Organisms respond to changes in their internal and external environments

Cell body

Contains the nucleus, lots of RER, mitochondria, ribosomes.

Required for protein + ATP production for synaptic transmission - active transport + neurotransmitter production.

Dendrites

At the start of a neurone, receive neurotransmitters + chemical messages from synaptic transmission. Branched from dendrons.

Dendrons

Structure which moves towards the cell body or axon + carries signal towards neurone.

Axon

Long fibre extending across the whole neurone carrying impulse towards the axon terminals.

Myelin sheath

Schwann cells which have been tightly wrapped around axon. Insulates axon and speeds up transmission.

Nodes of Ranvier

Junction between Schwann cells. 2-3 micrometers. Speeds up transmission as requires impulse to go through saltatory conduction.

Axon Terminal / Terminal buttons

Marks the end of the pre-synaptic neurone. Neurotransmitter released from here + re-absorbed.

Motor neurone structure

Cell body connected to dendrons + at the start of neurone. Connects to axon, moves down to terminal buttons and then attached to an effector (e.g. muscle).

What is the resting potential?

-70mV (millivolts) when no stimulus present.
Obtained through K+Na+ pumps (active transport) and K+ and Na+ voltage-gated channels.
Pump actively transports potassium into the cell and sodium out of the cell (3Na+ out for 2 K+ in), causing inside to be more negative than outside. K+ channels open to allow for facilitated diffusion (K+ outside)

What is the electrochemical gradient?

The difference between outside of the membrane and inside of the membrane caused by concentration of K+ and Na+ ions inside / outside.

How is an action potential created?

A stimulus is detected and causes some Voltage-gated Na+ channels to open. Na+ ions diffuse into the axon, causing depolarisation (charge becomes less negative).
Is depolarisation reaches threshold value (-55mV) then all Na+ voltage-gated channels to open. Loads more Na+ diffuse into membrane and membrane potential reaches +40mV.
At +40mV, K+ voltage-gated channels open (Na+ close) and repolarisation occurs and K+ enter membrane into axon.
Once -70mV is re-reached, K+ channels slowly close and hyperpolarisation occurs (membrane potential is very negative, overshoots resting potential). Causes refractory period.
In order for membrane potential to return to resting, K+Na+ pump is activated.

Refractory Period

When membrane potential experiences hyperpolarisation and the time taken to return membrane potential to resting = refractory.
Ensures action potentials and firing doesn't overlap and impulses travel in one direction (uni directional).

All-or-nothing principle

Once the threshold potential has been exceeded, all Na+ voltage-gated channels open and an action potential is conducted (all) and if the threshold is not reached, nothing happens (nothing).

How does an impulse travel down an axon?

Na+ ions diffuse into the neuron at one area on the axon. Depolarisation occurs an the Na+ ions (inside the neurone) diffuse into the next section of the neurone, reaching the threshold potential, opening voltage-gated Na+ channels and causing depolarisation in the next section. Process is repeated and impulse travels down axons as a self-propagating wave.
Unidirectional as the prev section has to be return to resting potential before re-stimulation; cannot accept Na+ - refractory period.
Self-propagating wave.

How is the strength of a stimuli measured

Through impulse frequency (rather than magnitude or size). If stimuli does not reach threshold, no impulses created. If stimuli just reaches threshold, impulse frequency lower than if stimuli exceeds the threshold.

Saltatory conduction

Process by which an impulse travels down a myelinated neurone. Depolarisation is only required at gaps between Schwann cells (Nodes of Ranvier). Impulse jumps from Node to Node; speeds up impulse transmission through neurone.

Axon diameter impacts

Larger axon diameter > Small diameter. Large diameter allows for more cytoplasmic room so more ions are able to diffuse into the axon - threshold more easily reached.
Less resistance for ions to diffuse in - more room for organelles to live.
Less ion leakage / ions diffusing back outside the neurone.

Temperature impacts

Higher temp > Lower temp. Increase temp = higher kinetic energy in ions to diffuse, causing faster depolarisation and repolarisation as well as a shorter refractory period (process can be repeated faster).
Increased enzyme for respiration to produce ATP more rapidly; faster return to resting potential by pump.
Too high temp: transport proteins through membrane are denatured. (Voltage gated carriers and pump).

Pre-synaptic neurone

The neurone located before the synaptic cleft. Fire's neurotransmitters as a result of an action potential in order to transmit an impulse across a synapse.

