Ch. 11 lecture PP nerve physiology

neural tissue

A. cells

A-1 neurons (information transfer)

*extreme longevity

*amitotic (lose ability to divide)

*very high metabolic rate (high O2 and glucose requirements)

Neural tissue

A. Cells

A-2 neuroglia (supporting cells)

*outnumber neurons

*retain ability to divide (mitotic)

wrap around or located near neurons

Neuron structure

1. Perikaryon

cell body (soma)

contains neurofilaments

contains microtubules

neuron structure

organelles

a. typical (mitochondria, lysosomes, etc.)

b. specialized:

-Nissl bodies= aggregations of free and fixed ribosomes

-that will contribute to gray matter of N.S. (not in axons)

Note: following differentiation, most neurons become incapable of undergoing mitosis (cannot replace themselves after cell death)

neuron structure

2. dendrites:

•  Highly branched processes

•  Provide large surface area for receiving input

•  Transmit electrical changes toward body (soma)

neuron structure

3. Axon with axon hillock (area of the axon closest to the soma)

•a long axon also called a nerve fiber.

•  Myelin Sheath:  Whitish, fatty (protein-lipid), segmented sheath around most long axons

o  It functions to:

- Protect the axon

-  Electrically insulate fibers from one   another

- Increase the speed of nerve impulse   transmission

neuron structure

4. collaterals

axonal branches

neuron structure

5. telodendria

fine extensions of a collateral (terminal branches)

neuron structure

6. synaptic knobs (aka terminals):

expanded ends of the axon

neuron classification

C-1. “structural”

1. multipolar

large, several dendrites, single axon

  common in CNS, eg). motor neurons

neuron classification

C-1. “structural”

2. unipolar:

•soma off to one side

•  dendritic and axonal processes are   continuous

  ex). sensory neurons (general)

neuron classification

c-1. “structural”

3. bipolar:

•one dendrite, one axon

•  soma between

  ex). sense organs, (eye, ear)

neuron classification

C-2 “functional”

1. afferent (sensory)-

carry impulses toward CNS

neuron classification

C-2. “functional”

2. efferent (motor)-

carry impulses away from CNS

neuron classification

C-2 “functional”

3. interneurons (association)

•Located between sensory and motor n. in CNS only   (brain & sp. chord)

•  Will “shuttle” impulses

•  99% of neurons in the body of this type

nueroglia classifications

CNS neuroglia

1. astrocytes

•largest, most numerous

•  functions include:

  a. maintenance of BLOOD BRAIN   BARRIER (isolates brain

  from general circulation, will discuss in   later chapter)

  b. create 3-D framework (via         microfilaments)

  c. regulate interstitial fluid composition

neuroglia classifications

CNS neuroglia

2. oligodendrocytes:

•large, few slender extensions

•  many layers of membranous wrapping creates MYELIN in CNS

•  gaps between the myelinated segments of the axon are called nodes of Ranvier

Note:  myelinated segments are glossy white

* will appear white and thus make up the "white   matter" of the CNS

* whereas, "gray matter" constitutes the area   where cell bodies predominate

  * axons without myelin = unmyelinated axons

CNS neuroglia

3. Microglia

•small mobile cells (macrophages), move toward debris

•  phagocytotic WBC that have migrated into the CNS….migrate,  

•   enlarge, then engulf

CNS neuroglia

4. Ependymal cells

•will line the central canal and ventricles which are filled with CSF = cerbrospinal fluid

•  will aid in CSF circulation

PNS Neuroglia

1. satellite cells:

  surround cell bodies of ganglia cells   (sensory cells)

PNS Neuroglia

2. Schwann cells:

•create neurolemma around every peripheral axon

•  will myelinate a single axon (as shown with   oligodendrocytes)

Membrane channels: (proteins within the cell membrane)

1. chemically (ligand) regulated

a. will open or close when specific chemicals bind the channel

eg.) ACh receptor of the skeletal muscle cell

b. produce graded potentials

-which may be depolarizing or hyperpolarizing

c. found at dendrites, soma, presynaptic surfaces of axon, the motor end plate

Membrane channels: (proteins within the cell membrane)

2. voltage regulated

a. will open or close in response to changes in the transmembrane potential.

Eg. 1) sarcolemma channels in response to depolarization

Eg. 2) axonal channels

b. produce action potentials

Generation of an ACTION POTENTIAL

1. Graded potential brings membrane to threshold (approx. -55 mV)

• results in activation (opening) of voltage-regulated

  Na+ channels

• Na+rushes IN!

• results in rapid depolarization of membrane

(up to +30mV)

• “All or None” phenomenon means if threshold is reached, an AP is always generated, if threshold not reached, NO AP generated.

2. Na+ channels are closed (inactivated), due to new (+30)

transmembrane potential

Generation of an ACTION POTENTIAL

3. Voltage regulated K+ channel activation (opening)

K+ rushes OUT!

• results in repolarization of the membrane (back toward -70mV)

4. Normal permeability is established:

• Na+ channels closed but capable of opening

• K+ channels begin closing, will allow for a hyperpolarization before permeability returns to normal

Action potential conduction:

1. proceeds away from the point of initial stimulation

2. affects the entire membrane surface via a “chain reaction”

3. velocity of AP depends upon:

a. presence or absence of myelin sheath

b. diameter of the axon (larger axon is faster)

4. types of conduction:

a. continuous, along unmyelinated axons

b. saltatory, on myelinated axons, is 7X faster

Multiple sclerosis (MS)

• An autoimmune disease that mainly affects young adults

• symptoms include visual disturbances, weakness, loss of muscular control, and urinary incontinence

• nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses

  • shunting and short circuiting of nerve impulses occurs

Multiple sclerosis: Treatment

• The advent of disease modifying drugs including interferon beta-1a and -1b, Avonex, Betaseran, and Copazone

• Hold symptoms at bay

• Reduce complications

• reduce disability

synaptic communcation-

1. characteristics of a synapse

junction of information transfer from neuron to another cell

synaptic communication-

characteristics of a synapse

types- (note: presynaptic vs postsynaptic cells)

• based on location: axondendritic, axosomatic, axoaxonic

• based on mechanism of information transfer: electrical or chemical

Synaptic communication-

Termination of neurotransmitter (NZ) effects:

a. Degradation of NZ by enzymes (in cleft)

b. Removal from the synapse (reuptake)

c. Diffusion of NZ away from synapse

Postsynaptic potential and integration

a. EPSP: excitatory postsynaptic potential

*a graded potential that moves the transmembrane potential closer to threshold (a depolarization)

Ex) chemical binds and opens sodium channels, causes depolarization

Postsynaptic potential and integration

b. IPSP: inhibitory postsynaptic potential

*a graded potential that moves the transmembrane potential further from threshold (hyperpolarization)

Ex) chemical binds and opens potassium or chloride channels, causes hyperpolarization

Classification of neurotransmitters-

a. Acetylcholine, Ach (cholinergic synapses)

*produces graded potential via ion movements, usually depolarizing (excitatory)

*located at neuromuscular junctions, brain, and peripheral autonomic neurons

*acetylcholinesterase present to degrade Ach

Classification of neurotransmitters-

b. biogenic amines (adrenergic synapses)

*includes norepinephrine, dopamine, serotonin, histamine

*effects alter metabolism through second messenger molecules

*can be excitatory (depolarizing) or inhibitory (hyperpolarizing)

Classification of neurotransmitters-

c. amino acids- *includes GABA, Glycine, glutamate

d. peptides - *includes endorphins, substance P

e. others- *purines (ATP and adenosine)

*Dissolved gases: Nitric oxide, carbon dioxide (“signaling gases”)