Topic 15 - Nervous coordination and muscles

• There are two main forms of coordination in animals - the nervous system and the hormonal (endocrine) system.
• The nervous system uses nerve cells to pass electrical impulses along their axons. They stimulate target cells by secreting neurotransmitters.
• The nervous system uses rapid communication between different parts of an organism, however these responses are often short-lived and restricted to a localised region of the body.
• The hormonal system produces hormones that are transported in the blood plasma to their target cells. These cells have specific receptors on their cell-surface membranes, and respond to changes in the concentration of a hormone.
• Hormonal communication results in a slower, less specific form of communication between parts of an organism. These responses are often long-lasting and widespread.
• Neurones are specialised cells adapted to rapidly carry electrochemical charges called nerve impulses in the body.
• A mammalian motor neurone is made up of a cell body, dendrons, an axon, Schwann cells, myelin sheath and nodes of Ranvier.
• The cell body of a motor neurone contains a nucleus and rough endoplasmic reticulum, which can be associated with the production of proteins and neurotransmitters.
• The dendrons of a motor neurone are extensions of the cell body that subdivide into dendrites. They carry nerve impulses towards the cell body.
• The axon of a motor neurone is a single long fibre that carries nerve impulses away from the cell body.
• The Schwann cells of a motor neurone surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis (removing cell debris) and play a role in nerve regeneration.
• Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.
• The myelin sheath forms a covering of the axon and is made up of the membranes of Schwann cells. These membranes are rich in a lipid known as myelin.
• Neurones with a myelin sheath are called myelinated neurones.
• Nodes of Ranvier are constrictions between adjacent Schwann cells where there is no myelin sheath. The constrictions are 2-3 um long and occur every 1-3mm in humans.
• There are three types of neurone: sensory, intermediate and motor.
• Sensory neurones transmit nervous impulses from a receptor to an intermediate or motor neurone. They have one very long dendron which carries their impulse towards the cell body. They have a long axon which carries the impulse away from the cell body.
• Motor neurones transmit nerve impulses from an intermediate/relay neurone to an effector, such as a gland or muscle. Motor neurones have a long axon and many short dendrites.
• Intermediate/relay neurones transmit impulses between neurones (e.g. sensory to motor neurones).
• A nerve impulse may be defined as a self-propagating wave of electrical activity that travels along the axon membrane.
• A nerve impulse is the temporary reversal between two states - the resting potential and the action potential.
• Two factors allow for the control of the movement of sodium and potassium ions across the axon membrane : channel proteins in the phospholipid bilayer and the sodium-potassium pump.
• The phospholipid bilayer of the axon plasma membrane prevents sodium/potassium ions from diffusing across it unless channel proteins are used. Some of these have ’gates’, so are closed sometimes and open others, allowing sodium and potassium ions to move in via facilitated diffusion.
• Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon. This is known as the sodium-potassium pump.
• The resting potential refers to how the inside of an axon is negatively charged relative to the outside, and is -65mV in humans.
• When the axon is in resting potential, it is said to be polarised.
• When a stimulus of a sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the changes on either side of the axon membrane.
• If the stimulus is great enough, the negative charge of -65mV becomes a positive charge of 40mV. This is known as the action potential, and the axon membrane is said to be depolarised in this state.
• Depolarisation of the axon membrane occurs because the channels in the axon membrane change shape, hence opening or closing, depending on the voltage across the membrane. These are known as voltage-gated channels.
• The myelin sheath acts as an electrical insulator, preventing the myelin regions from forming an action potential. Instead, saltatory conduction between adjacent nodes of Ranvier occurs, increasing the speed of action potential from 30 m s-1 in unmyelinated axon to 90 m s-1.
• There are three factors affecting the speed of action potentials : the myelin sheath, the diameter of axon and the temperature.
• As the diameter of the axon increases, so does the speed of conductance. This is because there is less leakage of ions from larger axons, making it easier for the membrane potential to be maintained.
• Temperature affects the rate of diffusion of ions, so the higher the temperature, the faster the nerve impulse. Also, higher temperature increases enzyme activity, which increases the rate of respiration. This provides more energy for the active transport in the sodium-potassium pump, helping to increase the speed of action potential.
• The all-or-nothing principle highlights that a threshold value of the stimulus must be met in order for an action potential to be generated. Below threshold = no action potential = no impulse. Above threshold = action potential = impulse.
• Since all action potentials are the same size, there are two different ways to determine the size of a stimulus : measuring number of impulses in a given time, or by having different neurones with different threshold values.
• The larger a stimulus, the more frequent the impulses are in a given time.
• The refractory period is known as the period after an action potential has been generated, where further influx of sodium ions is inhibited due to the closure of the sodium voltage-gated channels. As a result, no further action potentials can be generated during this time,
• The refractory period serves three main purposes : (1) it ensures that action potentials are only propagated in one direction, (2) it produces discrete impulses, and (3) it limits the number of action potentials.
• The refractory period is important because it ensures that action potentials are unidirectional. Action potentials can only pass from an active region to a resting region, therefore cannot be propagated in a refractory region.
• The refractory period is important because it produces discrete impulses. This means that a new action potential cannot be formed immediately behind the first one, ensuring that they are separate.