Neurons are found through out the body. There are 3 main types of neurons, and we can divide the entire neural network into 2 parts (CNS & PNS)

Lets start with the neurons

Types of neurons

How do they look

Where are they found

How do they talk to each other (Synapse)

How do we use neurons (reflex)

Neurons

&

Nervous System

Neurons are cells, which transmit signals throughout the body.

Like all cells they have a nucleus, mitochondria, vesicles and other organelles.

There are 3 main types, each looking and functioning slightly differently.

1. Sensory Neurons

From the edge of your finger tips, to your nose, sensory neurons are everywhere.

At the top of these neurons are specialised receptors. We have sensory neurons for vision, hearing, smell, taste, pain, temperature and touch.

We can break these sensory neurons up into 4 main 4 main categories.  

1a Chemo-receptors

Chemo receptors are stimulated by chemicals from the external and internal environment.

External such as in the nose and mouth. Detecting smell and taste.

Internal such as Carbon dioxide levels in the blood (carotid bodies).

1b Mechano-receptors

When your body is stretched, compressed, distorted or vibrated, mechanoreceptors activate and detect this change.

Touch receptors (near the surface of the skin), Pressure receptors (deep in the skin), Balance receptor (sensitive to vibrations in the ear) Stretch receptors (how stretch muscles are).

1c Photo-receptors

Photoreceptors are located in your eyes. They detect changes in light intensity and colour.

There are 2 main types, Rods and Cones.

Rods detect the presence of light. They are 1000 times more sensitive than cones and much more abundant in the eye.

There are roughly 6 million Cones in the human eye, which detect colour. Red cones (60%), Green cones (30%) and Blue cones (10%)

1d Thermo-receptors

Thermo receptors detect changes in temperature. Located on the skin and in the brain the can detect changes in temperature externally and internally, respectively.

Within the brain changes as small as 0.01 degrees Celsius can be detected

2 Relay neurons

Relay neurons connect a sensory neuron to a motor neuron. They make up over 97% of the human nervous system.

Essentially they are like an extension cord, they don’t create a stimulus or response but they help to carry the stimulus between sensory and motor neurons.  

Some of these relay neurons are very long, while other can be small. The long neurons are myelinated, allowing them to pass messages quickly over long distances. Some short neurons, like those found in the brain, are un-myelinated.

Since the signal only travels a short distance, there is no need for myelination.

3 Motor neurons

Motor neurons carry the signal from the CNS to the effector (muscle or gland).

A motor neuron connects to a muscle or gland and stimulate it.

Try the quiz bellow

Short answer questions

1. What are 4 types of sensory neurons?

2. Explain how the Rods and Cones operate in the human eye?

3. What are 2 differences between sensory neurons and motor neurons?

4. How does Motor Neuron Disease (MND) affect the human nervous system (6 marks)

How do they look

If we look at our 3 types of neurons, you’ll notice they all have something's in commons

All neurons have Cell bodies (Soma), Nucleus, Axon, Axon endings and Dendrites.

Most neurons have myelin, wrapped around the axon.

Dendrites are short projections from the cell body. They receive and carry nerve impulses towards the cell body.

The Soma contains all the important parts of the cell. Nucleus, mitochondria and other organelles.

The axon is the is the long structure, stemming from the soma.

At the end of the axon, are the Nerve ending or axon terminals. Its here where neuro-transmitters are released.

Surrounding the axon is a fatty tissue called Myelin. It serves 2 main functions:

1. Speeds up impulses down the axon.

2. Prevents the impulse from jumping to adjacent nerve cells

In infants, myelin isn’t fully developed. This is why they have poor co-ordination and movement.

Now that we understand the structure and types of neurons in our body, how are they ordered in the body?

All the neurons in your body are connected to either, another neuron, muscle or gland. We can break up all these connections into 2 parts.

The Central Nervous System (CNS) Peripheral Nervous System (PNS).

The sequence of actions is as follows:

  1. Stimulus (flame)

  2. Receptor

  3. Sensory neuron

  4. CNS

  5. Motor neuron

  6. Effector (biceps)

  7. Response (move hand away)

The PNS includes all the neurons in your body, EXCEPT those in the spine and brain.

The PNS consists of motor and sensory neurons, which carry impulses too and from the CNS.

The CNS consists of the brain and spinal cord.

Try the quiz bellow

Short Answers Questions

1.

      Select the two most appropriate words from the list given in part Y and complete         the labels 1 and 2 in the spaces provided on the diagram.

2. Define the CNS and PNS

3. Draw a neural network. Include a sensory, relay and motor neuron.

4. Draw and label a sensory and relay neuron

5. Explain how myelin affects neurons

6. Why would some neurons not be myelinated

7.