13 - Neural Communication Flashcards
What is a stimulus?
A change in the internal or external environment of an organism
What is a sensory receptor?
Specialised nerve cell that is designed to respond to a specific sensory stimulus
What are the two main features of any sensory receptor?
- Acts as a transducer 2. Is specific to a single stimulus
What is a pacinian corpuscle?
They are nerve endings in the skin, responsible for sensitivity to deep pressure touch
What type of energy do sensory receptors convert stimuli into?
Electrical energy
What are the 3 main parts of a neurone?
Cell body, dendrons, axons
Why do Pacinian Corpuscles stop responding when pressure is constant?
They are only sensitive to changes in pressure
What is the structure of a Pacinian Corpuscle?
Oval shaped, with many concentric rings of connective tissue which are wrapped around a nerve ending
How do Pacinian Corpuscles work?
Pressure on the skin deforms the rings of connective tissue, which then press against the nerve ending
What 2 organelles are very prevalent within a neurone’s cell body and why?
Mitochondria and endoplasmic reticulum, in order to produce lots of neurotransmitters
What are dendrons responsible for?
Transmitting electrical impulses towards the cell body
What are axons responsible for?
Transmitting electrical impulses away from the cell body
How many axons and dendrons does a sensory neurone have?
One of each
How many axons and dendrons does a relay neurone have?
Many short ones of each
How many axons and dendrons does a motor neurone have?
One long axon and many short dendrons
What is a myelinated neurone?
One covered in a myelin sheath (many layers of cell membrane)
What is the function of a Schwann Cell?
Produce myelin sheaths by growing around the neurone many times
What is the resting potential of a neuron?
-70mV
How many times quicker can a myelinated neurone conduct a nerve impulse than an unmyelinated neurone?
Up to 100 times quicker
What is the purpose of a myelin sheath?
Electrically insulates the axon of a neuron and increases speed of nerve impulse conduction.
What is a node of Ranvier?
A small gap in the myelin sheath of a nerve, between adjacent Schwann cells.
Why is the transmission of electrical impulses so much faster in myelinated neurones?
The electrical impulse ‘jumps’ between the nodes of Ranvier, which is much quicker than being continuously transmitted along an unmyelinated neurone
What neurones are myelinated in mammals?
Around 1/3 of peripheral neurones
What neurones are not myelinated in mammals?
All of them in the CNS and approximately 2/3 of the peripheral nervous system
What are the 4 main types of sensory receptor in an animal?
- Mechanoreceptor 2. Photoreceptor 3. Chemoreceptor 4. Thermoreceptor
What special type of ion channel is found within Pacinian Corpuscles?
Stretch-mediated sodium channels
How is a generator potential created within a Pacinian Corpuscle?
The change in pressure causes stretch-mediated sodium channels to become permeable to sodium, causing an influx of positive sodium ions which depolarises the membrane
What is resting potential?
The difference in electric charge between the inside and outside of a neuron’s cell membrane when it is not transmitting an action potential
When at its resting potential, why is a membrane said to be polarised?
Because there is a potential difference across it (i.e. a different charge on each side)
Movement of what creates the resting potential?
Sodium and potassium ions across the membrane of the neurone
What distance do myelinated and unmyelinated neurones tend to transmit nerve impulses?
Myelinated tend to transmit further, hence the need for quicker transmission than the shorter, unmyelinated neurones
Why can’t sodium or potassium ions travel across the neuronal membrane via simple diffusion?
They cannot pass through the phospholipid bilayer
How does the sodium potassium pump work?
ATP causes the membrane to transport 3 sodium ions out of the neurone, and this causes 2 potassium ions to enter via the same pump
What does the action of the sodium potassium pump cause in terms of diffusion of sodium and potassium ions?
Potassium ions try to diffuse out of the neurone, sodium ions try to diffuse back in
How is resting potential maintained?
There are more gated potassium channels open in the neuronal membrane than gated sodium channels. This means that more potassium can diffuse out than sodium can diffuse back in, giving a relative negative charge within the neurone
At resting potential, is the inside of the neurone more negative or positive than the outside?
More negative
What happens to the charges on the sensory neurone’s membrane when a stimulus is detected?
They are temporarily reversed
What is the membrane’s charge during an action potential?
40mV
What is depolarization of a membrane?
The change of the membrane’s charge from negative to positive
What is repolarization of a membrane?
The return of the membrane’s charge from positive to negative, restoring the resting potential
What is hyperpolarisation?
