Neuron Structure and Function » Sensory Neurons

What you'll learn this session

Study time: 30 minutes

The structure of sensory neurons and how they differ from other neurons
The function of sensory neurons in transmitting information from receptors to the CNS
How sensory neurons respond to different stimuli
The role of sensory neurons in the reflex arc
Real-world examples of sensory neuron function and dysfunction

Introduction to Sensory Neurons

Sensory neurons are amazing cells that act like messengers, carrying information from the world around us to our brain and spinal cord. They're the reason you can feel a gentle breeze on your skin, taste your favourite ice cream, or hear your friend calling your name. Without sensory neurons, we'd be completely cut off from our environment!

Key Definitions:

Sensory neuron: A specialized nerve cell that transmits information from sensory receptors to the central nervous system.
Receptor: A specialized cell or structure that detects specific stimuli from the environment.
Central Nervous System (CNS): The brain and spinal cord, which process information from sensory neurons.
Dendrite: The branched extensions of a neuron that receive signals from other cells.
Axon: The long, slender projection of a neuron that conducts electrical impulses away from the cell body.

👁 Sensory Neuron Structure

Sensory neurons have a unique structure that sets them apart from other types of neurons:

Cell body: Contains the nucleus and is located in a structure called the dorsal root ganglion, which sits just outside the spinal cord.
Dendrites: In sensory neurons, these are often very long and extend to sensory receptors in the skin, muscles, or organs.
Axon: Carries electrical signals toward the central nervous system.
Myelin sheath: A fatty layer that insulates the axon and speeds up signal transmission.
Nodes of Ranvier: Gaps in the myelin sheath where the electrical signal jumps, making transmission faster.

📝 What Makes Sensory Neurons Special

Unlike motor neurons (which send signals from the CNS to muscles) or interneurons (which connect neurons to each other), sensory neurons have some special features:

They are pseudounipolar - their axon splits into two branches from a single process
Their cell bodies are located outside the CNS in ganglia
They only send signals in one direction - from receptors to the CNS
They can have extremely long dendrites - those reaching from your toes to your spinal cord can be up to a metre long!

How Sensory Neurons Work

Sensory neurons are constantly at work, turning physical stimuli into electrical signals that your brain can understand. This process happens in a few key steps:

The Journey of a Sensory Signal

💡 Step 1: Stimulus Detection

A stimulus (like heat, pressure, or light) activates a sensory receptor. For example, when you touch something hot, temperature receptors in your skin are activated.

⚡ Step 2: Signal Generation

The receptor converts the stimulus into an electrical signal (called a generator potential). If this reaches a threshold, it triggers an action potential in the sensory neuron.

🚀 Step 3: Signal Transmission

The action potential travels along the axon of the sensory neuron to the spinal cord or brain, where it can be processed and responded to.

Types of Sensory Receptors

Sensory neurons connect to different types of receptors, each specialised for detecting specific stimuli:

📌 External Receptors

These detect stimuli from the outside world:

Mechanoreceptors: Detect touch, pressure, vibration and sound
Thermoreceptors: Detect temperature changes
Photoreceptors: Detect light (in your eyes)
Chemoreceptors: Detect chemicals (taste and smell)
Nociceptors: Detect pain or potential tissue damage

📍 Internal Receptors

These monitor conditions inside your body:

Proprioceptors: Detect body position and movement
Baroreceptors: Detect blood pressure
Chemoreceptors: Detect oxygen and carbon dioxide levels in blood
Osmoreceptors: Detect water concentration in blood
Stretch receptors: Detect stretching in organs like the stomach or bladder

Sensory Neurons in the Reflex Arc

One of the most important roles of sensory neurons is in reflex actions - automatic, rapid responses that don't require conscious thought. The classic example is pulling your hand away from something hot before you even realise what's happening.

In a reflex arc:

A stimulus activates a sensory receptor
The sensory neuron carries the signal to the spinal cord
An interneuron in the spinal cord receives the signal
The interneuron passes the signal to a motor neuron
The motor neuron carries the signal to a muscle, which contracts

This happens incredibly quickly - often in less than 0.1 seconds - because the signal doesn't need to travel all the way to the brain for processing. This speed can be life-saving in dangerous situations!

Case Study Focus: Phantom Limb Sensation

Sometimes sensory neurons can behave in unusual ways. People who have had a limb amputated often report still feeling sensations from their missing limb - a phenomenon called phantom limb sensation. This happens because the sensory neurons that used to connect to the limb are still present and can sometimes become active, sending signals to the brain that seem to come from the missing limb.

Neuroscientist V.S. Ramachandran developed a treatment using a "mirror box" that tricks the brain by showing a reflection of the remaining limb where the missing one would be. This visual feedback can help reduce phantom limb pain by realigning what the sensory neurons are reporting with what the person can see.

Adaptation in Sensory Neurons

Have you ever noticed how you stop feeling your clothes against your skin after wearing them for a while? This is due to sensory adaptation - a process where sensory neurons reduce their response to a constant stimulus over time.

There are two types of sensory adaptation:

Rapid adaptation: Neurons respond strongly at first but quickly reduce their firing rate (like when you first get into a hot bath)
Slow adaptation: Neurons maintain a steady response over longer periods (like those that monitor body position)

Adaptation is incredibly useful - it allows us to filter out background information and focus on new or changing stimuli that might be more important.

Disorders Affecting Sensory Neurons

When sensory neurons don't work properly, various conditions can result:

🩸 Peripheral Neuropathy

Damage to sensory neurons can cause symptoms like tingling, numbness, or pain. This can be caused by:

Diabetes
Vitamin deficiencies
Certain medications
Physical injuries
Autoimmune conditions

🔬 Sensory Processing Disorders

Sometimes the problem isn't with the sensory neurons themselves, but with how the brain processes their signals. This can lead to:

Hypersensitivity to stimuli (finding normal sounds, lights, or textures overwhelming)
Hyposensitivity (reduced response to stimuli)
Difficulty integrating information from multiple senses

The Importance of Sensory Neurons

Sensory neurons are essential for our survival and quality of life. They allow us to:

Detect dangers in our environment
Maintain balance and coordination
Enjoy pleasurable sensations like tasty food or a gentle touch
Monitor our internal body state to maintain homeostasis
Communicate and interact with others

Without properly functioning sensory neurons, we would be unable to experience the world around us or respond appropriately to environmental changes. The next time you feel the warmth of the sun on your skin or smell your favourite food cooking, remember to thank your sensory neurons for making these experiences possible!

🧠 Test Your Knowledge!