Post-synaptic neurone

Neurone located after the synaptic cleft. Receives neurotransmitters from the synaptic cleft through holding receptor proteins attached to Na+ channels connected in the post-synaptic membrane. Generates an action potential in order to continue message transmission through summation.

(synaptic) Vesicles

Store neurotransmitters in pre-synaptic neurone.

Neurotransmitter

e.g. Acetylcholine - used in cholinergic synapses. Chemical messengers that carry messages across a synapse in order to continue transmission.

Synaptic cleft

A 20nm gap between 2 neurones, which neurotransmitters travel across.

Receptor proteins

Located on Na+ ion channels on post-synaptic membrane. Complementary to neurotransmitters.

Na+ channels

Facilitate the diffusion of sodium channels across the neurone membrane. Have neurotransmitter receptors attached to it.

How does an electrical impulse travel between neurones?

Action potential arrives at axon terminal / pre-synaptic neuron. Voltage-gated Na+ channels open & Na+ ions diffuse into membrane.
Na+ ions fuse vesicles with the presynaptic membrane, release vesicle contents (neurotransmitter Acetylcholine) into synaptic cleft.
ACh diffuse across the synaptic cleft.
ACh binds to complementary receptor proteins attached to Na+ ion channels on post-synaptic membrane, causing channels to open.
Na+ ions diffuse into post-synaptic neurone, reaching the threshold and producing an action potential. Message continues to transmit.

Acetylcholinesterase role

Enzyme that breaks down neurotransmitter (acetylcholine) in order to be re-absorbed into pre-synaptic neurone and placed back into vesicles to be used again.

Inhibitory effects

If receptor proteins are attached to Chloride (Cl-) or Potassium (K+) channels in the post-synaptic neurone, the membrane potential will become more negative and an action potential cannot be formed / action potential is inhibited.

Excitatory effects

Receptor proteins are attached to Na+ ion channels so the membrane potential will become less negative, threshold will be exceeded and action potential will be formed.

Use of ATP in synaptic transmission

In the K+Na+ pump, when restoring membrane potential back to resting as an active (transport process).

How does transmission remain unidirectional?

Neurotransmitters are only located at the pre-synaptic neurone / terminal buttons of a neurone and neurotransmitter receptors are only located at the post-synaptic neurone / dendrites.

Role of summation

To allow an action potential to be generated even when there is low frequency or weak action potentials causing insufficient amounts of neurotransmitter to be released.

Spatial summation

Where there are multiple (2+) pre-synaptic neurones synapsing with 1 post-synaptic neurone. This means even with small amounts of neurotransmitter being released from 1, combination of all 3 will pass threshold potential.
However, if over 50% neurotransmitters are inhibitory from the pre, there may be no action potential generated.

Temporal summation

Where weak action potential frequency from 1 pre to 1 post is increased therefore generating neurotransmitter release more frequently so there is sufficient neurotransmitter in the cleft for depolarisation to pass threshold.
Lots of weak action potential happening in quick succession.

Drugs replicating neurotransmitter shape and binding to receptor sites.

1. They mimic the neurotransmitter and constantly stimulate the ion channels to facilitate the diffusion of (Na+) ion channels to then generate an action potential. Agonists.
2. They block the receptor sites to prevent ion channels being stimulated and ions diffusing into the membrane. No action potentials can be generated. Antagonists.

Drugs that replicate neurotransmitter shape and binding to Acetylcholinesterase (enzyme breaking down ACh).

Inhibits enzyme breakdown therefore build up of neurotransmitter in the synaptic cleft and neurotransmitters remain in receptor proteins for longer therefore more action potentials generated.

Drugs that block voltage gated ion channels.

Prevents (Na+) ions entering the pre-synaptic neurone to fuse vesicles to membrane and release neurotransmitter into the cleft to stimulate post channels which enables action potential generation.

Drugs that stimulate neurotransmitter release.

More neurotransmitter released into synaptic cleft, more receptors + channels activated, more (Na+) ions into post, faster threshold met, more frequent / stronger action potential generated.

Stimulus

Detectable change in the internal / external enviro that leads to a response.

Receptor

A cell or organ that detects change in internal / external environments.

Coordinator

Connects info between receptor and appropriate effector.

Effector

A cell, tissue, organ or system that carries out a response

Response

A change brought about due to stimulus.

Taxis

A directional movement in response to stimulus.

