6.2 Nervous coordination Flashcards
What is the nervous system?
- It uses nerve cells to pass electrical impulses along their length.
- They stimulate target cells by secreting neurotransmitters directly on to them.
- Resulting in rapid communication between specific parts of an organism.
- Responses are short-lived and restricted to a localised region.
What is the hormonal system?
- It produces chemicals that are transported in the blood plasma to their target cells.
- The target cells have specific receptors on their cell-surface membranes and the change in concentration of hormones stimulates them.
- Resulting in slower, less specific communication.
- Responses are long lasting and widespread.
What are the characteristics of the hormonal system?
- Transmission by blood stream, and relatively slow.
- Hormones travel to all parts of the body, but only target cells respond.
- Response is widespread, slow and long-lasting.
- Effects may be permanent and irreversible.
What are the characteristics of the nervous system?
- Transmission is by neurones and very rapid.
- Nerve impulses travel to specific parts of the body.
- Response is localised, rapid and short-lived.
- Effect is usually temporary and reversible.
What are neurones?
- Specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another.
- A motor neurone is made of a cell body, dendrons, an axon, Schwann cells, a myelin sheath and nodes of Ranvier.
What is a cell body?
- Contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum.
The RER is associated with the production of proteins and neurotransmitters.
What are dendrons?
Extensions of the cell body which subdivide into smaller branched fibres, dendrites, that carry nerve impulse towards the cell body.
What is an axon?
a single long fibre that carries nerve impulses away from the cell body
What is a Schwann cell?
- surround the axon, protecting it and providing electrical insulation.
- They also carry out phagocytosis and involved in nerve regeneration.
- They wrap themselves around the axon so that layers of membrane build up around it.
Name 3 processes which Schwann cells are involved in.
- electrical insulation
- phagocytosis
- nerve regeneration
What is a myelin sheath?
- It forms a covering to the axon and is made up of the membranes of Schwann cells.
- These membranes are rich in the lipid myelin.
- Neurones with a myelin sheath are myelinated neurones.
What are nodes of Ranvier?
- Constrictions between adjacent Schwann cells where there is no myelin sheath.
- The constrictions are 2-3 micro metres long, and occur ever 1-3mm in humans.
What are sensory neurones?
- They transmit nerve impulses from a receptor to an intermediate or motor neurone.
- They have one dendron that is often very long.
- It carries the impulse towards the cell body and one axon that carries it away from the cell body.
What are motor neurones?
They transmit nerve impulses from an intermediate or relay neurone to an effector.
They have a long axon and many short dendrites.
What are intermediate/relay neurones?
They transmit impulses between neurones.
They have numerous short processes.
How does an action potential pass along an unmyelinated neuron?
- stimulus leads to influx of Na+ ions. First section of membrane depolarises
- Local electrical currents cause sodium voltage-gated channels further along membrane to open, meanwhile the section behind begins to repolarise
- Sequential wave of depolarisation
Why does myelinated axons conduct impulses faster than unmyelinated axons?
Saltatory conduction - impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length
What are nerve impulses?
- It is a self-propagating wave of electrical activity that travels along the axon membrane.
- It is a temporary reversal of the electrical potential difference across the axon membrane.
- This reversal is between resting potential and action potential.
How is the movement of ions across the axon membrane controlled?
- The phospholipid bilayer prevents sodium and potassium ions diffusing across it.
- Channel proteins in the bilayer have ion channels which pass through them. Gates open and close so that these ions can move through them by facilitated diffusion at times.
- Some channels remain open all the time
- Some carrier proteins actively transport K ions in and Na ions out - the sodium-potassium pump.
What is resting potential?
- The potential difference across neuron membrane when not stimulated
- This control of ions mean the inside of the axon is negatively charged relative to the outside.
- Resting potential ranges from 50 to 90 millivolts, but is 65mV in humans.
- The axon is polarised.
How is resting potential established?
- membrane is more permeable to K+ than Na+
- Sodium-potassium pump actively transports 3Na+ out of a cell and 2K+ into cell
Establishes electrochemical gradient: cell contents more negative than extracellular environment
How is the potential difference of the axon established?
Sodium ions are actively transported out by the sodium-potassium pumps.
Potassium ions are actively transported in by the pumps.
3 sodium ions move in for every 2 potassium ions out.
So there’s more sodium ions in the tissue fluid surrounding the axon that in the cytoplasm, and more potassium in the cytoplasm that the tissue fluid, creating an electrochemical gradient.
The sodium ions diffuse back in naturally while potassium diffused back out.
Most of the channels for potassium are open, but sodium are closed.
