A2 Biology - Topic 8 - All You Need To Know - Part One

This hub series will contain everything that you need to know for Topic 8 of SNAB Biology (A2 Edexcel).

Key words are highlighted and information is accompanied with illustrations and split into sections.

Remember that SNAB Biology requires you to be able to answer any question from any of the previous topics too - so be sure to see my other hubs and other resources to revise for those parts too.

This hub covers:
1. The Nervous System
2. Neurones
3. Reflex Arcs
4. Pupil Dilation
5. Axon Voltage

For part two: A2 Biology - Topic 8 - All You Need To Know - Part Two

Breakdown of the Nervous System
Breakdown of the Nervous System

The Nervous System

The nervous system contains the:

  1. Central Nervous System (CNS)
  2. Peripheral Nervous System (PSN)

The Peripheral Nervous System
Contains the:

  1. Autonomic Nervous System
  2. Somatic Nervous System

The Autonomic Nervous System
Contains the:

  1. Sympathetic Nervous System
  2. Parasympathetic Nervous System

Click for Full Size

Diagram of a Motor Neurone
Diagram of a Motor Neurone
Diagram of a Sensory Neurone
Diagram of a Sensory Neurone

Neurones

The 3 Different Types of Neurones


Motor Neurone

  1. Cell body always found in the CNS
  2. Carries impulses from the CNS to effector cells. (Effector cells = muscles and glands)
  3. Also known as "effector neurones"

Sensory Neurone

  1. Conduct impulses from sensory cells (e.g. photoreceptors in the eye) to the CNS.

Relay Neurone

  1. Have many connections to other neurones
  2. Also known as "interneurones" and "connector neurones"


An Overview of Neuronal Structure

An overview of the structural differences between neurones
An overview of the structural differences between neurones
Diagram to show what the myelin sheath is made up of.
Diagram to show what the myelin sheath is made up of.
Diagram to show Nodes of Ranvier
Diagram to show Nodes of Ranvier

The Myelin Sheath

The myelin sheath is a fatty layer of insulation around a neurone's axon.

Not all animals have 'myelinated' axons. Invertebrates do not have myelin sheaths and neither do certain vertebrates.

The myelin sheath is made out of lots of small units called 'Schwann cells'.

Between the Schwann cells are gaps called Nodes of Ranvier which serve to speed up the conduction of nerve impulses. This speeding up is caused by forcing the impulse to jump across the gaps due to voltage differences between them. This jumping is something known as saltatory propagation.

A Reflex Arc

A simple diagram to illustrate a reflex arc
A simple diagram to illustrate a reflex arc | Source

Reflex Arcs

A reflex arc is a simple nerve pathway involving just a few neurones. Reflex arcs are responsible for our reflexes - rapid involuntary reactions to certain stimuli. They are important as they allow for the speedy escape of potentially fatal events e.g. when you put your hand near fire or lean it on something sharp, your hand will retract from the danger involuntarily due to a reflex arc. Our reflexes are so fast that it may seem like you had moved before even realising you were in danger.

The following is the usual way in which a reflex arc works:

  1. Receptors detect stimulus and generate a nerve impulse.
  2. Sensory neurones conduct the nerve impulse to the CNS.
  3. The sensory neurone forms a synapse with a relay neurone to which it passes its nerve impulse along.
  4. The relay neurone forms a synapse with a motor neurone which passes the nerve impulse to the effector cells.
  5. The effector cells cause a response in an attempt to eliminate any further danger/stimulus.

It must also be noted that even in simple reflex arcs like the one above, there is usually the added complexity of some motor neurones being inhibited so that only the desired movement would occur (moving away from danger) and not a mixture of signals which could have resulted in an uncoordinated movement more towards the danger.

Pupil Dilation

The iris controls the size of the pupil.

It contains a pair of antagonistic muscles which are controlled by the autonomic system. These muscles are known as the "radial" and "circular" muscles.

  1. Radial muscles are controlled by the sympathetic reflex and make the pupils dilate
  2. Circular muscles are controlled by the parasympathetic reflex and make the pupils constrict.

The following is the way in which the eyes react to different levels of light:

  1. High light levels strike the photoreceptors located in the retina.
  2. A nerve impulse is created and is passed along to the CNS
  3. There, the impulse is passed around the coordinating cells in the midbrain.
  4. The impulses then pass along motor neurones to the circular muscles in the iris, causing them to contract.
  5. At the same time, the radial muscles relax
  6. This results in the constriction of the pupil, restricting the amount of light that can enter the eye.

Sodium - Potassium Pump
Sodium - Potassium Pump
Diagram to help illustrate the pumps allowing the potential difference in axons.
Diagram to help illustrate the pumps allowing the potential difference in axons.

Axon Voltage

An axon's inside has a voltage of -70 millivolts (mV).

Because the outside of an axon's membrane is not charged and the inside is at -70mV, the membrane of an axon has a potential difference across it. The membrane of an axon is therefore said to be polarised.

There is an uneven distribution of ions across the cell membrane of an axon than would be expected from just diffusion occurring. This means that the axon is controlling what concentration of ions it keeps inside of itself.

It does this via sodium - potassium pumps in its cell surface membrane.

Sodium - potassium pumps pump sodium out of the cell and potassium into it.

These pumps work against the concentration gradient found in the axon cell surface membrane, having more potassium in the cell than out of it, but still pumping more in (working against diffusion). This requires energy from the hydrolysis of ATP.

Additionally, substances called anions (e.g. negatively charged amino acids) are too large to leave the cell and so create an electrical gradient across the cell surface membrane. Because these are negatively charged substances, negative chloride ions leave the cell in order to balance (in this case trying to make the cell less negative) the charge now made across the cell surface membrane.

Once the electrical gradient has balanced out via the loss of chloride ions and no difference in charge is seen between the inside and outside of the cell, potassium leaves the axon via potassium channels due to the sodium-potassium concentration gradient created by the sodium-potassium pumps.

Since potassium ions are positive, the axon becomes more negative due to its loss of potassium. Because of this difference in charge between the inside and outside of the cell membrane, some potassium is drawn back into the axon to make it less negative again. However, this force is not as strong as the sodium-potassium gradient that has been created. This gives the cell its resting potential of -70mV.

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