Can be +ve or -ve

‘T’ has 2 straight lines… directional movement

List the prefix’s for -taxis and -kinesis and their meanings

Gravi- Gravity

Photo- Light

Hydro- Water

Chemo- Chemicals

Thigmo- Touch

Thermo- Temperature / heat

Kinesis

A non-directional, random movement in response to a stimulus.

Intensity of stimulus determines the speed of movement and rate of direction change.

‘K’ has multiple lines… multiple directions + random movement

Tropism

A growth movement in plants in response to a directional stimulus controlled by auxin, IAA (plant growth factor / hormone)

Positive tropism: towards stimulus.

Negative tropism: away from stimulus.

Why do tropisms take place

Optimise photosynthesis rate, increase mineral absorption, reduce water loss, prevent stem collapsing, optimise reproduction rates

Positive phototropism in shoots

IAA produced in plant tip which is initially spread evenly down shoot and stimulates cell growth in stem.

As light source is detected and moves (during day), IAA is repelled away and settles in the shaded area of the plant tip.

IAA in shoots stimulates cell elongation in the area of exposure, the shaded side.

This means plant shoots will grow towards light source as elongation determines direction of growth.

Shoots show negative gravitropism.

Role of IAA

Indoleacetic acid. Plant growth factor (auxin) that stimulates or inhibits elongation (growth) of plant cells.

Positive geotropism in roots

IAA is produced in root tip and dispersed down all sides of roots.

Gravity causes IAA to accumulate on lower side of the root.

In roots, IAA inhibits cell elongation and area of exposure (underside of root) does not grow.

This means plant roots will bend downwards as growth is occurring only on upper side.

Roots show negative phototropism.

Reflex

A rapid, automatic response to a stimulus to prevent damage to body tissues. Uses a direct link between receptor and effector and brain is not always needed for response. Reflexes are not learned.

Reflex arc sequence of events

Stimulus → Receptor → Sensory Neurone → Relay Neurone → Effector → Response

Describe signalling by Pacinian Corpuscle

Resting potential is present; less positive inside neurone compared to outside. -70mV

A stimulus is applied / pressure is applied to the corpuscle, distorting lamellae and opening the stretch-mediated Na+ (sodium) channels in membrane. This allows Na+ to enter the corpuscle and depolarise the membrane.

The greater the pressure applied (greater stimulus), the more Na+ channels open and the more Na+ enter the corpuscle. Eventually a generator potential is reached and if threshold is reached (-50mV) the all-or-nothing principle occurs. All Na+ voltage-gated channels open in axon and action potential is reached (+40mV)

Label the eye

Sclera

Vitreous humour

Choroid layer

Optic nerve

Blind spot (no photoreceptors)

Fovea (high conc of cone cells)

Retina (covered with rod and cone cells)

Iris (made from radial and circular muscles)

Aqueous humour

Lens

Pupil

Cornea

Suspensory ligaments (tighten and loosen to regulate lens thickness)

Conjunctiva

Ciliary muscle

Pacinian corpuscle structure and location

They are mechanoreceptors found deep in the skin which detect strong pressures. Consists of a single sensory neurone surrounded by many layers of connective tissues (lamellae) at the end of the sensory neurone which acts as a receptor.

Describe the physiology of vision

Light enters the eye and hits photoreceptors connected to the retina (and fovea). Light is then absorbed by a photosensitive pigment which is ‘bleached’ and broken down.

This eventually stimulates the release of neurotransmitter which is absorbed by a connected bipolar neurone and a generator potential is produced.

If threshold is reached, all-or-nothiwqng occurs and an action potential is propagated along the bipolar neurone, releasing neurotransmitters which synapse with a ganglion cell.

Axons of all ganglion cells join to form optic nerve and visual info is then transmitted to brain.

Rod cells + info transmission

Rod cells are responsible for b&w vision (they are monochromatic).

Highest concentration on cornea (outside of fovea and blind spot).

Rod cells work in low / dim light but have low activity as many rod cells will synapse with 1 bipolar neurone (spatial summation) so enough neurotransmitter for generator potential to reach threshold.

As spatial summation occurs (multiple rods to 1 bipolar), image is not clear and in b&w.

Multiple rod cells = 1 singular transmission to brain.

Photoreceptor pigment (that is ‘bleached’) = rhodopsin.

Cone cells + info transmission

Cone cells are responsible for colour perception (they are trichromatic). They are blue sensitive, green sensitive and red sensitive.

