Neuronal Communication and the Nervous System

 

Neurotransmitters and Receptors

A neurotransmitter is a chemical messenger that allows neurons to communicate with one another.

A receptor is a protein on the surface of a neuron or another cell that receives neurotransmitters.

When an electrical signal reaches the end of a neuron:

1. The action potential reaches the axon terminal.
2. Neurotransmitters are released into the synapse (the tiny gap between two neurons).
3. The neurotransmitters travel across the synapse.
4. They attach (bind) to matching receptors on the next neuron, like a key fitting into a lock.
5. This causes the receiving neuron to either continue or stop the nerve signal.
6. Once the message has been delivered, the neurotransmitters are removed from the synapse.

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The Synapse

The synapse is the tiny space between two neurons where communication occurs.

Steps in Synaptic Transmission

1. Electrical signal travels along the neuron.
2. Signal reaches the axon terminal.
3. Neurotransmitters are released.
4. Neurotransmitters cross the synapse.
5. Neurotransmitters bind to receptors.
6. A new electrical signal may begin in the next neuron.
7. Remaining neurotransmitters are broken down or recycled.

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Neuronal Communication

Neurons communicate using both electrical and chemical signals.

• Electrical signals travel within the neuron.
• Chemical signals (neurotransmitters) travel across the synapse.

This process allows the brain and body to communicate quickly and accurately.

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Membrane Potential

The membrane potential is the electrical difference between the inside and outside of a neuron.

Neurons use this electrical charge to send messages.

There are three important stages:

• Resting potential
• Threshold of excitation
• Action potential

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Resting Potential

The resting potential is the neuron's normal resting electrical charge when it is not sending a signal.

Characteristics

• Inside of the neuron is more negative than the outside.
• Usually about −70 millivolts (mV).
• The neuron is ready to receive information.

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Threshold of Excitation

The threshold of excitation is the minimum level of stimulation needed to trigger a nerve impulse.

Characteristics

• Usually around −55 mV.
• If the threshold is reached, the neuron fires.
• If the threshold is not reached, no action potential occurs.

This is known as the all-or-none principle: a neuron either fires completely or not at all.

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Action Potential

An action potential is the electrical impulse that travels along the axon.

Steps

1. Stimulus reaches threshold.
2. Sodium ions move into the neuron.
3. The inside becomes positively charged (depolarization).
4. The electrical impulse travels down the axon.
5. Potassium ions leave the neuron (repolarization).
6. The neuron returns to its resting potential.

Action potentials allow messages to travel rapidly throughout the nervous system.

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Reuptake

After neurotransmitters have delivered their message, many are taken back into the sending neuron through a process called reuptake.

Functions

• Recycles neurotransmitters.
• Stops continuous stimulation.
• Prepares the neuron for the next message.
• Helps maintain normal communication.

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Neurotransmitters and Drugs

Many medications and drugs affect neurotransmitters.

Some drugs:

• Increase neurotransmitter release.
• Block neurotransmitter release.
• Block receptors.
• Increase reuptake.
• Prevent reuptake.

Examples

Drug Type	Effect
Antidepressants (SSRIs)	Reduce serotonin reuptake, increasing serotonin in the synapse.
Local anesthetics	Block nerve impulses so pain signals cannot travel.
Stimulants	Increase dopamine and norepinephrine activity.
Sedatives	Increase the effects of GABA, reducing brain activity.

Because neurotransmitters regulate mood, movement, memory, sleep, learning, and attention, drugs that alter neurotransmitters can significantly affect brain and body function.

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The Nervous System

The nervous system is divided into two main parts:

1. Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)

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Central Nervous System (CNS)

The Central Nervous System (CNS) consists of:

• The brain
• The spinal cord

Functions

The CNS:

• Processes information.
• Makes decisions.
• Controls thoughts, emotions, and memory.
• Coordinates movement.
• Sends instructions to the rest of the body.

The spinal cord acts as the main communication pathway between the brain and the body.

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Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) includes all the nerves outside the brain and spinal cord.

It is made up of bundles of axons (nerve fibers) that connect the CNS to the rest of the body.

The PNS carries messages:

• From the brain to muscles and organs.
• From the body back to the brain.

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Two Major Divisions of the PNS

1. Somatic Nervous System (SNS)

The Somatic Nervous System (SNS) controls voluntary movements and carries sensory information to the brain.

Functions

• Skeletal muscle movement.
• Conscious sensation.
• Touch.
• Pain.
• Temperature.
• Pressure.
• Vibration.
• Position of the body.

The somatic nervous system allows you to move, walk, write, and interact with your environment.

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Sensory (Afferent) Nerves

Sensory (afferent) nerves carry information from the body to the brain and spinal cord.

They detect information from receptors in the skin, muscles, joints, and sense organs.

They carry information about:

• Light (vision)
• Sound (hearing)
• Touch
• Pressure
• Vibration
• Temperature
• Pain
• Taste
• Smell
• Body position

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The Five Main Senses

The sensory nervous system receives information through the five traditional senses:

1. Sight (Vision)
2. Hearing (Audition)
3. Taste (Gustation)
4. Smell (Olfaction)
5. Touch (Somatosensation)

Touch also includes sensations such as:

• Pressure
• Vibration
• Temperature
• Pain
• Texture

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Proprioception

Proprioception is the body's ability to sense the position and movement of muscles, joints, and limbs without looking at them.

It is sometimes called the "sixth sense."

Functions

• Balance
• Coordination
• Walking
• Standing
• Posture
• Smooth movement
• Knowing where your arms and legs are

For example, you can touch your nose with your eyes closed because proprioceptors in your muscles and joints continuously send information to your brain.

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2. Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) (sometimes mistakenly written as "automantic") controls involuntary body functions—activities that happen automatically without conscious effort.

Functions

• Heart rate
• Breathing
• Blood pressure
• Digestion
• Sweating
• Body temperature regulation
• Pupil size

The ANS has two main divisions:

• Sympathetic nervous system ("fight or flight"), which prepares the body to respond to stress.
• Parasympathetic nervous system ("rest and digest"), which helps the body relax, recover, and conserve energy.

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Summary

• Neurotransmitters are chemical messengers released into the synapse.
• They bind to receptors on the next neuron to continue or modify the nerve signal.
• Membrane potential is the electrical charge across a neuron's membrane.
• Resting potential is the neuron's resting state (about −70 mV).
• Threshold of excitation is the minimum stimulation needed to trigger an action potential.
• Action potential is the electrical impulse that travels along the axon.
• Reuptake removes and recycles neurotransmitters after signaling.
• The Central Nervous System (CNS) consists of the brain and spinal cord.
• The Peripheral Nervous System (PNS) consists of nerves outside the CNS and connects the brain and spinal cord to the rest of the body.
• The Somatic Nervous System (SNS) controls voluntary movement and conscious sensation.
• Sensory (afferent) nerves carry information from the body to the CNS.
• Proprioception is the unconscious awareness of body position and movement.
• The Autonomic Nervous System (ANS) controls involuntary functions and includes the sympathetic ("fight or flight") and parasympathetic ("rest and digest") divisions.