Unit 3.1 The Nervous System Flashcards
How much does the nervous system weigh and how much of the body mass does it account for?
The nervous system weighs barely 2 kilograms and only accounts for around 3% of the body mass, yet is perhaps the most complex of all the body systems.
What does the nervous system use to coordinate the body and maintain homeostasis?
It uses electrical impulses.
What are the specialised cells which are responsible for direct communication between different groups of cells called?
Never cells, or neurones.
Neurones form a network of highly specific connections between different groups of cells in order to do what 3 things?
Gather information from sensory receptors.
Process information and create memory in the central nervous system (CN)
Stimulate effector cells (for example to cause voluntary movement, or to cause the release of secretions from a gland)
What organ is the most anatomically complex system in the body?
The brain.
What 2 ways does the brain function in?
As a coordination and control centre for many parts of the body (for example controlling the respiratory system)
And also as a self contained unit (for example in the development of memory)
Although the brain and CNS are highly complexing the way they function, which 5 types of cell are they made up of?
Neurones, Glial cells, microglial cells, connective tissue and blood vessels.
Neurones are specialised cells capable of developing electrical impulses and transmitting the impulses along the length of the cell. What do they all posess?
They possess a cell body, axons and dendrites (dendrons), myelin sheath, nodes of ranvier, synapse, synaptic knob.
How many neurones are there in the body?
There are between 10 and 20 billion neurones in the body.
What size do neurones range from?
Ranging in the size from a few thousandths of a millimetre (micron) to over a metre in length,
Why do neurones have a high level of metabolic activity?
Energy is needed to maintain neurones in a state of readiness for conducting impulses.
What do neurones contain many of?
They contain many mitochondria to supply the ATP they need for aerobic respiration.
How are electrical impulses in neurones generated?
By changes in the permeability of the cell membrane to different ions.
What can neurones be classified according to?
The direction in which they conduct impulses.
What do sensory neurones transmit?
Sensory neurones transmit nerve impulses from sensory cells towards the spinal cord and the brain, also known as afferent neurones.
What do motor neurones transmit?
Motor neurones transmit nerve impulses from the brain and the spinal cord towards the muscles and glandular organs, also known as efferent neurones.
What does the cell body of a neurone contain?
The contains the nucleus and the normal organelles associated with cell metabolism.
What are the axons of neurones?
These are fibres which conduct nerve impulse away from the cell body.
How many axons per neurone?
A neurone has a single axon, which may be as long as a metre in length.
What is at the end of an axon?
At the end of an axon there are hundreds or even thousands of branches that end at axon terminals called synaptic knobs.
What are dendrites?
These are the connecting fibres of each neurone and look like branching hair lie processes.
How many dendrites do each neurone have?
Each neurone has hundreds and sometimes many thousands of dendrites.
What does the extensive branching allow dendrites to do?
The extensive branching allow dendrites to communicate with many different neurones.
What is the function of the myelin sheath?
This is an insulating sheath around the axon which increases the speed at which an impulse is conducted.
What are the nodes of Ranvier?
The myelin sheath is interrupted at approximately one millimetre intervals by gaps known as the nodes of Ranvier.
What do the nodes of Ranvier provide?
The gaps increase the speed of transmission of impulses along the axon, as the change in potential jumps from one node to another.
What is the synapse?
The synapse is a very small gap between neurones. Often the gap its between 2 dendrites but it may be between a dendrite and the cell membrane of a cell body.
What does the synapse contain?
Interstitial fluid
Billions of never cells meet a synaptic junctions but do not touch, therefore what is the communication at the synapses?
The electrical nerve impulses cannot jump the gap so communications at the synapses is chemical.
What is the time taken to cross a synapse?.
0.5 milliseconds.
What is the synaptic knob?
This is the terminal end of the neurone.
What doe the synaptic knob contain?
It contains a chemical (neurotransmitter) which is released in response to nerve impulse arriving in the knob.
What do the neurotransmitters do?
The neurotransmitters diffuses across the gap and binds to receptors on the postsynaptic membrane stimulating never impulses in the postsynaptic neurone. Different synapses shave different neurotransmitters but they all facilitate communication between different neurone.
Around how many different neurotransmitters have been identified and how many in each neurone?
Many neurones contain two or three types of neurotransmitters each having different effects upon the postsynaptic cells. Around 100 different neurotransmitters have been identified, including acetylcholine, noradrenaline, serotonin and dopamine.
What are nerves often categorised on?
Nerves are often categorised on the main neurotransmitter they release, for example nerve cells that release acetylcholine are called cholinergic neurones.
A severed bacon accent be repaired or re-grown, but neurones do have a capacity to adapt, known as what?
Plasticity.
What does plasticity involve?
Plasticity involves the sprouting of new dendrites, synthesis of new proteins and changes in synaptic contact with other neurones.
In which nervous system its plasticity greater?
Plasticity is greater in the peripheral nervous system than it is in the central nervous system.
What do space do glial cells occupy?
Glial cells occupy the space between neurones and carry out support functions.
What is the function of glial cells?
