Lesson 2: Neuron Structure

Question 1

The Neuron

Answer 1

The structural and functional unit of the nervous system is the neuron.

The CNS and the PNS are both composed of a series of
interconnected neurons.

The PNS is made up of nerves, which are numerous neurons held together by connective tissue.

The CNS is made up of 90% of all the neurons in the body.

Question 2

Neuron Structure

Answer 2

A neuron consists of 3 parts:

1. Cell body
2. Dendrites
3. Axon

Question 3

The Cell Body

Answer 3

The cell body contains a large, centrally located nucleus with a large nucleolus.

The cytoplasm contains numerous mitochondria for energy production.

Neurons are capable of surviving for over 100 years since most do not undergo cell division after adolescence.

Question 4

Dendrites

Answer 4

Dendrites are the primary site for receiving signals from other neurons.

A neuron may have one or thousands of dendrites depending on neuron function.

Question 5

Axon

Answer 5

The axon is a long cylindrical extension of the cell body that can range from 1mm to 1m in length.

When the neuron receives a sufficiently strong stimulus, the axon transmits impulses or waves of depolarization along its length.

At the end of the axon are specialized structures that release chemicals (neurotransmitters) that stimulate neighbouring
neurons.

The axon is covered by a fatty layer called the myelin sheath, which helps to speed the transmission of the impulses along the axon.

The myelin sheath is formed by Schwann cells which line up along the length of the axon.

Schwann cells deposit a layer of phospholipid bilayer every time they grow.

Once the Schwann cell has stopped growing, there may be 20 layers.

Between each Schwann cell is a gap called the node of Ranvier, where the membrane of the axon is exposed.

Question 6

Speed of Signal Conduction

Answer 6

A simple neuron without myelination can carry a signal at a speed of ~ 1 m/s.

Myelination and the node of Ranvier allow for saltatory
conduction which can carry a signal at a speed of ~ 100 m/s.

Saltatory conduction occurs when the signal jumps from node to node.

Question 7

Types of Neurons

Answer 7

Neurons can be separated into 3 classes:

1. Sensory neurons - take information from a sensory receptor to the CNS.
2. Motor neurons - take information from the CNS to an effector (muscle or gland)
3. Interneurons - exchange
   information between neurons (found in the CNS).

Question 8

Nerves

Answer 8

A bundle of nerve fibres enclosed in a protective
sheath.

Vary in size based on the number of nerve fibres
and the what proportion is myelinated. The largest
is the sciatic nerve.

Most contain both sensory and motor neurons.
Some specialized nerves will only contain sensory
(optic nerve) or motor (oculomotor nerve)