A2 Biology: Nervous system + coordination

• The ability of living things to respond to changed in the environment is known as irritability or sensitivity. 
• A change in energy levels in the environment which causes a response by an organsim is called a stimulus.

Receptors - detects the stimulus. E.g. eyes, ears and muscle stretch receptors.

Effectors - brings about the response. E.g. muscles and glands.

Coordinating system - to link together the receptors and effectors

The central nervous system (CNS) - made up of brain and spinal cord.

The peripheral nerves - Made up of the nerves which link the CNS to the receptor and effectors.

Receptors - specialised cells or organs they detect change in our surrounding. They are energy transducers that convert from one form of energy into another. Each is adapted to detect changes in a particular form of energy.

Receptor cells

• These trigger action potentials
• They convert energy from one form into electrical impulses in a neurone

Nerve cells

• The nervous system is made up of millions of nerve cells.
• The entire nervous system consists of two main types: neurones and neuroglia.

Neurones + Neuroglia

• Neurones are cells which are adapted to carry nerve impulses
• Neurones show considerable variation in size and shape but have the same basic structure
• Neuroglia which include Schwann cells, are cells which provide structural and metabolic support to neurones. 

Each neurones consists of:

• a cell body - containing the nucleus surrounded by granular cytoplasm
• cytoplasmic processes which branch from the cell body - a sing axon and one or more dendrites.
• The axon conducts impulses away from the cell body either to other neurones or to effectors, such as muscles.
• dendrites are highly branched processes which carry impulses from specialised receptors or from adjacent neurones with which they form synapses

Synaptic knobs

• The axon ends in synaptic knobs, these are where the neurone meets other nerve
• Large numbers of mitochondria and vesicles containing transmitter substances

Types of Neurones

• Motor neurones carry an action potential from the CNS to an effector such as a muscle or gland.
• sensory neurones carry the action potential from a sensory receptor to the CNS.
• relay neurons connect sensory neurones and motor neurones.

Myelinated neurones

• In the mammalian peripheral nervous system, specialised Schwann cells surround most axons. this process encloses the axon in a spiral layer of Schwann cell membrane. The overlapping pushes the nucleus and cytoplasm to the outside layer
• The covering formed by Schwann cell i referred to as the myelin sheath and axons which are covered in this way are myelinated.
• Between adjacent Schwann cells, there are short gaps where the axon is not covered by myelin. These gaps are known as the nodes of Ranvier and are at intervals of 1-3 mm along the axon.
• Around one third of neurones of the peripheral nervous system are myelinated.
• Neurones have specialised channel proteins in their cell surface membranes. They are specific to either Na+ or K+. They also possess a gate that can open or close the channel.

• They are known as voltage gated channel proteins. when open, the permeability of that membrane to the specific ion is increased; when closed the permeability is reduced. The channels are usually kept closed.

•  The myelin sheath is an insulating layer of fatty material. Sodium and potassium ions cannot diffuse through this layer.

• The larger the diameter of the axon the faster the speed it can conduct.
• In vertebrates the myelin sheath surrounding the axon increases the transmission speed
• when an action potential arrives at one node of Ranvier it sets up an electric current between this node and the next node.
• Nodes of Ranvier: The small uncovered areas of the axon between the Schwann cells Impulses jump from node to node.

Saltatory conduction

• Action potential appears to jump from one node to the next.
• This type of conduction is called salutatory conduction and results in the speeding up f the transmission of impulses They carry signals over long distances, the longest neurone in the human body is about 1m in length. 

Sodium Potassium pump

• They also have carrier proteins that  are able to actively transport Na+ out of the cell and K+ into the cell. For every 3 Na+ that goes out 2K+ goes in.
• This is called Na-K pump and it requires ATP.
• These maintain the potential difference cross the membrane, the membrane is polarised. 

Polarised

• A polarised membrane is one that has a potential difference across it
• It is the resting potential when there is no impulse

A nerve impulse

• A nerve impulse is created by altering the permeability of the nerve cell membrane to these ions.
• The movement of ions creates a change in potential difference (charge_ across the membrane. This is called depolarisation.

