Biol 2401 A & P I         Lecture Notes        Neural Tissue                        Dr. Weis                

NEURAL TISSUE

responsible for communication and regulation

arises from ectoderm ---> neuroectoderm to form a tubular structure that will eventually form the CNS

Extensions from the tube will form the PNS

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Anatomical Divisions ---->

CNS .....central nervous system = brain and spinal cord

PNS .....peripheral nervous system= Cranial nerves and Spinal Nerves

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Functional Divisions : Sensory Afferent (PNS)

Integrating Center (CNS)

Motor Efferent (PNS)

         The PNS has

a sensory afferent neuron to carry the message from the sensory input toward the CNS along the ascending pathways or tracks.

a motor efferent neuron to carry the impulse away from the CNS along the descending pathways or tracks

Divisions of the motor efferent ::

ANS has two subdivisions :

1. Sympathetic --> fight/flight

2. Parasympathetic --> rest/digest

c. Enteric - Gastrointestinal (GI) tract=s own internal nervous system which is ultimately controlled by ANS.

 

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Neural Tissue consists two cell types :  

1. Neurons  or nerve cells :: The functional unit of neural tissue

         excitable

         conduct nerve impulses (electrical events)

         amitotic (no centrioles or spindle fibers)

2. Neuroglia or glia  .....supporting cells : nonexcitable

(like the connective tissue that supports other cells in the body)

   Neuron Anatomical Structure

The plasma membrane surrounding the axon is called the axolemma.

May branch occasionally as axon collaterals

Ends in terminal branches called telodendria and these end in synaptic knobs or terminal buttons.

  These terminals represent the secretory component of the neuron. 

            Vesicles here contain neurocrine molecules.
When the impulse reaches the axon terminals, chemicals stored in the vesicles are released into the extracellular space called the SYNAPTIC CLEFT. 

These chemicals are called  neurotransmitters, neuromodulators, or hormones
and can EITHER excite or inhibit neurons or other cells at their postsynaptic membranes.

Neurons can be classified according to dendritic/axon relation

Therefore, for the PNS

Cell bodies of neurons in the CNS are called NUCLEI

Axons of neurons in the CNS are called TRACKS

Cell bodies of neurons in the PNS are called GANGLIA

Axons of neurons in the PNS are called NERVES

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2. Neuroglia  support cells

6 total :  4 for the CNS  and  2 for the PNS

CNS glial cells :

PNS glial cells :

Myelin in both the CNS & PNS improves the rate of conduction of the action potential has periodic constrictions called NODES.

In the disease Multiple Sclerosis there is a patchy destruction of myelin on the CNS, therefore creates a  failure of normal conduction.

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NEUROPHYSIOLOGY

Excitable tissues can generate and propagate electrical signals or nerve impulses.

Functional classification of neurons are based according to the direction in which the nerve impulse travels in relationship to the CNS. 
These are sensory afferents, interneurons, motor efferents.

Sensory Afferent Categories :

Somatic Sensory : pain, temperature, touch, pressure, position (proprioception)

Special Sensory : light, sound, smell, taste

Neurons used for pathways in between sensory and motor neurons are called ASSOCIATION NEURONS or INTERNEURONS

Signals can be sent through complex pathways.

Most interneurons are multipolar in structure and located in the CNS.

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Nerve Cells have a low threshold for excitation and are considered to be highly irritable. 
The stimulus can be ::

   electrical

   chemical

   mechanical

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Conduction of the impulse requires ENERGY and the stimulus must be of the correct strength and duration, as the nerve also operates on the ALL or NONE principle.

Once an action potential has been triggered, conduction continues along the axons, if myelinated, jumping from node to node.

The body fluids have electrolytes (charged particles in solution), so the nerves are operating in a good conducting medium.

Separation of electrical charges of the opposite sign have Potential Energy. 

The measurement of this potential Energy is  called VOLTAGE (V or mV).

The flow of charges from one point to another is called  CURRENT and can be used to do work. 
The opposition or hindrance to flow of charges (current) is termed RESISTANCE.

Substances with high resistance are called insulators, those with low resistance are called conductors.

The relationship between voltage (V), current (I), and  resistance (R) is OHM's LAW

Current  = Voltage/Resistance

or I = V/R

Electrical currents in the body reflect the flow of ions rather  than free electrons.

 The plasma membrane has membrane protein channels that change shape to open or close in response to various signals. 
Therefore there are ::

When channels open, ions will flow following their Electrochemical gradients.

The ion channels found in neurons are Na+, K+, Ca++, and CL-

The Na+ gated channels respond to     

The K+ gated and the Ca++ gated ion channels are voltage regulated

The Cl- gated ion channels are chemically regulated.

The flow of ions is down their electrical and concentration gradients (Electrochemical), thus K+ flows out, while Na+, Ca++ and Cl- flow into the cell.

The movement of ions in a neuron at rest can either hyperpolarize (open K+ or Cl-) Or depolarize (open Na+, Ca++) the membrane

Remember :: each channel is specific for a particular ion

  the potential difference across the plasma membrane @ rest is called the
Resting Membrane Potential (RMP) and ONLY exists across the membrane.

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Two types of electrical signals are produced by a change in RMP and involve opening or closing ion channels previously discussed.