Where, during the process of repolarisation, the membrane temporarily becomes more negatively charged than the resting potential
What causes an action potential?
The opening of voltage-gated sodium channels allows sodium ions to enter the neurone, which in turn changes the potential difference of the neurone and causes more voltage-gated sodium channels to open
What causes voltage-gated sodium channels to open, triggering an action potential?
The energy of the stimulus being, converted to electrical energy by the sensory receptor
What is the opening of more and more sodium ion channels during an action potential an example of?
Positive feedback
What occurs when the charge reaches +40mV due to depolarisation?
The voltage-gated sodium channels close and voltage-gated potassium channels open.
What causes repolarisation?
Opening of K+ channels allowing K+ out of the cell, whilst the closing of the Na+ channels means that no more can enter.
What causes hyperpolarisation?
So many potassium ions leave the cell that its charge is somewhat more negative than during resting potential
What occurs after hyperpolarisation?
The voltage-gated potassium channels close and the sodium-potassium pumps restore resting potential
What is a nerve impulse?
An action potential which starts at one end of the neurone and is propagated along it to the other end
How is a nerve impulse propagated along the neurone?
The local electrical currents created by the movement of Na+ ions during depolarisation of one end of the neurone causes the voltage-gated sodium channels a little further along the neurone to open, propagating the nerve impulse
What is the refractory period?
A short period of time after an action potential during which a neurone cannot be stimulated again, as the voltage-gated sodium channels remain closed
Why is the refractory period important in neurones?
Because it ensures that nerve impulses are not propagated back along the neurone as well as forwards, and also that the action potentials do not overlap, but rather exist as discrete impulses
What is saltatory conduction?
The propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.
Why is saltatory conduction more energy efficient?
Because you are opening fewer channels at the nodes, and so less ATP is used in return
What 2 things other than myelination affect the speed of transmission of a nerve impulse?
- Axon diameter 2. Temperature
How does axon diameter affect speed of conduction?
The bigger the diameter of the axon, the quicker the transmission, as there is less resistance to the flow of ions
How does temperature affect speed of conduction in neurones?
Generally, the higher the temperature, the faster the impulse, as ions diffuse faster at higher temperatures. However, this isn’t the case above 40°C as the membrane proteins begin to denature
What type of response are action potentials said to be?
All-or-nothing responses
What is the threshold value for a neurone?
The level of stimulus needed to trigger a response
What is the all-or-nothing principle?
The idea that, if the threshold value is reached, this will always cause an action potential to occur, and that the action potential will always be the same size no matter the size of the stimulus
How does size of the stimulus affect the number of impulses generated in a given time?
Larger stimuli cause impulses to be generated more frequently
Why are axons generally narrow?
To create greater resistance, and thus a greater temperature, which increases transmission speed
Can unmyelinated neurones be wrapped in Schwann cells?
Yes, but generally only one Schwann cell wrapped loosely around a few neurones to make the action potential travel along them in a wave
What are 4 ways a Synapse can be disrupted?
- Agonists- mimic the neurotransmitter 2. Blocking- prevent neurotransmitters reaching receptors 3. Enzyme inhibitors 4. Neurotransmitter inhibitors- block Ca2+ channels in the presynaptic membrane
What type of synapse disruptor is nicotine?
An agonist, as it mimics acetylcholine
What type of synapse disruptor is curare?
A blocker, as it blocks acetylcholine
What type of synapse disruptors are organophosphates?
Enzyme inhibitors
What type of synapse disruptors are opiates?
Neurotransmitter inhibitor
What is a synapse?
The junction between two adjacent neurones
What chemicals are used to convey messages in synapses?
Neurotransmitters
What is the gap between two neurones at a synapse called?
Synaptic cleft
What is the presynaptic knob?
The bulbous end of the presynaptic neurone, where the neurotransmitters are released from
What two organelles does the presynaptic neurone and why?
Mitochondria and endoplasmic reticulum, in order to help it manufacture neurotransmitters
What are synaptic vesicles?
Vesicles containing neurotransmitters which fuse with the membrane of the presynaptic neurone
What are the two main types of neurotransmitter?
Excitatory and Inhibitory
What is an example of an excitatory neurotransmitter?
Acetylcholine
What do excitatory neurotransmitters do?
Depolarise the postsynaptic neurone, approaching or exceeding threshold
What do inhibitory neurotransmitters do?