Highest conc found in fovea (none in blind spot).

Cone cells work in high / bright light intensity and have high activity as 1 cone cell synapses with 1 bipolar neurone, enough neurotransmitter is released from 1 cone cell for a generator potential to reach threshold on its own.

1 cone cell = 1 seperate transmission to brain.

Photoreceptor pigment (that is ‘bleached’) = idopepsin

SAN

Sinotrial node. Located in right atrium and receives information from the nervous system regarding heart rate.

AVN

Atrioventricular node. Prevents electrical impulse waves from travelling through atria directly to ventricles and allows blood to be pumped from atria to ventricles.

Bundle of His

A group of fibres connecting the AVN to the Purkyne tissues that travel through the centre of the heart.

Purkyne tissue

Fibers located up the side of the ventricles starting from the apex. When stimulated, they cause ventricles to contract.

Describe how heart rate is controlled

Heart rate is controlled by the autonomic nervous system, Signals are sent via the parasympathetic NS (decr.) or sympathetic NS (inc.) to the SAN. The SAN then sends out a wave of electrical impulses across the atria. As waves pass through the, this causes atria to contract.

Non-conducting tissue separates atria and ventricles so impulse is sent to the AVN. AVN delays impulse, allowing blood to be pumped into the ventricles, making sure that the atria and ventricles do not contract simultaneously.

AVN then passes another electrical wave down the bundle of his, towards the apex of the heart and then into the purkyne tissues which travel up the sides of the ventricles. When the signal travels through the purkyne tissues, this causes the ventricles to contract.

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Myogenic contraction = atria and ventricles contract autonomously.

Sequence of processes that occur to increase heart rate

(Chemo- or Baro-) -Receptors in aortic walls or carotid arteries detect a decrease in xxx. They send impulses to the cardioacceleratory centre in the medulla oblongata (MO).

MO produces more impulses that are sent via the sympathetic NS to the SAN (in

Sequence of processes that occur to decrease heart rate

(Chemo- or Baro-) -Receptors in aortic walls or carotid arteries detect an increase in xxx. They send impulses to the cardioinhibitory centre in the medulla oblongata (MO).

MO produces more impulses that are sent via the parasympathetic NS to the SAN (in

Chemoreceptors

Detect changes in blood pH

Baroreceptors

Detect changes in blood pressure

Response to increase + decrease in blood pressure

Receptor = Baroreceptor in carotid arteries + aorta

Inc. = Slow heart rate

Dec. = Faster heart rate

Response to increase + decrease in pH

Receptor = Chemoreceptors in carotid arteries + aorta

Inc. = Decrease heart rate as lots of O2

Dec. = Increase heart rate as lots CO2 so need to remove

Increased / Decreased exercise effects on pH

Inc. = Lower pH as more CO2 produced.

Dec. = Higher pH as little CO2 in blood and lots of O2 so no need to add O2 to more blood.

Function of muscles

Movement

Maintain balance and posture

Generate heat and contribute to homeostasis

Move substances around the body, e.g. peristalsis (wave of muscle contractions) down the esophagus to the stomach and intestines.

Gross structure of skeletal muscles

Many muscle fibres tog to create bundle of muscle fibres. 1 muscle fibre = cell. Each muscle fibre contains multiple myofibrils (organelles) which contain protein filaments.

Antagonistic pairs

2 muscles that work together, the agonist muscle contracts as the antagonist muscle relaxes and vise versa. e.g. bicep and tricep.

Maintain body or limb position and control rapid movement

Microscopic structure of skeletal muscle fibers

Cell membrane = sarcolemma. Folds inwards to create T-tubules.

Internal membranes = sarcoplasmic reticulum. Stores + releases CA^2+ for transmission.

Many mitochondria

Consists of many myofibrils (long, cylindrical organelles). Myofibrils hold 2 myofilaments; Actin and Myosin.

Ultra structure of a myofibril.

A myofibril is arranged into units called sarcomeres which contain strands of protein myofilaments, myosin and actin.

Z lines separate sarcomeres and actin myofilaments attach to.

I band consists of actin (no overlap), shows lighter.

A band is where actin and myosin overlap, shows darker.

H zone consists of only myosin, shows lighter.

M line is centre of H zone where myosin attaches.

Actin

Thin myofilament.

Consists of 2 strands coiled around each other.