1) Graded Potentials --> over a short distance

2) Action Potentials --> over a long distance

Graded Potentials :

types of Graded Potentials are -->

a) receptor potential sensory neuron excited by some form of Energy

b) postsynaptic potential neuron excited by neurotransmitter

Action Potentials : is a rapid change in membrane potentials

once generated, all AP are alike

Stimulus strength is coded for by the frequency of AP transmission

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There are refractory periods that can have an affect on a secondary stimulus. 

In the ABSOLUTE refractory period, no further stimulus will excite the nerve.

In the RELATIVE refractory period, a stronger than normal stimulus will excite the nerve.

The new action potential that is generated will begin at the axon hillock, and if traveling down a myelinated axon,
the impulse will occur at each node, and appear to jump from node to node. This is known as SALTATORY CONDUCTION.

If the new action potential that is generated travels down a non myelinated axon,
the impulse will occur at each ion channel and move at an apparent slower rate.
This is known as Continuous Conduction.

The stimulus must pass a threshold level in order for the membrane to depolarize and create an action potential.

Repolarization occurs to shift the transmembrane potential back toward resting levels.

The largest myelinated axons will carry the impulses quickly, while the smallest unmyelinated axons are the slowest. 
They are referred to as A, B, C fibers

The average resting membrane potential for nerves is  -70mV (but can also go up as high as -90mv in larger nerves),
created by the ionic charges across the membrane by the active transport of sodium ion and potassium ion.  
ENERGY is required as sodium ion is transported OUT & potassium ion is transported IN and the ratio is 3:2 (better known as the sodium pump)

Calcium ion is also important ::

SYNAPSES :

Impulses are transmitted from one nerve cell to another at synapses

Transmission is primarily CHEMICAL

Two definitions are given to cells involved in synapses

Ends of presynaptic fibers generally enlarge to form terminal buttons (knobs).
Inside the Knobs are small vesicles (granules) that contain the chemical transmitter. 
This transmitter is released and crosses the junction to bind with receptors on the post synaptic cell surface and release its chemical contents.

Therefore, the conduction of impulses by chemical transmitters will involve only one direction.....from presynaptic cells to postsynaptic cells.

Two classes of synapses are

Here are the events when a nerve impulse reaches an axon terminal

1) Ca++ gates open in presynaptic axonal terminals and Ca++ floods into the terminal from ECF

2) Vesicles containing neurotransmitter move to the membrane and the chemical is released by exocytosis. 

The more Ca++ in, the more vesicles release their chemicals

3) Neurotransmitter binds to postsynaptic membrane receptors

4) Ion (protein) channels open in the postsynaptic membrane.

The resulting current flow causes local changes in RMP.

Depending on the neurotransmitter and protein channel, the effect on

the postsynaptic membrane may be excitatory or inhibitory.

The rate limiting step in the transmission of impulses reflects the time required for release, diffusion across the cleft, and binding to receptors.

This is called the SYNAPTIC DELAY.

During the action potential, if the excitability of the neuron to the other stimulus is increased,
it is called an EXCITATORY POSTSYNAPTIC POTENTIAL  or EPSP.  
This will result from the make-up of the neurotransmitter to excite the post synaptic membrane.

If the excitability of the neuron to other stimuli is decreased, it is called an
INHIBITORY POSTSYNAPTIC POTENTIAL or an IPSP. 
This will result when the neurotransmitter inhibits the action potential from continuing due to HYPERPOLARIZATION.

Therefore a presynaptic neuron can contain one of two types of neurotransmitters, either excitatory or inhibitory.

Both EPSPs and IPSPs can occur over a period of time (usually in milliseconds) known as
temporal summation or they can occur on different locations on the same nerve cell membrane and is known as spatial summation.

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CHEMICAL TRANSMITTING AGENTS :

I. Acetylcholine (ACH)

II. Biogenic Amines

A. Catecholamines

B. Indolamine

Biogenic amines are

Norepinephrine

Dopamine

Serotonin

Problems :: mental illness

increases in dopamine --> schizophrenia

decreases in serotonin --> Alzheimers

III. Amino Acids

In CNS as neurotransmitters

  1. gamma amino butyric acid (GABA)

  2. Glycine

  3. aspartate

  4. glutamate

IV. Peptides (most are pain modulators)

1. Substance P

2. Endorphins

3. Enkephalins

4. Somatostatin

V. Others

1. ATP

2. Nitrous oxide (NO)

3. Carbon monoxide (CO)

Classification of Neurotransmitters ::

A. Functional

B. Direct vs. Indirect

When G proteins are activated they can ::

  Secondary messengers in turn can

DRUGS that affect Synapses

1. Nicotine... Binds to ACH receptor sites

2. Atropine...Competes with ACH on postsynaptic neurons

3. Barbiturates....decrease ACH release, causing muscular weakness, depression

4. Botulism toxin ... blocks ACH release, therefore paralyzes voluntary muscle

5. Insecticides ... prevents ACH breakdown by ACHase, causing sustained contraction of skeletal muscles and

other effects on smooth muscles and glands

NEURAL INTEGRATION

Neurons function in groups and each group has connections with other groups.

Organized into neuronal pools (functional groups)

The patterns of connections in neuronal pools are called Circuits.  Various types exist ::

Processing within circuits can be ::