Communication and Homeostasis

Part 2

Sensory Receptors

Receptors Energy Change Detected

Rods and Cones (in the retina) Light intensity and range of wavelengths

Olfactory Cells (line inner surface of nasal cavity)

Presence of volatile chemicals

Taste Buds (tongue), hard palate, epiglottis, first part of oesophagus

Presence of soluble chemicals

Pacinian Corpuscles (skin) Pressure on skin

Cochlea (inner ear) Vibrations in air

Proprioceptors (muscle spindles) Length of muscle fibres

Sensory receptors are specialised cells that can detect changes in our environment. They are energy transducers and each type is adapted to detect changes in a particular form of energy. They change this energy to a nerve impulse (electrical energy)

Generating a Nerve Impulse

A nerve impulse is created by altering the permeability of the nerve cell membrane. At resting potential, the cell membrane is polarised. As the sodium ion channels open the membrane permeability is increased and sodium ions can move across the membrane, down the concentration gradient into the cell. This movement creates a potential difference across the membrane, so the inside of the cell becomes less negative than the outside. This is depolarisation.

Motor Neurone

Sensory Neurone

How are they able to transmit the action potential?

• The Cell Body consists of a nucleus, mitochondria and ribosomes

• They maintain a potential difference across the plasma membrane

• They are long and so transmit the action potential over a long distance

• Both have dendrites connected to other neurones

• The myelin sheath insulates the neurone from the electrical activity of nearby cells

• The plasma membrane has many gated ion channls to control the entry and exit of sodium, potassium and calcium ions

• Sodium and potassium ion pumps (using ATP) actively transport sodium ions out, and potassium ions in.

• A: At rest; both the sodium and potassium ion channels are closed. The outside is positive compared to the inside.

• B: The rising phase. This is depolarisation (threshold value of -50mV) . The sodium ion channels open and some sodium ions diffuse into the cell. Eventually, voltage gated sodium ion channels open and many sodium ions flood in. The inside becomes positive compared to the outside. The potential difference reached +40mV.

• C: The falling phase (repolarisation). Sodium ion channels close and potassium ion channels open. Potassium ions diffuse out of the cell so the inside is once again negative compared to the outside.

• D: The refractory period (Undershoot/Hyperpolarisation). The time after an action potential it is possible to stimulate the cell membrane to reach another action potential. This period allows the cell to recover and for the action potential to be transmitted in one direction.

• E: the original potential difference is restored so the cell returns to resting state.

Local Currents (movements of ions along the neurone)

1. An action potential triggers sodium ion channels to open so sodium ions diffuse across the membrane

2. This movement upsets the balance of ionic concentrations

3. The concentration of sodium ions inside the neurone rises at the point where the sodium ions are open

4. Sodium ions diffuse sideways away from the region of increased concentration

• The myelin sheath is an insulating layer of fatty material, created by a series of Schwann Cells, which ions can’t diffuse through. The gaps in the myelin sheath are gaps between the Schwann Cells and are called nodes of Ranvier.

• In myelinated neurones, the action potential ‘jumps’ from one node of Ranvier to the next. This is Saltatory Conduction. An advantage of this is that it speeds up the transmission of the action potential.

• In non-myelinated neurones, there is no saltatory conduction. Several neurones are enshrouded in one loosely wrapped Schwann Cell so the action potential moves along the neurone in a wave.

Advantages of myelination

• Transmit an action potential much quicker

• Carry signals over a long distance

(combined, this means a quicker response to a stimulus)

Nerve Junctions

A synapse is a junction between two or more neurones. It is where a one neurone can communicate with, or signal to, another neurone. Between the two neurones is a small gap called the synaptic cleft (20nm). An action potential is produced by the movement of ions across the neurone membrane; it can’t bridge the gap between the two neurones. So, the presynaptic action potential causes the release of the transmitter substance which diffuses across the gap and generates a new action potential in the postsynaptic neurone.

Synaptic Knob

This is a swelling at the end of the presynaptic neurone.

It has specialised features…

• Many mitochondria

• Lots of SER

• Vesicles of acetylcholine (transmitter substance)

• Voltage-gates calcium ion channels in the membrane

The postsynaptic membrane contains specialised sodium ion channels that can respond to the transmitter substance. They consist of 5 polypeptide molecules; 2 have a special receptor site specific to acetylcholine so it can fit and bind to the site. When it binds to the 2 receptors, the sodium ion channels open.

The transmission of a signal across a synaptic cleft.

1. An action potential arrives at the synaptic knob

2. Voltage-gated calcium ion channels open

3. Calcium ions diffuse into the synaptic knob

4. They cause the vesicles to move to and fuse with the membrane

5. Exocytosis releases acetylcholine

6. Acetylcholine molecules diffuse across the cleft

7. Acetylcholine molecules bind to receptor sites on sodium ion channels

8. Sodium ion channels open

9. Sodium ions diffuse across the membrane into the neurone

10. An excitatory postsynaptic potential generator is created

11. If sufficient generator potentials combine, then the potential across the postsynaptic membrane reaches the threshold potential

12. A new action potential is created in the postsynaptic neurone

Acetylcholinesterase

This is found in the cleft. It hydrolyses the acetylcholine to ethanoic acid and choline. This stops the transmission of signals so the synapse won’t produce ATP. The acid and choline are recycled and re-enter the knob by diffusion. They are recombined to acetylcholine using ATP from respiration in the mitochondria.

An action potential is an all or nothing response. Once the action potential starts, a neurone will conduct it all along its length. At the end of the neurone, the presynaptic knob releases a transmitter substance into the synaptic cleft. The signal sent to the next neurone is an acetylcholine molecule.

The role of synapses• Filter out unwanted low-level signals

• Connect two neurones together so a signal can be passed

• Several presynaptic neurones might converge to one postsynaptic neurone. Signals from different parts of the nervous system can create the same response

• The creation of specific pathways within the nervous system is thought to be the basis of conscious thought and memory

• One presynaptic neurone might diverge to several postsynaptic. One signal can be transmitted to several parts of the nervous system

• Low-level signals can be amplified by summation. Summation is the interaction of many small potentials to create one large potential that may pass the threshold and create an action potential

• Ensure signals are transmitted in the right direction

• Acclimatisation: after repeated stimulation of a stimulus, vesicles may run out which contain transmitter substances. Therefore, overstimulation of an effector is avoided (this is how we get used to a smell for example)

Frequency

When a stimulus is at high intensity, the sensory receptor will produce more generator potentials. This will cause more frequent action potentials in the sensory neurone. When these arrive at the synapse, more vesicles will be released, creating a higher frequency of action potentials in the postsynaptic neurone.

So, the more intense the stimulus, the higher the frequency of action potentials.