Myosin

Thick myofilament

Rod-shaped fibers connected with globular heads.

Tropomyosin

Associated with acid.

Very thin fibrous strands wrapped around the actin filament.

Troponin

Associated with tropomyosin.

Are protein heads that are attached to tropomyosin.

Neuromuscular junction and transmission

A specialised cholinergic synapse between a motor neurone and a muscle fibre.

Vesicles fuse w the pre-synaptic membrane + release acetylcholine (neurotransmitter) into the synaptic cleft.

ACh then diffuse across the synapse and bind to receptors on the sarcolemma, causing depolarisation.

The depolarisation travels down T-tubules and leads to release of Ca^2+ from the sarcoplasmic reticulum.

Calcium ions then bind to proteins in muscle fibres which causes contractions (see other card).

Acetylcholinesterase in the cleft then hydrolyses the ACh to inhibit contraction, so that contraction only happens when impulses arrive continuously.

Slow Twitch Muscles

Used for endurance work as contractions are slow.

Example: Soleus muscle in lower legs (calf) and back

Large store of myoglobin (high O2 affinity)

Rich blood vessel supply so can obtain oxygen readily for aerobic respiration.

Many mitochondria for aerobic respiration in order to supply muscles with ATP.

Fast Twitch Muscles

Used for intensive exercise and fast contractions.

Example: eyelids / eyes.

High concentration of glycogen and enzymes as a store of Glucose for aerobic respiration.

Large store of phosphocreatine (used to recycle ADP and source of ATP).

Uses aerobic respiration as provided enough ATP for demand of myosin heads for contraction.

Phosphocreatine

Used for maintenance and recycling of ADP to form ATP.

Myoglobin

Protein with a single polypeptide, has a prostatic heme group used for reversibly binding oxygen. Has high oxygen affinity.

Myofibril structure changes during contractions

Every thing shortens, apart from A bands, which stAys the sAme.

How does muscle contraction occur?

Action potential arrives at neuromuscular junction. Ach is released and sarcolemma of muscle fibers is depolarized.

Depolarization travels through T-tubules causing calcium ions to diffuse out of sarcoplasmic reticulum into myofibrils.

Calcium ions attach to troponin molecules (on tropomyosin), changing their shape. This moves the tropomyosin and exposes the binding on the actin.

Myosin heads then attach to actin binding sites and forms cross bridges (actinomyosin bridges).

As cross bridge forms, myosin heads release ADP and Pi which bends the myosin (power stroke), pulling the actin along a short distance.

ATP molecule then attaches to each myosin head, separating it from the actin as changes myosin head shape.

ATP is hydrolysed via enzyme (ATPase) in myosin head which causes recovery stroke and head returns to og position.

Myosin heads repeat process; attaches onto another binding site and slides along the actin more.

Once stimulation of muscles cease (aka no action potential is present), calcium ions are actively transported to sarcoplasmic reticulum and sarcomeres return to og length.

How is ATP supplied to muscle fibers?

Aerobic respiration (mostly oxidative phosphorylation in mitochondria, occurs when plentiful oxygen supply).

Anaerobic respiration (small amnts of ARP formed rapidly via glycolysis, occurs with high intensity exercise)

Phosphocreatine

Phosphocreatine role

ADP + PCr → ATP + Cr (Creatine)

Donates phosphate to combine with ATP to form ATP to supple to muscles.

Generation is v quick during intensive exercise.

Does not require oxygen (therefore anaerobic) however does not produce lactic acid as a by-product.

Homeostasis definition

The maintenance of a constant internal enviro despite changes in the external enviro.

What controls homeostasis?

The nervous system and endocrine system.

Glucoregulation

Blood glucose regulation

Thermoregulation

Body temp regulation

Osmoregulation

Blood water regulation

Why is it important to control body temperature and pH?

To provide an optimum environment for enzyme action and maintain proteins tertiary structure.

Why is it important to control blood glucose concentration?

Provides a stable supply of respiratory substrate for cells and regulates water potential of body fluids.

Why is it important to control blood water content?

To maintain correct water potential for cells.

Describe a negative feedback

A counter action of an initial response which reduces the initial effect of the stimulus and returns the factor level back to its normal value. As the factor gets closer to normal value, level of correlation reduces.

Receptor

Detects a stimulus involved w/ a condition / physiological factor.

Coordination system

Transfers info between different parts of the body.

E.g. nervous system or endocrine system.