In the case of injury or disease, neruoglial cell multiple and fill in spaces formally occupied by neurones..
What is bigger and more common in the nervous tissue, the neurones or glial cells?
Glial cells are vernally smaller than neurones and about 50 times more numerous in the nervous tissues.
What function do some glial cells have?
Some neuroglial cells have an immune system function.
What disease is commonly derived from glial cells?
Primary brain tumours.
What is a nerve impulse?
A nerve impulse is an electrical sign which is transmitted along the length of the neurone.
What are the two major functional properties of neurones?
Irritability and conductivity.
What is irritability the ability to do?
Irritability (seen mainly in sensory neurones) is the ability to generative nerve impulses in response to a stimulus,
What is conductivity the ability to do?
Conductivity is the ability to transmit the impulse to other neurones, muscles or glands.
What is the plasma membrane of an inactive neurone said to be?
Polarised - the inside of the cell is more negative than the outside.
What happens to the plasma membrane after a stimulus such as neurotransmitter being received?
The permeability of the plasma membrane to sodiums and potassium ions is altered for a very short time. Depolarisation occurs.
What state is a cell in when it has been depolarised?
The inside of the cell becomes positive compared with the outside.
What is depolarisation caused by?
Sodium ions flood into the cell through the cell membrane.
What does the entry of sodium ions into the cell cause?
An action potential.
What is the membrane of an axon relatively impermeable to and relatively freely permeably to?
The membrane of an axon is relatively impermeable to sodium ions (Na+) but relatively freely permeable to potassium ions (K+).
What does the difference in permeability gives the neurones?
They’re conducting properties.
What pump dose the axon membrane also contain?
A very active sodium/potassium pump.
How does the sodium potassium pump work?
This pump uses ATP to move sodium ions out of the axon and potassium ion into the axon.
What is the result of the sodium/potassium pump?
As a result, the concentration of sodium ions inside the axon is lowered because the ions are pumped out but cannot move back into the axon bu diffusion.
What can the potassium ions do once pumped into the axon?
Although the sodium pump moves potassium ions into the axon, they diffuse out again through the membrane down a concentration gradient.
What is the result of the ion movement?
The inside of the axon is left slightly negative relative to the outside.
What does the polarised membrane have a potential difference of?
The polarised membrane has a potential difference of -70mV which is known as the resting potential.
What occurs when a nerve impulse travels along an axon, or any nevre fibre?
The big change is in permeability of the axon membrane to sodium ions.
In a sensory neurone, what does the change in permeability happen in response to?
A stimulus such as light, sound, touch, taste or smell.
In a motor neurone, what is an impulse triggered by?
A neurotransmitter chemical.
When happens when a neurone is stimulated and there is a sudden big increase in the permeability of the axon membrane to sodium ions?
Very specific sodium ion channels open and sodium ions diffuse rapidly into the axon down concentration and electrochemical gradients.
What does the diffusion of sodium into into the cell result in?
This results in a brief reversal of the potential across the axon membrane - it becomes depolarised with he inside of the cell become positive relative to the outside.
What is the potential difference across the membrane, what is is known as?
+40mV and this is the action potential.
How long does the action potential last for?
About one millisecond.
What does the all or nothing response mean in terms of the action potential?
Every action potential is exactly the same size, it either happens or it does not.
What needs to happen to the membrane after the action potential? What is this process called?
After the action potential the membrane needs to return to its resting state so that another impulse can be passed. This is the re-polarisation.
How does the re-polarisation take place?
The sodium ion channels close and the sodium/potasssium pump quickly pumps out the excess sodium ions, an active transport and therefore uses ATP.
In re-polarisation, what happens to the permeability of the membrane?
The permeability of the membrane to potassium increases briefly o potassium into also diffuse out of the axon down both concentration and electrochemical gradients.
After re-polarisation the inside of the cells is returned to its resting state, what is this?
With the inside negative compared to the outside, as the positive sodium and potassium ions leave the cell
How long does re-polarisation take and what is it known as?
It takes a few milliseconds and this is known as the refractory period.
During the refractory period, what can not happen to that area of the axon? What 2 things does this help make sure?
It cannot be simulated again. This helps to make sure that the impulse travels in only one direction along the axon and limits the number of impulses which can travel along a neurone in a given period of time.
For the nerve system to function, one neurone must be able to pass an impulse to another, they must be able to communicate - what are neurones linked by and what is the action potential conducted along until it reaches?
A synapse
The electrical impulse cannot jump across the synaptic gap to stimulate the next neurone, what chemicals are used to cross the gap?
Chemicals known as neurotransmitters are used to cross the gap.
When an impulse arrives in the synaptic knob, it increased the permeability of the pre-synpatic membrane to what?
Calcium ions (Ca 2+)
How do calcium ions move into the pre-synaptic knob?
Down a concentration gradient.
What effect does the movement of calcium ions into the pre-synaptic knob have on the neurotransmitters?
As a result, synaptic vesicles full of neurotransmitters move to the pre-synpatic membrane here they fuse with the membrane and release their transmitter molecules into the synaptic gap.
