15.1. Control and co-ordination in mammals basics Flashcards
Types of information transfer used to coordinate the body’s activities (IN ANIMALS)
1) nerves that transmit information in the form of electrical impulses (CNS and PNS)
2) chemical messengers called hormones that travel in the blood (endocrine system)
The mammalian nervous system is made up of:
1) Central Nervous System (CNS) - brain and spinal cord
2) Peripheral Nervous System (PNS) - the cranial and spinal nerves
Where do cranial and spinal nerves attach to?
- Cranial nerves are attached to the brain
- Spinal nerves are attached to the spinal cord.
- Information is transferred in the form of nerve impulses, which travel along nerve cells at very high speeds.
Receptor Cells
A cell that responds to one such stimulus by initiating
an action potential
- are transducers: they convert energy in one form – such as light, heat or sound – into energy in an electrical impulse in a neurone
- found in sense organs
Neurones (Nerve Cells)
highly specialised cells that are adapted for the rapid transmission of electrical impulses called action potentials, from one part of the body to another
Sensory Neurones
the receptor cells detect and impulse and send it to the CNS
Sensory Neurones Structure
- receptor cell at one end, dendrites at the other of the neurone connected by an axon
- the axon is surrounded by Schwann Cells (with nucleus) which make Myelin Sheaths
- there are Node of Ranvier which are gaps between the Myelin Sheath
- cell body (with a nucleus) attached to axon around the middle
Intermediate Neurones (also known as relay or connector neurones)
transmit impulses from sensory neurones to motor neurones
- found entirely within the central nervous system.
Intermediate Neurones Structure (also known as relay or connector neurones)
- cell body (with nucleus) at one end, dendrites at the other with axon connecting them
- NO Myelin Sheaths
Motor Neurones
transmit impulses from the CNS to efectors
Motor Neurones Structure
- cell body (with nucleus) at one end, dendrites at the other with axon connecting them
- dendrites connect to a neuromuscular junction
- the axon is surrounded by Schwann Cells (with nucleus) which make Myelin Sheaths
- there are Node of Ranvier which are gaps between the Myelin Sheath
Neurones Special Features
MOTOR
1) Highly branched dendrites - give a large surface area for the endings of other neurones.
2) Axon is much longer - conducts impulses over long distances
ALL
1) Cell body - dark specks can be seen in the cytoplasm. These are small regions of rough endoplasmic reticulum that synthesise proteins.
2) Axon has many mitochondria and vesicles containing chemicals called transmitter substances, involved in passing impulses
Schwann Cells and Myelin Sheath
- Myelin is made when Schwann cells wrap themselves
around the axon all along its length. - The Schwann cell spirals around, enclosing the axon in
many layers of its cell surface membrane. - This enclosing sheath, called the Myelin sheath, is made largely of lipid, together with some proteins.
- Sheath affects the speed of conduction of the nerve impulse
- Not all axons are protected by myelin.- about 2/3 of motor and sensory neurones are unmyelinated
Nodes of Ranvier
- The small, uncovered areas of axon between Schwann cells
- occur about every 1–3mm in human neurones.
Reflex Arc definition
the pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain.
Reflex Arc process
1) Information picked up by a receptor is transmitted to the central nervous system as action potentials travelling along a sensory neurone
2) The impulse may then be transmitted to a relay neurone, which lies entirely within the brain or spinal cord. (not all arcs have relay neurone)
3) The impulse is then transferred to many other neurones, one of which may be a motor neurone. This has its cell body within the CNS and carries the impulse all the way to the effector
Transmission of Nerve Impulses
- Neurones transmit electrical impulses.
- These impulses travel very rapidly along the cell surface membrane from one end of the cell to the other, and are not a flow of electrons like an electric current.
- Rather, the signals are very brief changes in the distribution of electrical charge across the cell surface membrane called action potentials, caused by the very rapid movement of sodium ions and potassium ions into and out of the axon.
Resting Potential
a difference in electrical potential between the outside and inside of the cell membrane of a neurone
- usually at about -70mV inside
- In a resting axon, it is found that the inside of the axon always has a slightly negative electrical potential compared with the outside
Maintenance the Resting Potential
- resting potential is produced and maintained by the sodium–potassium pumps in the cell surface membrane
- 3 sodium ions are removed from the axon for every 2 potassium ions brought in.
- higher sodium ion conc outside, and higher potassium conc inside
- membrane has protein channels for both ions, but way more for potassium than of sodium (more permeable) so potassium can more easily diffuse back out
- there are many large, negatively charged molecules inside the cell that attract the potassium ions reducing the chance that they will diffuse out.
- The result of these effects is an overall excess of negative ions inside the membrane compared with outside.
Action Potential
rapid, fleeting change in potential difference across the membrane caused by changes in the permeability of the cell surface membrane to sodium ions and potassium ions.