Cause hyperpolarisation of the postsynaptic neurone, preventing further transmission of the action potential
What is an example of an inhibitory neurotransmitter?
GABA
What two places are cholinergic synapses common?
- The CNS of vertebrates 2. Neuromuscular junctions
What is the neuromuscular junction?
Meeting point of a motor neurone and a muscle cell
What neurotransmitter do cholinergic synapses use?
Acetylcholine
What enzyme breaks down acetylcholine?
Acetylcholinesterase
What does Acetylcholinesterase break acetylcholine down into?
Ethanoic Acid and Choline
What happens to the broken down products of acetylcholine?
They reenter the presynaptic bulb seperately by diffusion, and recombine to form acetylcholine
What are the 3 main functions of synapses?
- Ensures one-way transmission of nerve impulses 2. Interconnection of nerve pathways 3. Memory and learning
What is the sequence of events at a synapse?
- Action potential causes voltage-gated calcium channels in the presynaptic bulb to open 2. Calcium ions cause acetylcholine vesicles to exocytose out of the pre-synaptic bulb 3. Acetylcholine diffuses across synaptic cleft 4. 2 acetylcholine molecules bind to each ligand-gated sodium ion channel on the post-synaptic neurone, causing them to open 5. This causes an action potential 6. Acetylcholinesterase recycles the acetylcholine
Why is the ion used in the presynaptic bulb calcium and not sodium?
To prevent the neurone firing all the time as it would if they were sodium channels
What is synaptic convergence?
When many neurones connect to one neurone information can be amplified (made stronger)
What is synaptic divergence?
When one neurone connects to many neurones information can be dispersed to different parts of the body
What do inhibitory neurones do?
Stop the action potentials of other neurones
What is summation?
Where the neurotransmitter from one action potential is not enough to trigger an action potential in the postsynaptic neurone, so it needs to build up
What does spatial summation prevent?
Overreaction to minor stimuli
What is spatial summation?
Where several neurones connect to one postsynaptic neurone in order to release enough neurotransmitter to cause a response
What does spatial summation allow?
Signals from multiple stimuli to be coordinated into a single response
What is temporal summation?
Where the same presynaptic neurone transmits two or more action potentials in quick succession, increasing the chance that there will be enough neurotransmitter to cause an action potential in the postsynaptic neurone.
What is the peripheral nervous system?
The sensory and motor neurons that connect the central nervous system to the rest of the body
What is the central nervous system?
The brain and spinal cord
What two functional categories is the nervous system organised into?
Somatic and Autonomic
What is the somatic nervous system?
That which is under voluntary control
Where does the somatic nervous system carry signals to?
Skeletal muscles
Where does the autonomic nervous system carry signals to?
Smooth muscle, glands, cardiac muscle
What is the autonomic nervous system?
This regulates events that are automatic, or involuntary. This is often referred to as the involuntary nervous system.
What 2 categories is the autonomic nervous system divided into?
Sympathetic and parasympathetic
What does the sympathetic nervous system do?
Increases bodily functions (in response to fight or flight)
What does the parasympathetic nervous system do?
It creates a reverse response of the sympathetic nervous system to help slow the body down and reduce activity, known as rest and digest
What two things physically protect the brain?
The skull and the meninges (protective membranes)
What is an advantage of having the brain as a central control centre for the body?
Communication between the different neurones is much quicker than if the control centres were spread throughout the body
What are the 5 main areas of the brain?
- Cerebrum 2. Cerebellum 3. Medulla oblongata 4. Hypothalamus 5. Pituitary gland
What does the cerebrum and cerebral cortex control?
Voluntary actions such as learning
What does the cerebellum control?
Unconscious functions such as balance and coordination
What does the medulla oblongata control?
Autonomic control of things such as heart and breathing rate
What does the hypothalamus control?
Temperature, autonomic nervous reflexes, glucose and hormone levels, and the visceral reflexes (digestion, sweating, hunger, thirst, and sleep).
What does the pituitary gland do?
Stores and releases hormones that regulate many body functions
How does the cerebrum coordinate responses?
It receives sensory information, interprets it with respect to prior experiences, then sends impulses along motor neurones to relevant effectors
Why is the cerebrum highly folded?
To increase surface area, in order to increase its capacity for complex activity
Where do the most sophisticated processes of the brain take place?
The frontal and prefrontal lobes of the cerebral cortex
What two things is the cerebrum split into?
