8.2 How the nervous system works.
N S made up of interconnected neurones (nerve cells). They carry impulses from special receoptor cells, giving info about internal and external environment. Neurones also carry impulses to specialised effector cells.
Groups of receptors have evolved to work in sensory organs (i.e the eye)
Some neurones only carry info from internal or external environment into central processing areas of N S - known as sensory neurones.
Central nervous stystem, C N S, area where incoming info is processed, and where impulses are sent through motor neurones which carry impulses to effector organs. - in vertebrates the C N S consists of brain and spinal cord.
Neurones - each has long nerve fibe which carries impulse. Nerves are bundles of nerve fibres. Some carry only motor fibres - motor nerves. Some carry only sensory fibres - sensory nerves. Others carry a mix - mixed nerves.
Nerves have: cell body containing - nucleus, mitochondria, other organelles, and Nissl's granules which are rough endoplasmic reticulum and ribosomes needed for synthesis of neurotransmitter molecules.
Cell body has dendrites which connet to neighbouring nerve cells.
Nerve fibre that is extremely long and thin and carries the nerve impulse.
Fibres that carry impulses away from nerve cell body -called Axons. Fibres which transmit impulses towards cell body (in sensory neurones) - called Dendrons.
Short relay / connector neurones are in C N S and connect motor and sensory neurones. Known as bipolar neurones (because 2 fibres leave same cell body).
Myelinated nerve fibres.
Vertebrate neurones are associated with Schwann cell.
The Schwann cell membrance wraps repeatedly around nerve fibre. Forms fatty layer called myelin sheath. Gaps between Schwann cells called nodes of Ranvier. Myelin sheath protects nerves, and speeds up nerve impulse.
Speedy nerve impulses.
Speed impulse carried depends on:
Diameter of nerve fibre. thicker - rapid.
Presence or absence of a myelin sheath. Myelinated - faster than unmyelinated.
Invertebrates nerve impulses quite slow. Have no myelin sheaths and thin.
But need to react quick to danger. Many have envolved giant axons - which are nerve fibres around 1 milimeter. Travel at 100 mili seconds.
Vertebrates have myelinated and unmyelinated nerves.
Voluntary motor neurones that transmit impulses to voluntary muscles are myelinated. Autonomic neurones that control involuntary muscles have some unmyelinated fibres.
Myelin sheath speeds up impulse.
Investigating nerve impulses.
Some nerve impulses are electrical events. Record and measure the electrical changes. Pair of electrodes placed on nerve given a controlled stimulus. Impulses which result recorded by electrodes and shown on screen.
External electrodes record responses of entire nerves, made of lots of different nerve fibres. Very in thickness and sensitivity, so results difficult to interpret.
To be more correct, recordings made within individual nerve fibres. Using axons.
Sensory nerve fibres run from sense organ in head to brain, or from sensory receptors in skin to spinal cord, making them hard to get at.
Motor axons run directly to muscles, in large motor nerves. Makes them easy to access. Effect of stumulating them seen immediately with twitch of muscle.
Axons are extremely small so recording from inside is hard.
Axons in invertebrates (that are around 1 mili meter) very rapid impulse.
Work on these by Hodgkin and Huxley. Microelectrodes inserted into giant axon. Another electrode recorded the electrical potential from outside.
Allowed changes that occur in passage of nerve impulse to be recorded accurately. This technique has now been refined so internal electrodes used with almost any nerve fibre.
8.2 Nerve Impulses.
Concentration of sodium, potassium and other charged particles is different outside axon than in.
Membrane of axon - partially permeable. Difference in permeability to positively charged sodium and potassium ions gives special conducting properties.
Axon is 'at rest' when not conducting impulse. At rest, axon membrane is relatively impermeable to sodium ions, but freely permeable to potassium ions.
Contains very active sodium / potassium ion pump. Uses A T P to move sodium ions out of axon and potassium ions in. This lowers concentration of sodium ions inside axon. Pumped out and cannot diffuse back. Potassium ions are moved in, but diffuse out again along concentration gradient through open potassium ion channels. Eventually the movement of positively charged potassium ions out of cells along conc gradient is opposed by electrochemical gradient.
As results, inside of cell is left slightly negative relative to outside - Polarised.
Potential difference across membrane of around minus 70 m V. known as Resting potential.
Impulse along axon, change in permability of cell membrane to sodium ions that occurs in response to stimulus.
Neurone stimulated, axon membrane shows sudden increase in permeability to sodium ions. Specific sodium ion channels / gates open. Allow sodium ions to diffuse rapidly down concentration and electrochemical gradients.
Potential difference across membrane briefly reversed.
Cell becomes positive on inside. Depolarisation. lasts about 1 millisecond. The potential difference across membrance is about + 40 m V. Known as action potential.
After this depolarisation, sodium ion channels close. Excess sodium ions pumped out by sodium pump. Active transport uses A T P. Permeability of membrane to potassium ions is temporarily increased as voltage-dependent potassium ion channels open.
Potassium ions diffuse out of axon, down concentration and electrochemical gradient. Attracted by negative charge outside of membrane. As positive sodium and potassium ions leave cell, inside becomes negative relative to out once again. Takes a few milliseconds before resting potential is restored.
Threshold of any nerve fibre - the point at which sufficient sodium ion channels open for rush of sodium ions into axon to be greater than outflow of potassium ions. Threshold reached - action potential occurs. All or nothing response.
- recovery time of an axon. time it takes for area of axon membrane to recover after an action potential. Time it takes for ionic movements to repolarise membrane and restore resting potential.
Depends both on sodium/ potassium pump and on membrane permeability to potassium ions.
Impossible to restimulate fibre after action potential. Sodium ion channels are completely blocked and resting potential not restored. - known as absolute refractory period.
After this, period when axon may be restiumulated, but will only respond to stronger stiumlus than before. The threshold has effectively been raised. - known as relative refractory period.
During this time, voltage-dependent potassium ion channels still open. Not until they close that normal resting potential can be restored.
Refractory period limits rate impulses flow along fibre. Ensures impulses flow in one direction along nerves.
Until resting potential is restored, part of nerve fibre that impulse has just left cannot conduct another impulse. Impulse can only continue travelling in same direction.