8.2 How the nervous system works

8.2 How the nervous system works

The nervous system

The nervous system is the network of nerve cells and fibres that transmits impulses around the body. The nervous system is divided into the central and peripheral nervous systems.

Central nervous system = brain and spinal cord

Peripheral nervous system =

Sensory nerves – carry sensory information from the receptors à CNS
Motor nerves – carry motor commands from CNS à effectors

Peripheral nervous system is sub-divided into:

Somatic nervous system – voluntary, stimulates skeletal muscle
Autonomic nervous system – involuntary, stimulates smooth muscle, cardiac muscle and glands

Autonomic nervous system divided into:

Sympathetic – Prepares body for fight or flight response – increases activity e.g. speeding up heart rate
Parasympathetic – Prepares body for rest and digest – decreases activity e.g. lowering breathing rate

The nervous system carries messages around the body using neurones. Neurones are nerve cells which are highly specialised and adapted for the rapid transmission of electrical impulses (action potentials) around the body. Most neurones have a similar basic structure – they:

Most are long – can transmit the action potential over a long distance
The cell surface membrane has gated ion channels that control the movement of Na, K or calcium ions
Have Na/K pumps that use ATP for active transport
Maintain a potential difference across their cell surface membrane
Have a cell body containing the nucleus, mitochondria and ribosomes

Sensory neurones:

Carry impulses from receptors to CNS
Cell body is attached to the middle

Motor/effector neurone:

Conducts impulses from CNS to effectors (muscles or glands)
In CNS
Can have long axons
Cell body is at the end

Relay/connector neurones:

Connect sensory and motor neurones
Mostly in CNS
Large number of connections with other nerve cells
Cell body is in the middle of the axon

Stimulus à Receptor cells à Sensory neurone à CNS à Motor neurone à Effectors à Response

There are two types of main extensions from the cell body of a neurone:

Dendrites – Conduct impulses towards cell body
Axons – Transmits impulses away from cell body

Myelin sheath:

Some neurones have a fatty insulating layer around the axon
Made of many layers of Schwann cells wrapped around the axon
Acts as an electrical insulator
Between Schwann cells are exposed patches of membrane – nodes of Ranvier
Speeds up the transmission of action potentials – impulse jumps from node to node (saltatory conduction)

Nerve impulses

Neurones have sodium-potassium pumps in their surface membranes. These actively pump Na+ (sodium) ions out of the cell by active transport. This uses ATP as ions are moved against their concentration gradients.

When a neurone is not transmitting impulses, it is at rest. K+ (potassium) ions are pumped inside through channels. K+ and Na+ ions diffuse back down their concentration gradient but K+ diffuses faster back out than Na+ can diffuse back in – there is a net movement out of the cell, making the inside more negative. This is the resting potential (-70mV) and the membrane is polarised.

Generation of action potential:

Depolarisation…
- There is a change in permeability, causing Na+ channels to open so Na+ to enter the cell down the concentration gradient – inside of cell becomes more positive than outside
- This reverses the resting potential, the membrane depolarises. The potential difference is +40mV (action potential)
- If this reaches the threshold level, an action potential is generated and an impulse is fired. If it does not, nothing happens
Repolarisation…
Na+ gates close and membrane permeability to Na+ ions decreases
Voltage-dependent K+ channels open, K+ ions move out of cell down electrochemical gradient
Inside of cell becomes more negative – drops below the resting potential value (hyperpolarisation)
Restoring resting potential…
K+ channels close, sodium-potassium pump restarts, restoring the normal distribution of ions either side of the cell surface membrane
Potassium ions diffuse into axon

The refractory period is a time delay between action potentials, when the axon restores its resting potential after the action potential. Ion channels are recovering and cannot be opened and the axon is unable to generate another action potential until the refractory period is over and all voltage-dependent K+ and Na+ channels are closed, ensuring impulses only travel in one direction.

When the threshold is reached, the action potential always fires with the same voltage, no matter the size of the stimulus. If it isn’t reached, the action potential will not fire. A larger stimulus does not cause a larger action potential but it causes more frequent action potentials.

