15. Control and Coordination Flashcards
Describe the features of the endocrine system, with named examples [3]
The endocrine system consists of:
- endocrine glands, which release hormones into the blood
- hormones, which are specific to hormone receptors on target cells
ADH, glucagon and insulin are peptide/protein hormones that are water-soluble (hydrophilic) and bind to receptors on the outside of their target cells’ membranes
Compare the features of the nervous system and the endocrine system [5]
Nervous System:
1. its components are neurones in nerves, brain and spinal cord
2. imformation is transmitted by electrical impulses in neurones
3. the speed of transmission is very rapid
4. the duration of the effect is often very brief
5. often only affects only a few effector cells - localised effect
Endocrine system:
1. its components are secretory cells in endocrine glands
2. information is transmitted by hormones carried in blood plasma
3. the speed of transmission is usually relatively slow
4. the duration of the effect is often longer term
5. often affects several different target organs - widespread effect
Describe the structure and function of a sensory neurone [5]
- has a cell body, containing the cell’s nucleus, branching off in the middle of the cell
- has myelinated axons (have Schwann cells) on either side of the cell body
- the sensory axon ends with dendrites connecting the cell to receptors
- the other axon ends with dendrites and synaptic knobs connecting the cell to the central nervous system (CNS) across synapses
- their function is to carry impulses from receptors to the CNS
Describe the structure and function of a motor neurone [4]
- has a cell body, containing the cell’s nucleus, at the central nervous system (CNS) end of the cell
- has many highly-branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones
- has a myelinated axon (have Schwann cells) with dendrites connecting the cell to an effector
- their function is to carry impulses from the CNS to effectors
________ connect _______ neurones and _____ neurones
(relay / intermediate) neurones, sensory, motor
Outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones [5]
A sensory receptor cell is one that responds to a stimulus:
- they are transducers (converting energy from one form into energy in an electrical impulse within a sensory neurone)
- receptor cells are often found in sense organs
- some are specific eg. taste receptors (chemoreceptors)
- some receptors are simply the ends of sensory neurones themselves
Describe and sequence the events that result in an action potential in a sensory neurone, using a chemoreceptor cell in a human taste bud as an example [7]
- chemoreceptors in the taste buds that detect salt NaCl respond directly to sodium ions
- when salt is present in the food being eaten, sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptors
- so the membrane of the chemoreceptor is depolarised
- the increase in positive charge inside the cell is known as the receptor potential
- if there is sufficient stimulation by sodium ions and so sufficient depolarisation of the membrane, the receptor potential becomes large enough to stimulate calcium ion voltage gated channels to open
- so calcium ions enter cell and stimulate exocytosis of vesicles containing neurotransmitters from the basal membrane
- the neurotransmitter stimulates and action potential in the sensory neurone, which transmits an impulse to the brain
Describe and explain how the resting potential of neurone membranes is maintained [5]
- resting potential is about -70mV
- sodium-potassium pumps in the axon membrane move 3 sodium ions out of the axon for every 3 potassium ions they move into the axon, using energy from the hydrolysis of ATP to move these ions against their concentration gradients
- many large, negatively charged molecules (anions) remain inside the axon (so more negative inside), attracting potassium ions so they are less likely to diffuse out of the axon, down their concentration gradient
- the impermeability of the axon membrane to ions keeps ions mostly on the side of the membrane they were pumped to, so sodium ions cant diffuse back in when the neurone is at rest
- the closure of voltage-gated channels (required for action potentials) in the axon membrane stops sodium and potassium ions diffusing through the axon membrane
Describe and explain the events that occur during an action potential [7]
- voltage-gated sodium ion channel proteins in the axon membrane open
- sodium ions move into the axon down the electrochemical gradient
- this reduces the potential difference across the axon membrane, as the axon becomes less negative in a process known as depolarisation
- if the potential difference reaches a threshold value of around -50mV or -55mV, many more channels open and allow sodium ions to enter until the inside of the axon reaches a potential of around +40mV
- an action potential is generated
