Week 1: Cells of the nervous system

Question 1

What are the two main types of cells in the nervous system?

Answer 1

Neurons & Glial cells (non-neural)

Question 2

What are neurons?

Answer 2

Excitable cells that transmit electrical signals

Question 3

What are non-neural/Glial cells?

Answer 3

These cells surround neurons. They work to support, nourish and insulate neurons and also remove waste products within and outside the neurons including after neural and synaptic transmission.

Account for over half of the brain’s weight.

Question 4

How many neurons and glial cells does the brain contain?

Answer 4

100 billion neurons
40-130 billion glial cells

Question 5

How many neurons can one single neuron be connected with?

And how many synapses can exist between these?

Answer 5

Each may be connected with up to 10,000 other neurons.

They pass signals to each other via as many as 1,000 trillion synapses.

Question 6

What are the 3 fundamental parts to a neuron?

Answer 6

Dendrites
Soma
Axon

Question 7

What are dendrites?

Answer 7

Short branched extensions of the soma which receive input from another nerve cell and conduct these electrical signals towards the soma.

Question 8

What is the soma?

Answer 8

Also known as the cell-body it contains the nucleus and is the primary site of protein synthesis in the neuron.

It contains all the organelles necessary for the survival of the neuron. The soma contains the axon hillock which is the part of the soma from which the axon originates. This area contains a lot of voltage gated iron channels – site where action potentials originate from.

Question 9

What clusters of soma known as in the CNS and PNS?

Answer 9

Clusters of soma in the CNS are called nuclei and in the peripheral nervous system are called ganglia.

Question 10

Role of the myelin sheath?

Answer 10

A white fatty protein based substance that surrounds the nerve cells. This myelin sheath allows electrical impulses to transmit quickly and efficiently along the nerve cells - this is because there are small gaps in the myelin called nodes of ranvier.

Question 11

Role of the nodes of ranvier?

Answer 11

The gaps between the myelin. The nodes of Ranvier allow the generation of a fast electrical impulse along the axon. This rapid rate of conduction is called saltatory conduction.

Question 12

What is action potential?

Answer 12

Generation of an electrical signal down the axon.

Question 13

How are neurons classified?

Answer 13

Unipolar, bi-polar or multi-polar.

Question 14

True unipolar neurons aren’t found in the adult nervous system. True or False?

Answer 14

True

Question 15

What neurons make up the peripheral nervous system?

Answer 15

Bipolar neurons and a variant pseduounipolar neurons make up all the primary sensory neurons of the peripheral nervous system.

Question 16

What are the two branches of bipolar neurons and pseduo-unipolar neurons?

Answer 16

Bipolar = axon & dendrite
Pseduo-unipolar = one branch travels peripherally and the other to the CNS. One end often has a receptor.

Question 17

What is a multi-polar neuron

Answer 17

These neurons have many variably branched processes extending in many directions. This is the most common type of neuron in the human nervous system. Multipolar neurons have one axon and many dendrites which allows them to communicate with many other neurons.

Question 18

What are the two types of axons?

Answer 18

Myelinated axons and unmyelinated axons

Question 19

What specialised cells create myelin?

Answer 19

Oligodendrocytes

Question 20

What is saltatory conduction?

Answer 20

The jumping of electrical signals

Question 21

Unmyelinated axons are slower, shorter and appear grey in colour. True or False?

Answer 21

True

Question 22

How are electrical signals propagated in myelinated vs unmyelinated axons?

Answer 22

To propagate the electrical signals the ion channels need to open and close all the way down the axon in the unmyelinated axon. But for the myelinated axon, ion channels only need to open and close at the nodes of Ranvier to propagate the electrical signal down the axon.

This is the reason why myelinated axons are a lot faster and retain the nerve impulse down the nerve. The unmyelinated nerve is a lot slower in conducting the electrical impulses and can lose strength down the nerve.

Question 23

What neurons are required for a basic reflex arc? And what is the process?

Answer 23

Sensory neuron (Pseduo-unipolar)
Interneuron
Motor neuro

The signal travels along the sensory neuron and enters the spinal cord, the axon terminals synapses with the dendrites of the multipolar interneuron and travels along the axon of the multipolar interneuron to the axon terminals which synapses with the dendrite and subsequent soma of the multipolar multi neuron. The motor neuron receives the signal and an actional potential travels down the motor neuron to the axon terminals which synapse at the neuromuscular junction which has the end result of a muscle contraction.