Summary:

Membrane is polarised (resting state)
Ion channels open, Na+ ions diffuse into cell
Membrane depolarises – less negative
Voltage-dependent Na+ channels open – more enter – positive
Potential difference reaches -40mV
Na+ ion channels close and K+ channels open
K+ ions diffuse out – negative inside – repolarisation
Potential difference overshoots – hyperpolarisation
Potential difference restored – returns to resting state

The action potential travels rapidly along the axon or dendron. This is because the repolarisation of one part of the membrane sets up local currents with the areas either side of it – these regions depolarise too as some Na+ flow sideways.

Propagation of an impulse along an axon:

At resting potential there is a positive charge on the outside of the membrane and negative charge on the inside, with higher sodium ion concentration outside and higher potassium ion concentration inside
When stimulated, voltage-dependent sodium ion channels open and sodium ions flow into the axon, depolarising the membrane. Localised electric currents are generated. Sodium ions move to the adjacent polarised (resting0 region causing a change in electrical charge (potential difference) across this part of the membrane
This change in potential difference initiates a second action potential. At the site of the first action potential, the voltage-dependent sodium ion channels close and voltage-dependent potassium ion channels open. Potassium ions leave the axon, repolarising the membrane. The membrane is hyperpolarised
A third action potential is initiated by the second. In this way, local electric currents cause the nerve impulse to move along the axon. At the site of the first action potential, potassium ions diffuse back into the axon, restoring resting potential

In a myelinated neurone, local currents cannot be set up where the myelin sheath is as the Na+ and K+ cannot flow through the fatty myelin sheath. Instead the action potential jumps from one node of Ranvier to the next where the ions move through the cytoplasm (saltatory conduction). This increases the speed at which it travels along the axon.

How is the impulse propagated along a myelinated axon?

Depolarisation occurs at Node of Ranvier
Local electric currents occur between nodes
Potential difference is reduced at next node, initiating another action potential
Impulses jump between nodes by saltatory conduction

Action potentials are conducted faster along axons with large diameters, as there is less resistance to the flow of ions so depolarisation travels faster. They also are faster at higher temperatures (up to 40oc).

Synapses

Two neurones are not in direct contact, there is a small gap called the synaptic cleft. The synapse is the junction between the two neurones. The presynaptic neurone has a swelling, called a synaptic knob which contains vesicles filled with neurotransmitter. The presynaptic knob has mitochondria to provide energy to make neurotransmitters and vesicles. The electrical impulse cannot cross the synaptic cleft, so a neurotransmitter is released at the end of the first neurone from the presynaptic membrane. It diffuses across the synapse, binds with the second neurone on the postsynaptic membrane, generating an action potential.

Neurotransmitter release à stimulation of postsynaptic membrane à inactivation of neurotransmitter

Action potential arrives at presynaptic membrane/synaptic knob
Membrane depolarises causing voltage-dependent calcium ion channels to open – calcium ions enter neurone down their concentration gradient
Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with presynaptic membrane
Neurotransmitter released into synaptic cleft by exocytosis and diffuses across the cleft
Neurotransmitter binds with complementary receptor proteins on postsynaptic membrane, causing cation channels to open so Na+ ions flow through into the cytoplasm of the neurone
Postsynaptic membrane depolarises, setting up an action potential in the postsynaptic neurone
To stop action potentials, the released neurotransmitter is taken up by the presynaptic membrane or diffuses away and is broken down by enzymes

Two examples of neurotransmitters are acetylcholine (ACL) and noradrenalin. They are synthesised in vesicles, which requires energy. Therefore the synaptic knobs have many mitochondria to produce ATP.

Although synapses slow the transmission of impulses, they are useful:

Ensure impulses travel only in one direction because receptors are only on the postsynaptic membrane
Allows neurones to connect with many other neurones – increases range of possible responses to a particular stimulus
Control nerve pathways an give flexibility of response
Integrate information from different neurones to give a coordinated response

Extent of depolarisation:

Depends on how much neurotransmitter reaches the postsynaptic membrane
– Depends partly on the frequency of impulses reaching the presynaptic membrane
A single impulse usually won’t release enough neurotransmitter to depolarise and postsynaptic membrane
Also depends on the number of receptor sites on the postsynaptic membrane

Excitatory synapses (depolarise)

Make the postsynaptic membrane more permeable to Na+ ions

Makes it more likely that an action potential will be generated

Several impulses together release enough – summation

Inhibitory synapses (hyperpolarise)

Inhibitory synapses make it less likely that an action potential will result.