Once the neurotransmitters are in the synaptic gap, what happens next?
The diffuse across the gap and become attached to specific protein receptor sites on the post-synaptic membrane.
When neurotransmitters attach to protein receptor sites on the post synaptic membrane, what happens?
This opens sodium ion channels so sodium ions move into the post synaptic knob.
What happens if enough receptors are stimulated?
The threshold level of sodium ions inside the cell is reached and a new action potential is set up in the post synaptic cell. It usually needs a number of impulses to arrive in pre-synpatic neurones to trigger an action potential in the post synaptic neurone.
In healthy individuals, what happens to neurotransmitters in the synaptic cleft once they have done their job?
The are destroyed by enzymes in the synaptic clift so they do not continue to stimulate the post synaptic neurone.
What can affect the transmission of nerve impulses?
A range of drugs, including alcohol, sedatives and anaesthetics.
Some drugs affect the sodium ion channels in neurones, what affect can this have?
It can prevent the sudden inrush of sodium ions which causes the action potential.
Other drugs affect the sodium/potasium pump mechanism. without this, what cannot be maintained?
Without the sodium potassium pump the resting state of a neurone cannot be maintained and so it will not be able to transmit an impulse.
Some drugs affect the synapses between neurones, examples being?
They may replace natural neurotransmitters, destroy natural neurotransmitters or enhance their action.
What does cold or continuous pressure also obstruct impulse conduction?
Because it interrupts blood flow to neurones.
What various forms can information about the external world and the internal environment of the body exist?
Pressure, temperature, light, smell, sound waves.
What specialised cells can convert various stimulus into nerve impulses in sensory neurones?
Specialised receptor cells.
There are several different types of sensory receptors which respond to external and internal stimulus, what do exteroceptors receive?
Exteroceptors receive external stimuli (from outside the body) such as light, sound, chemical changes, temperature changes and pain.
What do Interoceptors receive?
Interoceptors receive internal stimuli such as water levels in the body (thirst) nutrient levels (hunger) carbon dioxide concentration in the blood and blood pressure.
What is adaption to a stimulus?
In the presence of a constant stimulus, neurones eventually top responding and sensitivity i great reduced - this is adaption to a stimulus - this is how we turn out repetitive low level sounds or chronic pain,.
What two types of adapting sensors are there?
There are fast-adapting sensors and slow adapting sensor.
What are nociceptors?
Nociceptors detect pain - these are slow adapting receptors are especially common in skin, joint capsules, bone and around the walls of blood vessels. Deep tissues and the visceral organs have few nociceptors.
What are thermoreceptors?
Thermoreceptos are fast-adapting and found in skin, skeletal muscles, the liver and the hypothalamus. Fat adaption means that when you enter an air-conditioned room, it feels initially quite cold but within a few minute you become conformation as the adaption process occurs.
Which thermoreceptors, hot or cold are more numerous?
Cold receptors are 3 or 4 times more numerous than heat receptors.
What are mechanoreceptors?
Mechanoreceptors are sensitive to stimuli that distort cell membranes, like a direct physical blow to a part of the body - these respond to forces that cause stretching, compression, twisting or other types of distortion to cell membrane.
What are the 3 types of mechanoreceptor?
Baroreceptors detect pressure changes in the walls of blood vessels - an interoceptor function.
Proprioceptors monitor the position of joints and muscles - these interoceptors are the most structurally complex of the mechanoreceptors.
Tactile receptor provide sensations to touch, pressure, vibration, itch or tickle - they are exteroreceptors.
What are chemoreceptors?
Chemoreceptors include exteroceptors that detect chemicals in the mouth (taste), nose (smell) and many interoceptors that detect chemicals in body fluids.
What bare osmoreceptors?
Osmoreceptors are interoceptors that detect the water potential (osmotic pressure) of body fluids.
What are photoreceptors?
Photoreceptors arre exteroceptors which detect light some of them adapt rapidly and some of them are very slow adapting.
What is the cause of every impulse in sensory neurones?
Stimulation of receptors.
What is the pathways of communication from stimulation to response?
Stimulation of receptors is the cause of eery impulse in sensory neurones, information is coordinated in the central nervous system and impulses pass along motor neurones to reach effectors, where as response is made to the original sensory input. Typical examples of effectors are glands and muscles.
What does the Central Nervous system consist of?
The brain and spinal cord.
How are the brain and spinal cord protected?
The brain and spinal cored are so essential that they are fully encased in bone: the brain with the cranial cavity of the skull and the spinal cord within the vertebral canal of the vertebrae.
How much does the brain weigh?
About 1.5kg
How many neurones and supporting glial cells does the brain contain?
Over 12 billion neurones and 50 billion supporting glial cells.
The brain is protected within the cranial cavity of the skull by which 3 membranes?
The Dura Mater (outer layer) Arachnoid mater (middle layer) Pia mater (inner layer)
What is the dura mater?
The dura mater is a tough, leathery membrane that forms an inelastic bag that surrounds the brain and spinal cord.
