Which cell is adapted for transmitting messages

The cell body contains the nucleus and other cell organelles. Dendrites extend from the cell body and receive nerve impulses from other neurons. The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals.

These are the points where the neuron communicates with other cells. Types of Neurons Neurons are classified based on the direction in which they carry nerve impulses. Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain. Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands see Figure below. Interneurons carry nerve impulses back and forth between sensory and motor neurons.

Summary Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon. Neurons transmit nerve impulses to other cells.

Types of neurons include sensory neurons, motor neurons, and interneurons. Review What are the two main parts of the nervous system? List and describe the parts of a neuron. Action potentials : A neuron must reach a certain threshold in order to begin the depolarization step of reaching the action potential.

This process of depolarization, repolarization, and recovery moves along a nerve fiber from neuron to neuron like a very fast wave. While an action potential is in progress, another cannot be generated under the same conditions. In unmyelinated axons axons that are not covered by a myelin sheath , this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. Saltatory conduction is faster than continuous conduction.

The diameter of the axon also makes a difference, as ions diffusing within the cell have less resistance in a wider space. Damage to the myelin sheath from disease can cause severe impairment of nerve-cell function. In addition, some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves.

The amplitude of an action potential is independent of the amount of current that produced it. In other words, larger currents do not create larger action potentials. Therefore, action potentials are said to be all-or-none signals, since either they occur fully or they do not occur at all. The frequency of action potentials is correlated with the intensity of a stimulus. This is in contrast to receptor potentials, whose amplitudes are dependent on the intensity of a stimulus.

Reuptake refers to the reabsorption of a neurotransmitter by a presynaptic sending neuron after it has performed its function of transmitting a neural impulse. Reuptake is necessary for normal synaptic physiology because it allows for the recycling of neurotransmitters and regulates the neurotransmitter level in the synapse, thereby controlling how long a signal resulting from neurotransmitter release lasts.

The synapse is the site at which a chemical or electrical exchange occurs between the presynaptic and postsynaptic cells. The synapse is the junction where neurons trade information. It is not a physical component of a cell but rather a name for the gap between two cells: the presynaptic cell giving the signal and the postsynaptic cell receiving the signal. There are two types of possible reactions at the synapse—chemical or electrical. During a chemical reaction, a chemical called a neurotransmitter is released from one cell into another.

In an electrical reaction, the electrical charge of one cell is influenced by the charge an adjacent cell. The electrical response of a neuron to multiple synaptic inputs : Synaptic responses summate in order to bring the postsynaptic neuron to the threshold of excitation, so it can fire an action potential represented by the peak on the chart.

The process of a chemical reaction at the synapse has some important differences from an electrical reaction. Chemical synapses are much more complex than electrical synapses, which makes them slower, but also allows them to generate different results.

Like electrical reactions, chemical reactions involve electrical modifications at the postsynaptic membrane, but chemical reactions also require chemical messengers, such as neurotransmitters, to operate. Neurotransmitters are chemicals that transmit signals from a neuron across a synapse to a target cell.

Neurotransmitters are chemicals that transmit signals from a neuron to a target cell across a synapse. When called upon to deliver messages, they are released from their synaptic vesicles on the presynaptic giving side of the synapse, diffuse across the synaptic cleft, and bind to receptors in the membrane on the postsynaptic receiving side.

An action potential is necessary for neurotransmitters to be released, which means that neurons must reach a certain threshold of electric stimulation in order to complete the reaction. A neuron has a negative charge inside the cell membrane relative to the outside of the cell membrane; when stimulation occurs and the neuron reaches the threshold of excitement this polarity is reversed.

This allows the signal to pass through the neuron. When the chemical message reaches the axon terminal, channels in the postsynaptic cell membrane open up to receive neurotransmitters from vesicles in the presynaptic cell.

Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell that is, decreasing the voltage gradient of the cell, thus bringing it further away from an action potential , while excitatory neurotransmitters cause depolarization bringing it closer to an action potential.

Neurotransmitters match up with receptors like a key in a lock. A neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.

There are several systems of neurotransmitters found at various synapses in the nervous system. The following groups refer to the specific chemicals, and within the groups are specific systems, some of which block other chemicals from entering the cell and some of which permit the entrance of chemicals that were blocked before.

The cholinergic system is a neurotransmitter system of its own, and is based on the neurotransmitter acetylcholine ACh. This system is found in the autonomic nervous system, as well as distributed throughout the brain. The cholinergic system has two types of receptors: the nicotinic receptor and the acetylcholine receptor, which is known as the muscarinic receptor.

Both of these receptors are named for chemicals that interact with the receptor in addition to the neurotransmitter acetylcholine. Nicotine, the chemical in tobacco, binds to the nicotinic receptor and activates it similarly to acetylcholine. Muscarine, a chemical product of certain mushrooms, binds to the muscarinic receptor. Another group of neurotransmitters are amino acids, including glutamate Glu , GABA gamma-aminobutyric acid, a derivative of glutamate , and glycine Gly.

These amino acids have an amino group and a carboxyl group in their chemical structures. Glutamate is one of the 20 amino acids used to make proteins. Each amino acid neurotransmitter is its own system, namely the glutamatergic, GABAergic, and glycinergic systems. They each have their own receptors and do not interact with each other.

Amino acid neurotransmitters are eliminated from the synapse by reuptake. A pump in the cell membrane of the presynaptic element, or sometimes a neighboring glial cell, clears the amino acid from the synaptic cleft so that it can be recycled, repackaged in vesicles, and released again.

The reuptake process : This illustration shows the process of reuptake, in which leftover neurotransmitters are returned to vesicles in the presynaptic cell. Another class of neurotransmitter is the biogenic amine, a group of neurotransmitters made enzymatically from amino acids. They have amino groups in them, but do not have carboxyl groups and are therefore no longer classified as amino acids. A neuropeptide is a neurotransmitter molecule made up of chains of amino acids connected by peptide bonds, similar to proteins.

However, proteins are long molecules while some neuropeptides are quite short. Neuropeptides are often released at synapses in combination with another neurotransmitter. Dopamine is the best-known neurotransmitter of the catecholamine group. The brain includes several distinct dopamine systems, one of which plays a major role in reward-motivated behavior. Most types of reward increase the level of dopamine in the brain, and a variety of addictive drugs increase dopamine neuronal activity.

Other brain dopamine systems are involved in motor control and in controlling the release of several other important hormones. The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. If there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing more likely to reach an action potential or hyperpolarizing less likely to reach an action potential effect is also dependent on the receptor.

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