Neural Impulse and Synaptic Transmission

Objectives

1.To define neural impulse and synaptic transmission

2.To discuss the parts of a neuron and their functions

3.To discuss the events that take place during a

synaptic transmission and list important

neurotransmitters

4.To give examples of diseases that affect neural

impulse and synaptic transmission

5.To summarize the information disseminated during

the discussion

Definition of Terms

Neural Impulse

An electrical signal that travels along an axon. There is an

electrical difference between the inside of the axon and its surroundings,

like a tiny battery.

Synaptic Transmission

The process by which signaling molecules

called neurotransmitters are released by a presynaptic neuron and bind to

and activate the receptors a postsynaptic neuron.

The Neuron

A neuron is an excitable cell that processes and transmits information through electrical and chemical signals.

The signals between neurons occur via synapses, specialized connections with other cells.

Neurons are the core components of the brain and spinal cord of the central nervous system(CNS), and of the ganglia of the peripheral nervous system(PNS).

Parts of a Neuron

Soma

The body of the neuron. As it contains the nucleus, most protein synthesis occurs here. The nucleus can range from 3 to 18 micrometers in diameter.

Dendrites

The dendrites of a neuron are cellular extensions with many branches. This is where the majority of input to the neuron occurs via the dendritic spine.

Axon

A finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma.

Axon Hillock

The part of the axon where it emerges from the soma. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily excited part of the neuron.

Myelin Sheath

A dielectric (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell.

Schwann cells supply the myelin for the peripheral nervous system, whereas oligodendrocytes, myelinate the axons of the central nervous system.

The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier, which contain a high density of voltage-gated ion channels.

Axon Terminal or Terminal Boutons

Enlarged, often club-shaped endings by which axons make contact with other nerve cells or with effector cells (muscle or gland cells).

They contain synapses, specialized structures where neurotransmitters are released to communicate with target neurons. Axon terminals contain various kinds of neurotransmitters, sometimes more than one.

Stages of Synaptic Transmission

1. Synthesis of the neurotransmitter. This can take place in the soma, in the axon, or in the axon terminal.

2. Storage of the neurotransmitter in storage granules or vesicles in the axon terminal.

3. Calcium enters the axon terminal during an action potential, causing release of the neurotransmitter into the synaptic cleft.

4. After its release, the transmitter binds to and activates a receptor in the postsynaptic membrane.

5. Deactivation of the neurotransmitter. The neurotransmitter is either destroyed enzymatically, or taken back into the terminal from which it came, where it can be reused, or degraded and removed.

Types of Transmission

Axodendritic

Axosomatic

Axoaxonic

Important Neurotransmitters

Neurotransmitter Behavioral Effect

Acetyl Choline Arousal and cognitive enhancement

Beta-Endorphin Reduction of anxiety and tension

Dopamine Pleasure and appetite suppression

Gamma-Amino-Butyric Acid(GABA) Reduction of anxiety and tension

Glutamate Learning and memory enhancement

Norepinephrine Arousal and appetite suppression

Serotonin Mood modulation and appetite suppression

Clinical Correlation

Schizophrenia

Impairment of dopamine-containing neurons in the brain is implicated in schizophrenia , a mental disease marked by disturbances in thinking and emotional reactions. Medications that block dopamine receptors in the brain, such as chlorpromazine and clozapine , have been used to alleviate the symptoms and help patients return to a normal social setting.

Depression

In depression, there appears to be abnormal excess or inhibition of signals that control mood, thoughts, pain, and other sensations. Depression is treated with antidepressants that affect norepinephrine and serotonin in the brain.

Alzheimer's Disease

Characterized by memory loss and the eventual inability for self-care. The disease seems to be caused by a loss of cells that secrete acetylcholine in the basal forebrain (region of brain that is the control center for sensory and associative information processing and motor activities). Some medications to alleviate the symptoms have been developed, but presently there is no known treatment for the disease.

Attention-Deficit / Hyperactivity Disorder

People affected by attention-deficit/hyperactivity disorder(ADHD) experience difficulties in the areas of attention, overactivity, impulse control, and distractibility. Research shows that dopamine and norepinephrine imbalances are strongly implicated in causing ADHD.

Generalized Anxiety Disorder

People with generalized anxiety disorder(GAD) experience excessive worry that causes problems at work and in the maintenance of daily responsibilities. Evidence suggests that GAD involves several neurotransmitter systems in the brain, including norepinephrine and serotonin.

Summary Neurons are excitable cells that process and transmit information through

electrical and chemical signals.

A neural impulse is an electrical signal that travels along the axon of a neuron.

A synaptic transmission is the process by which signaling molecules called neurotransmitters are released by a presynaptic neuron and bind to and activate the receptors a postsynaptic neuron.

Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by receptors on other synapses.

Some of the most important neurotransmitters are ACh, B-Endorphin, Dopamine, GABA, Glutamate, Norepinephrine and Serotonin. They have different behavioral effects on humans.

Some diseases are associated with impairment and/or abnormal amounts of the above neurotransmitters.

Thank You!