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Rana J. Rahhal

Abdulrahman Al-hanbali

Faisal Mohammad

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I wrote everything important + what the doctor mentioned in the Lecture. Forgive me for any mistakes and GOOD LUCK

Creativity is intelligence having fun

Neuron Types & Neurotransmitters

Review :

Control systems of the body: 1. Nervous system 2. Endocrine system

Note that the response to the nervous system is much faster than that to the endocrine system.

Information is transmitted to brain by nerves.

There are two types of propagation: 1-Saltatory: these are with myelinated fibers (faster).

2-Continuous: these are with unmyelinated fibers (slower).

The velocity of this transmission depends on: -the size of the nerve fiber; nerve fibers come in all sizes between 0.5 and 20 micrometers in diameter, (larger diameter-> less resistance -> higher velocity). -myelination of the nerve fiber (myelinated nerve fibers ->saltatory propagation ->higher velocity).

READ ME:

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I)neuron types:

Classification of neurons according to diameter:

-In general classification, the fibers are divided into: A, C and B. The type A fibers are subdivided into α (the largest), β, γ and δ (the smallest) fibers.

-There is also an alternative classification in which the myelinated fibers are given the roman numbers (I, II and III) and (IV) for the unmyelinated fibers.

Type A fibers (type I ,II and III)are Myelinated fibers of varying sizes, they can transmit impulses at velocities as great as 120 m/sec (a distance in 1 sec that is longer than a football field!).

Type B fibers are Myelinated fibers, located in the autonomic nervous system.

Type C fibers (type IV) are the small Unmyelinated fibers, they can transmit impulses as slowly as 0.5 m/sec (it requires about 2 seconds to go from the big toe to the spinal cord).

Neuron Classification according to their function:

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Nerve cells are functionally classified as sensory neurons, motor neurons, and interneurons.

Sensory neurons (afferent neurons) are neurons that convert a specific type of stimulus, via their receptors into action potentials. This sensory information travels along afferent nerve fibers in an afferent or sensory nerve, to the brain via the spinal cord. The stimulus can come from outside the body, for example touch, pain, light and sound, or from inside the body, for example blood pressure or the sense of body position. Different types of sensory neurons have different sensory receptors that respond to different kinds of stimuli.

Association neurons (interneurons) enabling communication between sensory and motor neurons. They are located in the central nervous system (CNS).

Motor neurons (efferent neurons)an efferent neuron is a neuron whose cell body is located in the spinal cord and whose axon projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. Motor neurons axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects. Structural Classification of Neurons: Neurons can be classified according to the number of processes that extend from the neuronal cell body (soma).

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Multipolar neurons: are commonly seen in motor and sensory

functions. They have cell body (soma), axon (axonal terminal) and dendrites. Cell body has almost all organisms in the cell such as endoplasmic reticulum, mitochondria, Golgi…. EXCEPT the centrosomes (neurons do not undergo cell division). The first part of the axon is called the axon hillock, this area has the lowest threshold of action potential, because it has the highest density of voltage-gated sodium channels (positive feedback).

Bipolar neurons: have two processes, an axon and dendrite,

these are usually seen in sensory functions.

Unipolar neurons.

II)Neurotransmitters:

Neurotransmitter is a chemical substance that acts as the mediator for the transmission of impulse from one neuron to another neuron through synapses.

Transmission of information at chemical synapse involves: - 1. Release of neurotransmitter from presynaptic terminal. 2. Diffusion across synaptic cleft. 3. Binding to specific receptors to produce EPSP or IPSP.

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The membrane of the presynaptic terminal is called presynaptic membrane. It contains large numbers of voltage-gated calcium channels. When an action potential depolarizes the presynaptic membrane, these calcium channels open and allow large numbers of calcium ions to flow into the terminal. The calcium ions enter the presynaptic terminal, it stimulates transmitter vesicles to release its content into synaptic cleft. Then neurotransmitters bind to its receptors on postsynaptic membrane.

Classification of Synaptic Transmitters

Small-molecule, Rapidly acting transmitters

(acetylcholine, norepinephrine, dopamine, serotonin, GABA, glycine, glutamate, NO (lipid soluble)).

Review of AP in neuron

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Rapidly Acting (In milliseconds) by activation of Receptor Proteins.

by opening/closing the ion channels. (i.e. development of EPSP or IPSP).

Synthesized in the Cytosol of presynaptic terminals & stored in presynaptic vesicles.

Excreted in large amounts. Neurotransmitter can be recycled in presynaptic

terminal or can be broken down by enzymes within the cell.

Neuropeptides (Neuromodulator)

Slowly Acting. Cause Prolonged Actions. Synthesized in Ribosomes, ER, Golgi Apparatus of Soma.

Then transported very slowly to the axonal terminals. They can NOT be reused after secreting. Excrete in small amount. They are co-secreted with small-molecules NT.

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How these NT are released

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(The Dr.:” just read the figure”)

Each neuron has only one kind of small-molecule NT, but it can contain more than one neuropeptides.

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Removal of Neurotransmitter

1. Diffusion move down their concentration gradient.

2. Enzymatic degradation Acetylcholinesterase for (Ach), monoamine oxidase & catechol-O-methyltransferase for epinephrine & norepinephrine, and peptidases for neuropeptides

3. Uptake by neurons or glial cells NT can be reuptaken as a whole through neurotransmitter transporters or as degraded components.

4. Or a combination of 1,2and3. knowing the removal mechanisms allow us to synthesize drugs that prevent the removal of a certain NT which can cause the prolongation of the action potential (e.g. Prozac = serotonin reuptake inhibitor).

(Transmitter Inactivation: reuptake and enzymatic breakdown)

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Basic Concepts of NT and receptor Neurotransmitter: Endogenous signaling molecules that

alter the behavior of neurons or effector cells. Neuroreceptor: Proteins on the cell membrane (water

soluble) or in the cytoplasm (lipid soluble) that could bind with specific neurotransmitters and alter the behavior of neurons of effector cells.

Vast array of molecules serve as neurotransmitters The properties of the transmitter do not determine its

effects on the postsynaptic cells. They act as 1st messengers. The properties of the receptor determine whether a

transmitter is excitatory or inhibitory classical definition A neurotransmitter must:

Be synthesized and released from neurons. Be found at the presynaptic terminal. Have same effect on target cell when applied externally. Be blocked by same drugs that block synaptic transmission. Be removed in a specific way.

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