Synapses

• There are small gaps bettween neurones when they meet (approx 20 nm)
• These gaps are called synaptic clefts
• The gaps of the two ends of the neurones near it make up a synapes
• The signals is passed on by a chemical
• Action potential arriving causes release of a transmitter sbstance into the cleft
• Transmitter substace diffuses across the cleft
• Set up action potential in the membrane of the second neurone

1. An action potential arrives
2. The membrane depolarises. Ca2+ channels open and Ca2+ enter the neurones
3. Ca2+ causes synaptic vesicles containing neurotransmitter to fuse with pre synaptic membrane
4. Neurotransmitter i released into the synaptic cleft
5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Na+ flow through the channels.
6. The membrane depolarises and initiated an action potential
7. When released the neurotransmitter will be taken up across the pre synaptic membrane or it can diffuse away and be broke down.

• action potential reaches the synaptic knob
• voltage gated calcium ion channels in its membrane open to allow calcium ions diffuse into the knob
• Calcium ions cause synaptic vesicles, which contain a neurtransmitter to move to and fuse with presynaptic membrane
• Acetylcholin molecules are released by excocytosis and diffuse across the synaptic cleft
• —Acetylcholine molecules (have a specific shape to the receptor molecules) bind to receptor sites onthe sodium ion channels in the post-synaptic membrane, causing them to open.—
• Sodium ions diffuse in through the open channels in the post-synaptic membrane creating a generator potential or excitatory postsynaptic potential 
• —This depolarises the membrane. If sufficient generator potentials combine, then the potential across the post-synaptic membrane reaches the threshold potential and a new action potential is created in the post-synaptic cell.

Action potential

• When a neurone is not conducting an impulse it is said to be in the resting state
• The inside of the axon membrane has a negative charge relative to the outside
• This potential difference is called the resting potential and the membrane is polarised

Resting potential

• Na - positively charged Na+ are about ten times more concentrated on the outside of the membrane than they are inside of it.
• This is due to the activity of a sodium pump.
• Three sodium ions move for every two potassium ions that move in

K+

• K+ become more concentrated on the inside of the membrane
• but because the membrane is more permeable to K+ than Na+ some may move back out
• these are actively pumped back into the axon
• those that remain do so because they will be attracted back in the overall negative charge inside.

• So the differences in the concentrations of ions are due to the permeability of the axon membrane to these ions.
• in the resting state the permeability of the membrane to potassium is relatively high due to the presence of protein channels or gates in the membrane which allow K+ to pass through
• however there are no gates which allow the negatively charged organic ions to pass through so these remain trapped on the inside

• As a result the interior of the cell is maintained at a negative potential compared to the outside
• The membrane is said to be polarised and the potential difference across the cell surface membrane is about -60 mV (resting potential)

1. Resting potential: Na+ and K+ special voltage gated ion channels are closed and the  neurone is polarised  = negative inside
2. Some Na+ channels open (due to change in shape of protein itself makes protein more permeable to Na+) which allows some Na+ to diffuse into the neurone down the diffusion gradient.
3. Causing the inside of the neurone to become less negative: the membrane depolarises it reaches a threshold value of -50mV
4. Voltage gated sodium channels open and many sodium move in. The cell becomes positively charge inside compared to outside. Potential difference across the membrane is now +40 mV
5. Na+ channels now close and K+ gated channels now open
6. K+ diffuse out of the neurone down the electrochemical gradient so making the inside of the neurone less positive (more negative) again. The neurone is repolarised
7. So many K+ leaves axon that potential difference becomes even more negative than normal resting potential = hyperpolarised
8. the original potential differences is restored and the cell returns to its resting state

All or Nothing

Generator potentials in the sensory receptor are depolarisations of the cell membrane. A small polarisation will have no affect on the voltage gated channels unless the depolarisation is large enough to reach threshold potential it will open some nearby by voltage gated channels. This causes a large influx of Na+ and depolarisation reaches +40mV which is an action potential. Once this value is reached the neurone will transmit an action potential because many voltage gated sodium ions channels open. The action potential is self perpetuating once it starts are one point in a neurone it will continue along to the end of the neurone. This action potential does not vary in size nor intensity.

*Threshold point

1. Stimulus
2. Depolarisation
3. Action potential
4. Repolarisation
5. Refractory period - After an action potential the Na and K are in the wrong places. The Na+ and K+ ions which have diffused in/out of the cell are moved by actve transport (sodium-potassium pump) Until the Na+ and K+ are in their original positions no new action potential can be sent.
6. Resting potential

Voltage gated ion channels

• The gates on the channels further along the neurone membrane are operated y changes in the voltage across the membrane
• The movement of sodium ions along the neurone alters the potential difference cross the membrane
• When the potential difference cross the membrane is reduce the gates open
• This allows sodium ions to enter the neurone at a point further along the membrane, the action potential ha moved along the neurone.