Two hemispheres, each controlling half of the body
What are the hemispheres of the cerebrum split into?
Sensory areas, which receive information from receptor cells in sensory areas
What is the size of each sensory area or motor area of the cerebrum in proportion to?
The relative number of receptor cells or motor endings present in the body part
What are association areas?
A region of the cerebrum of the brain that connects sensory and motor areas, and that is thought to be concerned with higher mental activities.
Where do association areas pass impulses onto?
The motor areas of the cerebrum
What are the two centres of the hypothalamus for?
Controlling the sympathetic and parasympathetic nervous systems
What are the two sections of the pituitary gland?
Anterior and posterior
What does the anterior pituitary gland do?
Produces 6 hormones, including FSH
What does the posterior pituitary do?
Stores and secretes hormones that are made in the hypothalamus
What is a reflex arc?
A pathway of neurones that a nerve impulse follows to produce a reflex
What is a spinal reflex?
Where the neural circuit only goes to the spinal cord, not the brain
What is the stimulus for the knee jerk reflex?
The patellar tendon being stretched
What does the body use the knee jerk reflex for?
To help maintain posture and balance
What causes the leg to kick during the knee jerk reflex?
The extensor muscle is stimulated to contract by the reflex, while an inhibitory relay neurone inhibits the motor neurone of the flexor muscle, causing it to relax
What is the stimulus for the blinking reflex?
When the cornea is stimulated, such as by being touched, a bright light appearing or a loud noise occurring
What is a cranial reflex?
One which travels to the brain
What type of reflex is the knee jerk reflex?
Spinal reflex
What type of reflex is the blink reflex?
Cranial reflex
What is the optical reflex?
Blinking as a result of bright light
What is the corneal reflex?
Blinking in response to something touching the cornea
How does the blink reflex work?
The touch causes a sensory neurone (5th cranial nerve) to send a signal to the lower brain stem, where it passes along a relay neurone and along a motor neurone (7th cranial nerve) to initiate a motor response to close the eyelids of both eyes
Why are 4 reasons reflexes are important for survival?
- They are involuntary, so prevents the brain getting overloaded 2. They don’t have to be learnt 3. They have very short reflex arcs, and so are very fast 4. They control many everyday actions
What are the 3 types of muscle cell?
Smooth, cardiac, skeletal
What are skeletal muscles responsible for?
Voluntary movement
What special property do cardiac muscles have?
They are myogenic (can contract without a nerve impulse telling them to)
Are cardiac muscles only found in the heart?
Yes
What parts of organs are smooth muscle cells usually found in?
The walls
What are smooth muscles responsible for?
Involuntary movement
What is actin?
A protein in skeletal muscle that is thin and light
What is myosin?
Thick filament protein with a head and elongated tail, the heads form cross bridges with the thin filaments during muscle contraction
What is myoglobin?
A red pigment which stores oxygen in muscle cells and enables rapid diffusion of oxygen from blood to muscle
What is a myofibril?
Many of these units make up a single muscle fibre. These are made up of sarcomeres.
What is a sarcomere?
The contractile unit of the muscle fibre. From z-line to z-line.
What is the sarcolemma?
Plasma membrane of a muscle cell
What is the sarcoplasm?
Cytoplasm of a muscle cell
What is the sarcoplasmic reticulum?
The endoplasmic reticulum of a muscle cell
What does the sarcoplasmic reticulum do?
Stores and releases calcium
What type of muscle cell is not striated?
Smooth
Is cardiac muscle voluntary or involuntary?
Involuntary
How are skeletal muscles arranged?
Regularly, so that muscle contracts in one direction
How rapidly does each type of muscle contract?
Skeletal contracts most rapidly, cardiac intermediate speed, smooth muscle slowly
How long does each type of muscle contract for?
Skeletal contracts for a short time, cardiac intermediate length, smooth muscle can contract for a long time
What is the structure of skeletal muscle?
Long, tubular, multinucleate cells; obvious striations
What is the structure of cardiac muscle?
Faintly striated, short, branched, one or two nuclei per cell
What is the structure of smooth muscle?
Long and slender, no striations, spindle shaped with a single central nucleus
What are skeletal muscles composed of?
Bundles of skeletal muscle fibres plus connective tissues, nerves and blood vessels.
How are skeletal muscle cells different from most cells?
They contain a number of nuclei and are much longer than normal cells
Why are skeletal muscle cells multinucleate?