The neurotransmitters open channels for Cl- and K+ ions in the postsynaptic membrane
These move through the channels down their diffusion gradients
Cl- ions move into the cell and K+ ions move out
This causes a greater potential difference across the membrane as the inside becomes more negative than usual (-90mV)
This makes depolarisation less likely as more excitatory synapses are needed

If the stimulus is small, little neurotransmitter will be released and this might not be enough to excite the posysynaptic membrane to the threshold level – more than one synapse/neurone is needed to provide sufficient depolarisation – summation is when each impulse adds to the effect of another

Spatial summation: Impulses are from different synapses connect to one neurone, there is enough neurotransmitter for an action potential
Temporal summation: Several impulses arrive at a synapse one after another

The combined release of neurotransmitter generates an action potential in the postsynaptic membrane.

Generator potentials

Receptor cells respond to changes in the environment.

When a stimulus is detected, the membrane becomes more permeable – gated Na+ ion channels open and Na+ ions diffuse into the cell

Small change in potential – generator potential

The larger the stimulus, the more gated channels will open – the larger the generator potential

If enough Na+ ions enter, the potential difference changes significantly and will initiate an impulse or action potential.

Synapses can amplify or disperse information – one neurone connects to many other neurones – information dispersed around the body – synaptic divergence or many neurones connect to one neurone – information amplified (made stronger) – synaptic convergence.

Sensory receptors

Specialised cells that detect changes in the environment. They are specific to one type of stimulus
Energy transducers – convert one form of energy to another. Each type of transducer is adapted to detect changes in a particular energy form
May be a change in light levels/pressure on the skin
Each change in energy levels in the environment is a stimulus
Sensory receptors can convert any stimulus energy into a form of electrical energy (a nerve impulse)

Examples of receptors:

Receptors	Energy changes detected
Light sensitive cells (rods and cones) in eye retina	Light intensity and range of wavelengths
Olfactory cells lining the inner surface in the nasal cavity	Presence of volatile chemicals
Taste buds in the tongue, hard palate, epiglottis & oesophagus	Presence of soluble chemicals
Pressure receptors (pacinian corpuscles) in the skin	Pressure on skin
Sound receptors in the inner ear (cochlea)	Vibrations in air
Muscle spindles (proprioceptors)	Length of muscle fibres

The human eye

Light enters the eye through the pupil. The amount of light is controlled by the muscles of the iris. Light rays are focused by the lens onto the retina, which lines the inside of the eye and contains photoreceptor cells that detect the light. Nerve impulses are carried from the retina to the brain by the optic nerve.

Conjunctiva: Protects the cornea
Cornea: Bends light
Lens: Focuses light on retina
Iris: Controls amount of light entering eye by controlling pupil size
Sclera: Protective layer, allows attachment of external muscles
Blind spot: No light sensitive cells where optic nerve leaves eye
Fovea: Most sensitive part of retina
Retina: Contains light-sensitive cells
Vitreous humour: Transparent jelly
Choroid: Black layer prevents internal reflection of light
Cilary muscle: Alters thickness of lens for focusing
Optic nerve: Transmits impulses to brain
Pupil: Circular opening for directing light to the lens

How do photoreceptors convert light into an electrical impulse?

Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive pigments
Light bleaches the pigments, causing a chemical change and a change in membrane permeability to sodium
A generator potential is created, if it reaches threshold, a nerve impulse is sent along a bipolar neurone
Bipolar neurones synapse with ganglion neurones whose axons make up the optic nerve – bipolar neurones connect photoreceptors to the optic nerve
Optic nerve extends to several brain areas including the thalamus
Before reaching the thalamus, some of the neurones branch off to the mid brain
At the mid brain they connect to motor neurones that control pupil reflex and eye movement

There are two types of photoreceptors in the retina – rods cells and cone cells:

Rods – sensitive to dim light because many join to one neurone. They give low visual acuity and only give information in black and white. Rods are found in the peripheral parts of the retina.
Cones – Can only respond in bright light (less sensitive) as only one joins one neurone. They give high visual acuity because they are close together. They give information in colour. Cones are found packed together in the fovea.

	Rods	Cones
Number in retina	20:1
Where in retina	All over retina but not fovea	Only fovea
Light-sensitive pigment	Rhodopsin	Iodopsin
Vision	Black and white vision Both dim and bright light	Colour vision Only bright light
Sensitivity	Intensity	Wavelength

Rod cells

The outer segment of a rod cell contains membranes, staked up parallel. Sodium-potassium pumps act across the membranes and cation channels remain open, so some sodium and potassium ions leak back through.

The membranes also contain a pigment called rhodopsin (in vesicles) which is made of a retinal molecule and an opsin molecule. When light hits rhodopsin, the retinal changes shape. This causes sodium and potassium ion channels in the membrane to close, but the sodium-potassium pump keeps working. As the ions cannot leak back in or out of the neurone, a greater potential than usual builds up (negative inside) and the membrane is hyperpolarised.

When no light falls onto a rod cell and the potential difference is normal, it constantly releases transmitter substances, which diffuses to the next neurone. This stops that neurone generating action potentials. When light falls on the rod cell and hyperpolarises the membranes, it stops releasing the transmitter substances and the neighbouring neurone can generate action potentials. These are transmitted along axons to the optic nerve, which carry them to the visual centre in the brain.

The change in shape of retinal makes it unstable and it separates from opsin. If this happens to all the rhodopsin in all rod cells, you cannot see in dim light. If you walk from a sunny area to a dim room you cannot see much because our cones stop functioning in low intensity light and the rod pigments have become bleached. In dim light, retinal and opsin gradually combine again, forming rhodopsin – this is dark adaptation. Until rhodopsin is reformed, no more action potentials can be created in the bipolar cells so no more stimuli can be detected. This takes a few minutes. The brighter the light, the more rhodopsin molecules break down and the longer it takes for them to reform.

Dark

Na+ diffuses through open cation channels into outer segment
Na+ move down concentration gradient into inner segment
Na+ is actively pumped out of cell using ATP and ion pumps
Membrane slightly depolarised -40mV
Inhibitory neurotransmitter (glutamate) released from rod cells and binds to bipolar cell, preventing it from depolarising.

Light

Light energy breaks rhodopsin à opsin + retinal
Opsin binds to membrane of outer segment
Na+ cation channels close
Na+ still actively pumped out but cannot diffuse into outer segment
Inside of cell is more negative – membrane hyperpolarised (-90mV)
No inhibitory neurotransmitter (glutamate) is released
Cation channels in bipolar cell open and membrane is depolarised, generating an action potential in the neurone of the optic nerve à brain

Controlling pupil size

The iris is the coloured part of the eye surrounding the pupil and it controls the pupil size. Light passes through the pupil on its way to the retina. In bright light, the pupil is small (contracted) to limit the amount of light passing through to prevent damage to rods and cones. In dim light the pupil is large (dilated) to allow more light to reach the retina.

When bright light hits the retina, it is detected by photoreceptors which send nerve impulses along the optic nerve to the CNS of the brain along a sensory neurone. This causes action potentials to be sent along parasympathetic motor neurones to the muscles in the iris. Circular muscles contract and radial muscles relax, making the iris wider and the pupil narrower (pupil constricts).

In dim light, the circular muscles relax and the radial muscles contract to widen the pupil – the opposite reaction.

The radial muscles are controlled by sympathetic reflex

The circular muscles are controlled by parasympathetic reflex

Reflex arcs

Nerve impulses follow routes/pathways through the nervous system, these reflex arcs are responsible for our reflexes and they are controlled by the autonomic nervous system. Reflexes are fast and help to avoid damage to the body.

