Functions of the Nervous System: to collect, process and act upon information from the internal and external environment.
Neurones: specialised nerve cells that come in three types: sensory, relay and motor (efferent).
Stimulus: any environmental change that alters the electrical properties of neurone membranes, initiating a nerve impulse. Light, heat, sound and chemicals can all act as stimuli
Central Nervous System (CNS): this is comprised of the brain and spinal cord and receives all incoming information; the brain is the site of decision making.
Peripheral Nervous System: the rest of the neurone network- it links receptors to the CNS.
Receptors detect a change in stimuli.
Effectors: muscles or glands which bring about a response to the environment.
Nerve impulses: electrical impulses which pass along neurones.
Synapse: a gap where two neurones meet.
Chemical transmitters allow a nerve impulse to pass from one neurone to the next.
Now for more details…
Efferent/motor system is a part of the peripheral nervous system, and consists of two parts:
- Somatic – controls voluntary actions
- Autonomic– controls heartbeat, breathing, digestion and other involuntary functions.
The motor system is regulated by the medulla oblongata and cerebral cortex.
The Autonomic nervous system can be further subdivided into two parts:
- The sympathetic nervous system readies the body for sudden action and relies on the chemical transmitter noradrenaline, which is released by adrenergic nerves. Physical changes include dilation of pupils and bronchi; decreased salivation; increased heart rate; inhibition of gastric activity, pancreatic activity and peristalsis; conversion of stored glycogen into glucose and relaxation of the bladder.
- The parasympathetic nervous system inhibits action and relies on the chemical transmitter acetylcholine, which is released by cholinergic nerves. Physical changes include constriction of pupils and bronchi, increased salivation, decreased heart rate, stimulation of gastric activity, pancreatic activity and peristalsis, the conversion of glucose to glycogen in the liver and constriction of the bladder. The body is thus returned to normal “housekeeping” activities such as digestion.
The sympathetic and parasympathetic nervous systems are normally balanced. During an emergency, however, the sympathetic nervous system takes over. The parasympathetic will then take over after the emergency to normalise the body once more.
Structure of an effector neurone: Effector/motor neurones take information back out to effectors.
Sensory neurones take information in from receptors. They differ from the above structure in that the cell body extends from the axon partway along its length.
Connector/relay neurones are the “middle-men” of the nervous system (basically, they just make connections between the other parts of the nervous system). They differ from the above structure in that the cell body is in the middle of the axon.
Properties of neurones:
- Neurones are excitable, i.e. they can detect and respond to stimuli
- Neurones are conductive- they can transmit a signal from one end of an axon to another.
The axon at rest is polarised.
The fluid inside is negatively charged with respect to the outside due to an uneven distribution of Na+ and K+ ions, resulting in a resting potential difference of -70 mV between the inside and the outside. At rest, the membrane is relatively impermeable to Na+ ions, so the concentration of Na+ ions is very high outside the axon. It is possible, however, for some of the negative ion and K+ ions inside to diffuse out.
Generating an action potential (AP): stimuli cause subtle electrical changes to take place in the membrane of a neurone’s cell body. The resting potential becomes slightly less negative, and, if this change exceeds a threshold level, the membrane potential becomes positive and an action potential is triggered.
Depolarisation: the stimulus causes the membrane to become more permeable to sodium ions (sodium channels open). The membrane potential changes slowly to -50mV, then more quickly to +40mV as Na+ ions move down a concentration gradient. This is depolarisation.
Repolarisation: to return the membrane to its resting potential after an AP, potassium channels in the membrane open, and K+ ions diffuse out. This is repolarisation.
Refractory period: the time delay between the conduction of one AP and the next. It consists of two parts:
- The absolute refractory period. During this time, the sodium channels are closed and no impulse can be conducted.
- The relative refractory period. Here, the potassium channels are open as repolarisation begins. A stronger than usual stimulus can now generate a response.
Note: smaller neurones take longer to recover.
Functions of the refractory period:
- It imposes a limit on the frequency of nerve firing, thus preventing overload.
- It prevents the overlapping of APs.
Sodium/potassium pump: after several APs have been triggered, this “pump” actively transports Na+ and K+ across the axon membrane to restore their original concentration gradients.
Action potential graph:
All-or-nothing law: all action potentials have the same magnitude. They are stimulated if the initial stimulus is over the threshold level. Thus, the number of APs is important, not the size (since they either occur with a fixed size, or not at all).