- the depolarisation of the membrane at the site of the first action potential causes current to flow to the next section of the axon membrane, depolarising it and causing voltage-gated sodium ion channels to open
- so another action potential is produced in this section of the axon membrane, and in this way it travels along the axon as the process continues
Describe and explain how the resting potential of neurone membranes is restored during the refractory period [6]
- the refractory period lasts only 5-10 milliseconds in total
- there is a large excess of sodium ions inside the axon, so further influx cannot occur
- very shortly (about 1ms) after an action potential in a section of axon membrane is generated, all the sodium ion voltage-gated channels in this section close (and are actually inactivated), preventing any further sodium ions from diffusing into the axon
- potassium ion voltage-gated channel proteins in this section of the axon membrane now open, allowing potassium ions to diffuse out of the axon down their concentration gradient
- this returns the potential difference to normal (about -70mV) in a process known as repolarisation
- there is actually a short period of hyperpolarisation
Describe and explain the rapid transmission of an impulse in a myelinated neurone [6]
- myelin speeds up the rate at which action potentials travel, by insulating the axon membrane
- sodium and potassium ions cannot flow through the myelin sheath
- so, depolarisation cannot occur in parts of the axon which are surrounded by the myelin sheath
- action potentials can only occur at the nodes of Ranvier, where all the channel proteins and pump proteins are concentrated
- the local circuits exist from one node to the next, so action potentials ‘jump’ from one node to the next
- this is called saltatory conduction
Explain the importance of the refractory period in determining the frequency of impulses [4]
- the period of time when the sodium channels do not respond to depolarisation is the ‘refractory period’
- it ensures that discrete impulses are produced, so action potentials are separate from each other
- it ensures that action potentials travel in one direction only, stopping the action potential from spreading out in two directions which would prevent a response
- limits the number of impulse transmission, which is important to prevent over reaction to a stimulus and therefore overwhelming the senses
Describe the structure of a cholinergic synapse and explain how it functions [10]
- arrival of action potential at the synaptic knob
- calcium ion channels in the presynaptic membrane open
- calcium ions flow in from the synaptic cleft
- calcium ions cause vesicles of neurotransmitter, acetylcholine (ACh), to fuse with the presynaptic membrane and release ACh by exocytosis
- ACh diffuses across the synaptic cleft and binds to a receptor protein in the postsynaptic membrane
- this causes sodium ion channels to open in the postsynaptic membrane
- sodium ions flow into the postsynaptic neurone by diffusion
- depolarisation occurs, if the threshold potential is reached, an action potential is generated
- the enzyme cholinesterase catalyses ACh bound to the receptors into acetate and choline
- choline re-enters the presynaptic knob and combines with acetyl coenzyme A to form ACh once more, which is packaged and stored for re-use
Describe the roles of neuromuscular junctions, the T-tubule system and sarcoplasmic reticulum in stimulating contraction in striated muscle [4]
- the neuromuscular junction functions in the same way as a synapse, just between a motor neurone and an effector
- instead of the postsynaptic neurone, the sarcolemma is depolarised
- the depolarisation spreads from the sarcolemma down the T-tubules
- channel proteins for calcium ions open and calcium ions diffuse out of the sarcoplasmic reticulum, indirectly triggering muscle contraction
Describe the ultrastructure of striated muscle with reference to sarcomere structure [3]
Note: “myo-“ is a prefix for muscle
- a muscle is made of muscle fibres (multi-nucleated, but still only one cell), which are made of myofibrils, which are made of myofilaments
- a sarcomere is the repeating unit of the myofribril
- the two types of myofilaments are actin (thin filament) and myosin (thick filament)
Explain the sliding filament model of muscular contraction [7]
- nervous stimulation triggers the release of calcium ions from the sarcoplasmic reticulum
- calcium ions bind to troponin
- troponin changes shape, allowing
- tropomyosin to move and expose the myosin-binding sites on the actin filaments
- each myosin head uses a phosphate group from ATP to form a cross-bridge with thin, actin filaments
- the release of ADP from the myosin head is the trigger for the power stroke, when actin is pulled along, relative to myosin
- the sarcomere shortens
- when ATP binds to the myosin head, it detaches from the binding site