Receptors detect stimulus and generate nerve impulse
Sensory neurones conduct nerve impulse to CNS along sensory pathway
Sensory neurones enter spine
Sensory neurone synapses with relay neurone
Relay neurone synapses with motor neurone which leaves spine
Motor neurone carries impulse to effector producing a response

If there is a relay neurone involved, the reflex can be overridden by the brain.

Hormonal system

Made up of glands and hormones
Gland = group of cells specialised to secrete a useful substance
Hormones = chemical messengers, many are proteins or peptides
Hormones are secreted when a gland is stimulated
Hormones diffuse into the blood and diffuse around the body
Hormones trigger a response in target cells
Stimulus à Receptor à Hormone à Effectors à Response
Hormones are not released directly to the target cells, they travel in the blood which is slower
They aren’t broken down as fast as neurotransmitters – long-lasting
They are widespread – hormones are transported all over the body

Nervous system keywords

Nerve impulse: An electrical impulse sent along a nerve to allow information to travel to effectors to carry out a response to a stimulus
Neurone: A single cell which has dendrites, an axon, a cell body and terminal branches, making up part of the nervous system
Nerve: Contains a bundle of the axons of many neurones surrounded by a protective covering
Cell body: Contains the organelles and is found in different locations for different neurones with extensions called dendrites and the axon
Dendrites: Conduct impulses towards the cell body. Each neurone has many
Axon: Conducts impulses away from the cell body, a single long process
Motor neurone: Carry impulses from CNS to effectors
Sensory neurone: Carry impulses from sensory cells to CNS
Relay neurone: Have a large number of connections with other nerve cells
Myelin sheath: Fatty insulating layer around the axon. It affects the speed of the impulse. It is made of Schwann cells
Effector: Produce a response when an impulse is received from motor neurones
Reflex arc: Nerve impulses follow simple pathways
Reflexes: Rapid, involuntary responses to stimuli
Photoreceptors: Receptors in the retina that respond to light levels
Resting potential: -70mV, when the inside of the axon membrane is more negative due to the movement of potassium ions
Repolarisation: The charge is reversed, becoming positive on the inside (+40mV) because voltage-dependent Na+ channels open and Na+ flow in
Repolarisation: The Na+ channels close and K+ channels open, allowing K+ ions to flow out making the inside more negative
Action potential: The change in voltage across the axon membrane
Positive feedback: The opening of Na+ ion gates makes more open
All or nothing: There is either enough depolarisation or not enough to create an action potential
Hyperpolarisation: The K+ ion gates open to re-polarise the membrane and too many are left out, the potential reaches -90mV
Refractory period: A new action potential cannot be generated for a few milliseconds until all the gates are closed and resting potential is restored. This ensures the impulse only travels in one direction
Nodes of Ranvier: Gaps at regular intervals in the myelin sheath on the axon. They are the sites of depolarisation
Saltatory conduction: The action potential jumps across the axon to each node of Ranvier, making the impulse travel faster
Synapse: Where two neurones meet
Synaptic cleft: The gap between the two meeting neurones
Presynaptic neurone: Has Ca+ ion channels and neurotransmitter vesicles. It is the stimulating neurone that passes on the impulse
Postsynaptic membrane: Receives the neurotransmitter and the impulse, it fires off another impulse
Synaptic vesicles: Contain the neurotransmitter to depolarise the membrane
Neurotransmitter: When Ca+ ions enter the presynaptic membrane, vesicles fuse with the membrane and release the neurotransmitter into the synaptic cleft. They bind with channels on the postsynaptic membrane
Acetylcholine: A neurotransmitter, the first to be discovered
Summation: Each impulse adds to the effect of the others
Spatial summation: Impulses from different neurones
Temporal summation: Several impulses along one neurone
Inhibitory synapses: Make it less likely that an impulse will be fired in the postsynaptic neurone by allowing it to hyperpolarise by letting Cl- ions in and allowing K+ ions out
Excitatory synapses: Make the postsynaptic neurone more permeable to Na+ ions meaning it is more likely that depolarisation will occur and lead to an action potential