Direction of nerve impulse: it is important to note that nerve impulses can only flow in one direction down a particular axon.
Generator potential: this term describes a stimulus just great enough to stimulate an AP (i.e. one just above the threshold level).
Speed of nerve impulse (conduction velocity) depends on
- Axon diameter (the larger the diameter, the faster the impulse travels)
- Myelination (insulation of axon). This speeds up impulses by allowing saltatory conduction
The impulse can “jump” from node to node as the insulating myelin prevents the depolarisation of the entire neurone. Saltatory conduction is more energy and ion efficient as less energy is required for the sodium-potassium pump.
- Number of synapses. The more synapses there are, the slower the impulse travels; each synapse causes a delay as the electrical impulse must be converted to a chemical signal in order to cross the synapse.
The polysynaptic reflex arc: reflex actions are inborn, involuntary and invariable actions that involve three or more neurones. Obvious examples are the knee-jerk reflex, or the way we pull our hands away from a hot or painful object. The reflex arc works as follows (this example is for touching a hot pan, but other reflexes work in the same way):
- The stimulus is detected by heat/pain receptors in the skin.
- A stimulus over the threshold level propagates an AP in a sensory neurone.
- This impulse travels via the dorsal root to the spinal cord (CNS). A message is sent to the brain.
- The impulse now travels to the ventral root via a relay neurone.
- Motor/effector neurone is now stimulated by the impulse
- The effector (muscle) is now stimulated. The response is to move the hand away from the hot pan.
When an AP arrives at the synaptic bulb, it causes Ca2+ ions to diffuse into the synaptic bulb. The increase in Ca2+ ions causes the synaptic vesicle to fuse with the presynaptic membrane, releasing their neurotransmitter into the synaptic cleft. The neurotransmitter then takes 1 ms to diffuse across the synaptic cleft- this period is known as the synaptic delay.
The neurotransmitters then bind with receptors in the postsynaptic membrane. Some neurotransmitters cause sodium channels in the postsynaptic membrane to open, creating an excitatory postsynaptic potential (EPSP). Other neurotransmitters open potassium channels, creating an inhibitory postsynaptic potential (IPSP). For an AP to continue along the next neurone, the EPSP must be greater than the IPSP.
Once the neurotransmitter has acted on the postsynaptic membrane it is broken down by an enzyme. These breakdown products diffuse back into the presynaptic bulb. Mitochondria provide energy to rebuild the neurotransmitters and “repackage” them into synaptic vesicles.
The main neurotransmitters are noradrenaline and acetylcholine.
Functions of synapses:
- They allow impulses to pass from one neurone to another.
- The arrangement of membranes ensures that impulses travel in one direction only.
- They protect the nerve network by not firing when over-stimulated (fatigued).
- Low level stimuli (e.g. the sound of a clock ticking) can be filtered out.
- They aid information processing by summation (see below), i.e. the effects of several impulses are added together
- The next neurone can be excited or inhibited as required.
Transmission across synapses is graded, even though individual APs are an all-or-nothing event.
Facilitation: the first wave of neurotransmitter sets up an EPSP which may not be above the threshold level. As more neurotransmitter is released, further EPSPs are set up, so it is more likely that an AP will be triggered in the postsynaptic neurone. This process is called facilitation.
Summation: a term describing the additive effect of EPSPs and IPSPs e.g. as seen in the facilitation case above.
Convergence: this is where impulses from many presynaptic neurones stimulate one postsynaptic neurone.
Agonist: a presynaptic neurone that is likely to stimulate an AP in the postsynaptic neurone.
Antagonist: a presynaptic neurone that tries to inhibit an AP in the postsynaptic neurone. By working together, agonists and antagonists allow for fine control of impulses as they pass from neurone to neurone.
Taxis (plural taxes): movement in a specific direction as directed by a specific stimulus. A positive taxis occurs when an organism moves towards the stimulus (e.g. moving towards food), whilst a negative taxis occurs when an organism moves away from a stimulus (e.g. away from a predator). Types of taxis include phototaxis (light is the stimulus), thermotaxis (heat is the stimulus), geotaxis (gravity is the stimulus) and chemotaxis (chemicals are the stimulus).
Kinesis (plural kineses): changes in activity due to changes in conditions. Movement is random and not specifically directed by an stimulus. A good example is the movement of woodlice- they will move more in less humid conditions and settle down once they reach their preferred humid habitats. However, they do not specifically move from the less humid to the more humid zones.