- switches from −70mV to +30mV, and returns to normal after a brief ‘overshoot’
Action Potential in the axon
- An axon is stimulated with an electrical current (from receptor cells responding to stimuli) and causes an action potential to occur
- Channels called voltage-gated channels. open and close depending on the electrical potential (voltage). When at its resting potential, they are closed.
- Sodium ion voltage-gated channels in the cell surface membrane open which allow sodium ions to pass through.
- Sodium ions enter through the open channels into the axon as there is a higher concentration outside
- This changes the potential difference across the membrane, which becomes less negative on the inside.
- This Depolarisation triggers some more channels to open so that more sodium ions enter until the inside reaches a potential of +30mV compared to outside
- This is positive feedback as a small depolarisation leads to a greater depolarisation
- After about 1ms, all the sodium ion voltage-gated channels close, so sodium ions stop diffusing into the axon, and the potassium ion channels open.
- Potassium ions therefore diffuse out of the axon, down their concentration gradient.
- The outward movement of potassium ions removes positive charge from inside the axon to the outside, thus returning the potential difference to normal (−70mV). This is called repolarisation.
- There is then a refractory period
Refractory Period
the time taken for the axon to restore its resting potential after an action potential
- the axon is unable to generate another action potential until the refractory period is over
- it therefore places an upper limit for the frequency of nerve impulses
Consequences of the Refractory Period
- Action potentials are discrete events; they do not merge into one another.
- There is a minimum time between action potentials occurring at any one place on a neurone.
- Action potentials only go in one direction
- The length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones.
Transmission of Action Potentials
- An action potential or nerve impulse that is generated in one part of a neurone travels rapidly along its axon
- This happens because the depolarisation of one part of the membrane sets up local circuits with the areas either side of it, which also causes depolarisation with adjacent parts of the membrane
- the nerve impulse therefore sweeps along the axon
Peak value of action potentials
- However long an axon is, the action potential will continue to reach a peak value of +30mV inside all the way along.
- A very strong light shining in your eyes will produce action potentials of precisely the same size as a dim light.
- The speed at which action potentials travel does not vary according to the size of the stimulus. In any one axon, the speed of axon potential transmission is always the same.
How to determine the strength of a stimulus
determined by the frequency of action potentials
Features of a strong stimulus
- A strong stimulus produces a rapid succession of action potentials, each one following along the axon just behind its predecessor.
- Likely to stimulate more neurones than a weak stimulus and produce more action potentials in many neurones
Features of a weak stimulus
- A weak stimulus results in fewer action potentials per second
- Likely to stimulate less neurones than a strong stimulus so produces action potentials passing along just one or two neurones
Brain interpretation of action potentials
- Can interpret the frequency of action potentials arriving along the axon of a sensory neurone, and the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected.
- The nature of the stimulus, whether it is light, heat, touch or so on, is deduced from the position of the sensory neurone bringing the information.
- If the neurone is from the retina of the eye, then the brain will interpret the information as meaning ‘light’.
Saltatory Conduction
- in a myelinated neurone, local circuits cannot be set up in the parts of the neurone where the Myelin Sheath is present (sodium and potassium ions no entry)
- instead, the nerve impulse jumps from one Node of Ranvier (protein channels and pumps) to the next (1-3mm)
- this is saltatory conduction
- this greatly increases the speed at which the action potential travels along the axon
- Unmyelinated Neurones - speed of conduction
is slow, being as low as 0.5ms−1 - Myelinated Neurone - action potentials travel at
speeds of up to 100ms−1
Factors affecting Speed of Conduction of neurones
1) Myelinated or Unmyelinated
2) Diameter of neurone
How diameter of neurones affects speed of conduction
- Thick axons transmit impulses faster than thin ones, as their resistance is much less
What is the threshold potential of an action potential?
- action potential only generated if the potential difference reaches a value between −60mV and −50mV.
- If it is less than this, then an action potential does not occur.
Chemoreceptors
Are specialised cells found in taste buds which detect a specific type of stimulus and influence the electrical activity of a sensory neurone.
- Each chemoreceptor is covered with receptor proteins that detect these different chemicals.
- There are several types of receptor proteins, each detecting a different type of chemical and giving us a different sensation.
- There are fve tastes: sweet, sour, salt, bitter and umami (savoury).
Chemoreceptors in detecting stimuli and stimulating the transmission of nerve impulses in sensory neurones
1) Chemoreceptors in the taste buds that detect salt are directly influenced by sodium ions.
2) Sodium ions diffuse in the receptor cell, which stimulate calcium protein channels to open in the receptor cell. (when there enough sodium ions to allow calcium channels to open - receptor potential)
3) This allows calcium ions to diffuse in, which cause vesicles with neurotransmitter to diffuse to the cell surface membrane and fuse with the membrane by exocytosis.
4) They release the neurotransmitter cell from the receptor cell to the gap
5) Neurotransmitter diffuse across the gap and binds to neurotransmitter protein receptors
6) The binding of neurotransmitter causes the sodium ion voltage-gated channels to open
7) Sodium ions diffuse in
8) This generates an action potential