They are formed by the fusion of several embryonic muscle cells
What is an advantage of muscle fibres being formed of joined-up cells?
It is stronger as there are no junctions between cells to act as points of weakness
What part of the nervous system controls smooth muscle cells?
The autonomic nervous system
How are smooth muscle cells arranged?
In sheets
Why are cardiac muscles branched?
To ensure electrical stimulation occurs evenly across the walls of the heart
Where specifically in the heart are cardiac muscle cells located?
The walls of the atria and ventricles
What coordinates cardiac muscle cells?
The SAN
What gives skeletal muscle its striated appearance?
The arrangement of actin and myosin fibres
What things do ligaments connect?
Muscles to muscles
What things do tendons connect?
Muscles to bones
What are T-tubules?
Invaginations of the sarcolemma
What is the purpose of T-tubules?
They conduct impulses to the deepest regions of the muscle cell and every sarcomere, in order to ensure that they all receive the signal to contract at the same time
Why are myofibrils lined up in parallel?
To provide maximum contractile force
What makes up thin filaments?
Two twisted actin filaments
What two types of filaments make up myofibrils?
Thin (actin) and thick (myosin) filaments
What makes up thick filaments?
Myosin
Why are myofibrils striated?
Because of the alternating light and dark bands
What are light bands also known as?
I-bands
What are dark bands also known as?
A-bands
Why do light bands appear light?
They are the areas where actin and myosin filaments do not overlap
Why do the edges of dark bands look especially dark?
Because this is where actin and myosin filaments overlap
What is a Z-line?
A band in the middle of the light zone which signals the start/end of sarcomeres
What happens to the sarcomere and H-band when muscle contraction occurs?
They shorten
What is a H-band?
A lighter part of the dark band where only myosin filaments are present
What is wound round each thin filament?
A tropomyosin molecule
What model is used to describe muscle contraction?
The sliding filament model
What two binding sites are located on the myosin head?
ATP and actin
What does tropomyosin do?
Blocks actin-myosin binding site at rest, attached to troponin
What 3 things does troponin bind to?
Actin, tropomyosin and calcium
What structure allows myosin to move?
Hinged heads
What happens to the Z lines during muscle contraction?
They move closer together, shortening the sarcomere
What bonds do myosin heads form during muscle contraction?
Actin-myosin cross bridges
How do actin filaments move during contraction?
The myosin heads pull them over myosin molecules
What triggers muscle contraction?
An action potential arriving at a neuromuscular junction
Why are there many neuromuscular junctions along a muscle?
To ensure that the whole length of the muscle receives the signal to contract at the same time
What is a motor unit?
A single motor neurone and all the muscle fibres it innervates
What happens when an action potential reaches a neuromuscular junction?
Normal cholinergic synapse action occurs, and the sarcolemma is depolarised
What happens when the sarcolemma is depolarised?
Because the T-tubules are in contact with the sarcoplasmic reticulum, they stimulate calcium ion channels to open, which causes the sarcoplasm to be flooded with calcium ions from the sarcoplasmic reticulum
What do the calcium ions which are released from the sarcoplasmic reticulum do?
Bind to troponin, which changes shape and pulls the tropomyosin away from the actin-myosin binding site, unblocking it and allowing actin-myosin cross bridges to form
What does the myosin head do once bound to the actin filament?
Flex, pulling the actin fibre over the myosin fibre
What is released from the myosin head when it flexes?
An ADP molecule
What does the release of an ADP molecule from the myosin head allow?
An ATP molecule to bind to the myosin head
What does the ATP molecule binding to the myosin head cause?
The head to release from the actin-myosin binding site
What activity of the myosin, apart from muscle contraction, does calcium stimulate?
ATPase action
What does the ATPase activity of the myosin filament do?
Allows hydrolysis of ATP, which gives energy to return the myosin head to its original position
What happens when the myosin head releases from the actin-myosin binding site?
It can attach further along the actin filament to continue the contraction
What happens to calcium ions after muscle contraction?
They are rapidly pumped back into the sarcoplasmic reticulum, causing the muscle to relax
What 3 things are used as sources of energy for muscle contraction?
- Aerobic respiration 2. Anaerobic respiration 3. Creatine phosphate
What 2 things require energy in muscle contraction?
- Release of the myosin head 2. Active transport of calcium ions back into the sarcoplasmic reticulum
How is creatine phosphate used to generate energy?
It is used as a reserve supply of phosphate to make ATP
