

V) The Synapse

a) Can be electrical in which ions flow directly through protein channels from one neuron to another. They are the least common and neurons are said to be electrically coupled.  The result in rapid transmission and are abundant in embryos, and are eventually replaced by chemical synapses.

b) Chemical synapses are the most common and involves "chemical bridges" formed between neurons by neurotransmitters (table 11.3). The most notable neurotransmitters are acetylcholine and norepinphrine.   Electrical signals are changed into chemical signals (neurotransmitters) and converted back again at the postsynaptic dendrite.

VI) Information Transfer Across a Chemical Synapse (figure 11.8 page 409)

a) Calcium gates open in the presynaptic axonal terminal:

----Nerve impulse reaches axon terminal containing synaptic vesicles with neurotransmitter inside each vesicle---

---Calcium gates open: presynaptic axonal terminal (depolarization has opened Na+ channels too)---

---Calcium floods into the terminal from the extracellular fluid

b) Neurotransmitter released by exocytosis:

---Ca+ acts as a messenger instructing synaptic vesicles to fuse with axon membrane and empty into synaptic cleft.---

----The Ca+ also leaves the cell by Ca+ pump or taken up by the cell's mitochondria----

c) Neurotransmitter binds to postsynaptic protein receptors.

----these receptors change shape, causes an ion channel to open and Na+ flows into the next neuron creating an action potential----

c) Termination of neurotransmitter effects:

----Degradation of neurotransmitter by enzymes (example-acetylcholine by acetylcholenesterase).  This stops the impulse, because as long as the neurotransmitter is bound to the receptor, the impulse will continue---

----Removal of the neurotransmitter will occur by astrocytes, or by postsynaptic terminal where it is destroyed by enzymes (example would be norepinephrine)--

---Natural diffusion of neurotranmitter occurs away from the synapse.---

VI) Neuronal Pools

a) Found in the CNS and consists of groups of neurons carrying the same impulse, but arranged in four different types of circuits ( page 420 page 11.24)

----Diverging circuits are also called amplifying circuits in which  one neuron  triggers responses in ever increasing numbers of neurons farther and farther along in the circuit. An example would be when one neuron in the brain is stimulated to produce a complex movement involving many muscle fibers. Divergence can occur along single, or multiple pathways and can be sensory or motor.

---Converging circuits have a funneling or concentrating effect. Common in both sensory and motor pathways. An example is sight, smell, and hearing combining to form a flood of sensations.---

---Reverberating (oscillating) circuits involve positive feedback.  Impulses are sent through circuit again and again by collateral synapses. They involve rhythmic activities such as swinging your arms while walking. Another example is breathing.----

----Parallel after-discharge circuits occur when an impulse is divided up. In this type of circuit the impulse reaches the output cell at different times producing a "burst" of impulses (like fireworks) and lasts 15 ms after the initial impulse.  There is no positive feedback in that the impulse ends when the last burst occurs.  Examples include complex mental processing such as problem solving.----