Action potential
Interested in how the membrane potential changes along the axon during a nerve impulse? Watch this video!
Peripheral nervous system
The peripheral nervous system helps the central nervous system communicate with the rest of the body. Understanding how it works helps us see how nerves carry signals to muscles and organs, enabling movement and sensation. Use this resource to learn how the peripheral nervous system keeps us aware and responsive to our environment.
The peripheral nervous system (PNS) connects the central nervous system to the rest of the body, allowing for communication and response. It is the network of neurones that extend from the CNS, reaching every part of the body.
By transmitting signals between the CNS and other regions, the PNS allows our body to respond to stimuli, maintain balance, and coordinate actions, ensuring effective interaction with our environment.
Neurones
Did you know?
The longest neurone in the human body connects the base of the spine to the toes. It can be over a metre long!
There are a few types of neurones, but these cells have the same general structure. Their components are summarised in the table.
| Component | Description |
|---|---|
| Cell body (or soma) | Contains the nucleus and other organelles; processes incoming signals and generates outgoing signals |
| Dendrites | Branch out from the cell body; receive signals from other neurones and sensory receptors and pass information along to the cell body |
| Axon | A long, thin projection that transmits signals away from the cell body to other neurones, muscles or glands |
| Axon hillock | The part of a neurone that connects the cell body to the axon |
| Myelin sheath | A fatty layer of insulation covering some neurones; helps increase speed of transmission |
| Axon terminals | The end of an axon from which chemicals called neurotransmitters are released |
Neuron by Versal on Sketchfab, licensed under CC BY 4.0
An interactive three-dimensional model of a neurone with features labelled.
Neuron model
- Cell body (soma): The compact area of a nerve cell that constitutes the nucleus and surrounding cytoplasm, excluding the axons and dendrites; the cell body
- Axon: A tube-like structure that propagates signals to the axon terminals
- Dendrite: A short, branched extension of a nerve cell, along which impulses received from other cells at synapses are transmitted to the cell body
- Myelin sheath: An insulator that covers an axon in order to minimise dissipation of the electrical signal as it travels down the axon
- Node of Ranvier: A gap in the myelin sheath of a nerve; sites where the signal is "recharged" as it travels along the axon
- Axon terminal: Branch at the end of the axon that allows signals to be communicated to other cells
- Axon hillock: Portion of the neuron that connects the cell body, soma, to the axon. The impulses the neuron receives are summed up at the axon hillock to determine whether an action potential will be initiated
The PNS also consists of ganglia (singular: ganglion), which are clusters of neurone cell bodies. Neurones, on the other hand, are individual signalling cells.
Types of neurones
The three types of neurones are:
- sensory neurones which carry information from sensory organs to the CNS; also called afferent neurones
- motor neurones which send messages from the CNS to the muscles and glands, controlling our movement and reactions; also called efferent neurones
- interneurones which link sensory and motor neurones within the CNS and have a role in processing sensory information and making decisions about how our body should respond.
The sensory and motor neurones are found in the PNS, but the interneurons are found in the CNS, so the brain and spinal cord.
The position of the cell body in these neurones differs, giving them different appearances.
Diagram of an interneurone, sensory neurone and motor neurone
Neurotransmitters
Neurotransmitters are chemical substances that neurones use to communicate with sensory organs, muscles, glands, and other neurones. They are packaged into little capsules called vesicles that are released into gaps between a neurone and what it is communicating with.
This gap is called a synapse. This means the neurone releasing the neurotransmitters is called a pre-synaptic neurone and the neurone receiving the signals is called the post-synaptic neurone.
Diagram of the synapse between two neurones
Did you know?
The same neurotransmitter can produce different effects by binding to:
- different receptors on the same tissue
- the same receptor on different tissue types.
Nerve impulses
Nerve impulses are electrical signals that travels along neurones. They allow the nervous system to communicate quickly.
The process in generating a nerve impulse can be complicated, so let's break it down.
- Neurotransmitters bind to receptors in the dendrites and cause the membrane potential (the difference in charge between the inside and outside of the cell) to become less negative. At rest, the membrane potential is \(-70\textrm{ mV}\).
- If enough neurotransmitters bind to the cell, the membrane potential becomes negative enough and reaches a threshold of \(-55\textrm{ mV}\).
- Special ion channels in the cell membrane open to allow sodium ions to enter the cell. This causes the charge inside the cell to become even less negative. This depolarisation (where the membrane potential gets closer to \(0\)) creates an action potential.
- The action potential travels down the neurone's axon, moving as a wave of depolarisation.
- At the same time, once the cell becomes positive enough, ions channels that allow potassium ions to leave the cell open to help the cell regain its negative membrane potential. This repolarisation helps the action potential move as a wave.
- The cell actual becomes hyperpolarised, meaning that its membrane potential is even more negative that when how it started. But eventually, the potassium channels close to let the membrane potential return to normal.
- At the terminals, the impulse triggers the release of neurotransmitters into the synapse.
- These chemicals carry the message to the next neurone, continuing the signal until it reaches a target tissue (muscle or gland).
Watch this video to see how a nerve impulse is propagated through a neurone.