Question 24

Where are all the soma congregated in a reflex arc?

Answer 24

Dorsal root ganglia

Question 25

What elements do glial cells lack?

Answer 25

Axons & dendrites

Question 25

Explain the patella reflex…

Answer 25

With the quadriceps we have stretch receptors which are also known as muscle spindles. When the muscles stretch this receptor is activated. If we use a tendon hammer to hit the patellar ligament the stretch stimulus will go through the patellar ligament, through the patellar tendon to the quadriceps muscle where the stretch will be registered by the muscle spindle. The muscle spindle registers the stretch and the receptor converts the signal to an action potential which propagates down the sensory neuron (shown in yellow), goes through the dorsal root ganglion to enter the spinal cord with axons terminating and synapsing with the motor neuron and the interneuron within the spinal cord. In this instance the axon divides into two parts or it bifurcates. One part synapses with the motor neuron of the quadriceps and the other part synapses on an inhibitory neuron which subsequently synapses on the motor neuron of the hamstrings.

In this case the excitatory sensory neuron synapses onto an inhibitory neuron. Therefore the excitatory sensory neuron excites an inhibitory interneuron and this inhibitory neuron in turn inhibits the motor neuron of the hamstrings meaning no action potentials are sent through the motor nerve to the hamstrings meaning they don’t contract or in other words they relax. The signal in the quadriceps in this reflex arc is monosynaptic as the signal travels along the sensory neuron has one synapse, which is the one directly synapsing on the motor nerve of the quadriceps. The signal to the hamstrings is disynaptic as the signal is travelling along the sensory neuron, synapses and excites an inhibitory neuron which subsequently synapses and inhibits the motor neuron to the hamstrings. As there are two synapses, it’s called disynaptic. The net effect of this whole reflex arc is that the leg extends at the knee.

Question 26

What are the four glial cells in the CNS?

Answer 26

Astrocytes
Oligodendrocytes
Ependymal cells
Microglia

Question 27

What are the two glial cells in the PNS

Answer 27

Satellite cells
Schwann cells

Question 28

What are astrocytes & satellite cells?

Answer 28

Provide physical and nutritional support for the neurons in the CNS.

Functions of astrocytes include cleaning out brain debris, transporting nutrients to neurons, holding neurons in blood vessels in place ie providing structural support, digesting parts of dead neurons, regulating content of the extracellular space, promoting synaptic connections, clearing excess neurotransmitters, and ensuring the continued function of neurons. They also guide the migration of young neurons.

Satellite cells provide similar support roles in the PNS (equivalent of an astrocyte)

Question 29

What is the role of micro-glia?

Answer 29

• Phagocytose and remove foreign or damaged material cells or organisms.
• Small, relatively sparse cells, and they act as the brains resident clean-up squat by phagocytising apoptotic cells, plaques and pathogens.
• They can also prune and reshape synapses.
• Particularly involved in monitoring the integrity of synaptic function

Question 30

What are schwann cells and oligodendrocytes?

Answer 30

• Schwann cells are present in the peripheral nervous system and oligodendrocytes are present in the central nervous system
• These cells are involved in the myelin production. Myelin is composed of layered phospho-lipid membranes and serves to support and insulate axons allowing for faster impulse transduction. Saltatory conduction occurs as the impulses jump across nodes of Ranvier.

Question 31

What is one key difference between schwann cells & oligodendrocytes?

Answer 31

One oligodendrocyte myelinates multiple axons, however only one Schwann cell myelinates one axon.

*Also oligodendrocyte = CNS & schwann = PNS.

Question 32

What do ependymal cells create?

Answer 32

Cerebrospinal fluid

Question 33

What do ependymal cells line?

Answer 33

The ventricles of the brain and the central canal of the spinal cord

Question 34

Role of ependymal cells?

Answer 34

• Depending on where they are located ependymal cells also help to distribute neurotransmitters & hormones associated with the CNS
• Also the microvilli of ependymal cells can influence the flow of cerebrospinal fluid and let certain substances in and out of the brain.

Question 35

Composition of ependymal cells?

Answer 35

These cells are cuboidal to coloma and have cilia and microvilli on their surface to circulate and absorb cerebrospinal fluid