A typical neurone consists of a cell body, plasma membrane extensions called dendrites, an elongated fibre known as an axon, and an axon hillock, the trigger zone that releases a nerve impulse. The axon hillock maintains an excitation limit, or threshold, which determines whether or not a neuron will generate a nerve impulse. A nerve impulse is an electrical signal conducted by a neurone, causing a response in another neurone or target cell. When a neurone is at rest, its membrane is polarised because there are more positive ions outside the cell and more negative ions inside the cell, which creates a charge difference across the membrane. Active transport mechanisms called sodium-potassium pumps carry more sodium and less potassium ions across the membrane to maintain this charged difference. Even in a resting neurone, there is the potential for the charged difference to create an electrical current. This is called a resting membrane potential. When an electrical current flows through a dendrite, this is called a local membrane potential. When a dendrite detects a stimulus, a sodium channel in its plasma membrane opens and lets sodium into the neurone. This influx of positive ions reverses the charge across a particular section of the membrane in a process called depolarisation. To repolarise the membrane, potassium channels open and release potassium out of the neurone. Nearby, a sodium-potassium pump transports excess sodium amount and brings potassium in, which restores the resting membrane potential. The flow of reversing charges along the dendrite's membrane produces a wavelike electrical current toward the neurone's trigger zone. If the strength of the current meets or exceeds the threshold at the trigger zone, an electrical signal called an action potential or nerve impulse will occur. In a nerve impulse, the trigger zone sends an electrical signal down the axon toward the space between neurones called a synapse or to a target cell membrane.
Nerve impulses begin at a sensory organ, which activates a sensory neurone. This then synapses with an interneurone, which then stimulates a motor neurone. Finally, the motor neurone connects to a target tissue or effector, coordinating it to respond in the required way.
Division of the PNS
The PNS is divided into the somatic and autonomic nervous systems. This division allows the body to efficiently control different types of functions.
The somatic nervous system controls voluntary movements, like moving our arms and legs. Remember, "soma" means "body", thus "somatic" refers to the control of body movements.
The somatic nervous system does this by transmitting sensory and motor signals to and from the central nervous system, allowing us to consciously interact with our environment, move and react.
It is also involved in involuntary movements that are controlled by a reflex arc. One example of a reflex response is pulling your hand away from a hot candle flame.
Diagram showing a reflex arc
The autonomic nervous system regulates involuntary functions, such as heart rate, digestion, and breathing. The word "autonomic" comes from "autonomous", meaning independent.
By operating without conscious effort, the autonomic nervous system ensures vital processes continue smoothly, responding automatically to internal signals and maintaining a state of balance called Homeostasis.
The autonomic nervous system can be further divided into the sympathetic and parasympathetic nervous systems.
| Branch | Role | Physiological responses |
|---|---|---|
| Sympathetic nervous system | "Fight-or-flight" during stressful situations |
|
| Parasympathetic nervous system | "Rest and digest" to help body recover |
|
Watch this video to learn more about the sympathetic and parasympathetic nervous systems.
Say you're at the beach on a beautiful day, the Sun is shining, there's a gentle breeze. You dive into the water, but struggle to stay afloat. As you fight the current, your sympathetic nervous system or your fight or flight response kicks in. You are in survival mode and your body prepares for conflict. Your body signals danger by releasing adrenaline. Your heart starts racing. Your pupils expand and your mouth becomes dry. Your airways open up, sending as much oxygen as possible to your brain to keep you alert, and other bodily processes switch gears to help in your time of need. Your stomach and intestines stop digesting, your bladder relaxes, your reproductive system limits blood flow, and your liver releases glucose to give you a burst of energy. All these changes happen so quickly, you may not even be aware of them. When you make it back to shore, you start to calm down and relax. This is your parasympathetic nervous system or your rest and digest process at work. Those bodily processes that were at full attention before change course. Your heart rate slows down, your breathing returns to normal, your pupils constrict and you start to salivate. As your body realises it's no longer under threat, your stomach and intestines begin to digest. Your bladder constricts, and your reproductive system increases its blood flow. In other words, your body returns to its natural state. This is an example of your autonomic nervous system at work. The sympathetic nervous system and parasympathetic nervous system perform a vital balancing act that helps you survive and recover. It happens many times throughout the day, whether you're at the beach, hanging out at home, working from an office or anywhere in between. And while you may tend to think of your fight or flight response kicking in during major stressful situations like a car accident, it can also be triggered by minor things like spilling a glass of milk or getting a paper cut. You can also thank your rest and digest response for all those loving feelings, big and small, you may have from day to day. From cuddling your puppy, to professing your love to your partner. So the next time you are stressed and sense your body tensing up, or you find yourself relaxing on the couch after a busy day of work.
The differences between your sympathetic and parasympathetic nervous systems are so dramatic it can feel a bit like flipping a switch.
Summary
The organisation of the nervous system is summarised in the following flowchart.
Nervous system flowchart
- Nervous system
- Central nervous system
- Brain
- Spinal cord
- Peripheral nervous system
- Somatic nervous system
- Autonomic nervous system
- Sympathetic nervous system
- Parasympathetic nervous system
- Central nervous system
Exercise
See how well you understand the peripheral nervous system with a quick quiz.
Images on this page by RMIT, licensed under CC BY-NC 4.0
