Na+ linked symporters import amino acids and glucose into animal cells against high concentration gradients

Operation Model for the two-Na+/one glucose symport Ch 7 part II

Glucose transport against its gradient in the epithelial cells of intestine

Na+ linked antiport Exports Ca+2 from cardiac Muscle Cells In cardiac or muscle, Ca2+ ↑→contraction Normal cytosol is 10000 fold of Ca2+ concentration than Cardiac (10-6 → 10-2 M) 烏本 毛地黃素 Cardiac muscles contain 3Na+/ 1 Ca2+ antiporter Movement of three sodium is required to power the export of one calcium

+ +2 + +2 3Na out+ Ca in 3Na in+ Ca out

maintenance of low cytosolic Ca 2+ concentration i.e. inhibition of Na+/K+ ATPase by Quabain and Digoxin

raises cytosolic Na+ Electrochemical gradient lowers the efficiency of Na+/Ca+2 antiport

increases cytosolic Ca+2 More contraction ( used in cogestive heart failure)

1 Carrier proteins in the plasma membrane regulate cytosolic pH

(pHi) at about 7.2 Cotransporters that regulate cytosolic pH

+ - H2CO3 H + HCO H+ can be neutrolized by 1.Na+/HCO3-/Cl- antiport 2. Cabonic anhydrase

- - HCO3 CO2+OH 3. Na+/H+ antiport There are two mechanisms by which this pH is regulated -H+ is transported out of the cell Na+-H+ exchanger, an antiporter, couples the influx of Na+ to an efflux of H+ - + -HCO3 is brought into the cell to neutralize H in the cytosol + - - Na -driven Cl -HCO3 exchanger uses a combination The activity of membrane transport proteins that regulated the cytosolic pH of of the two mechanisms by coupling an influx of Na+ and memmalian cells changes with pH - + HCO3 to an efflux of Cl and H

Plant vacuole membrane Numerous transport proteins enable plant Movement of water vacuoles to accumulate metabolites and • pH 3—6 Osmosis: movement of water across semipermeable ions • Low pH, maintained by membrane V-class ATP-powered pump Osmotic pressure: hydrostatic pressure uses to stop pyrophosphate-hydrolyzing proton pump (PPi -powered pump) the net flow of water 1 and 2 only presented in plant The H+ pump inside → inside positive → powers mover negative ion move inside; High positive inside When CB concentration > CA →antiport many ion and More positive sucrose → inward

Osmotic pressure

π=RT( CB-CA)

2 Hypertonic solution: the concentration is higher than cytosol 圓鋸齒 Isotonic solution: equal to cytosol Hypotonic solution: lower; and water move to cytosol

Animal cell ace a problem in maintaining their cell volume within a limited range, thereby avoiding lysis

Plant cell: has cell wall → prevent cell shape Turgor pressure (膨壓): osmotic pressure, plasma membrane against water into the cytosol and then into the vacuole turgor pressure supplies rigidity The large forces of turgor pressure are resisted by the strength of cellulose microfibrils in the cell wall

Water channel protein ( aquaporin) Expression of aquaporin by frog oocytes increases their permeability Tetrameric protein Egg move to hypotonic environment •2-nm-long wate Injection aquaporin mRNA selective gate • 0.28nm gate width •Highly conserved arginine and histidine 6 α-helices for in the gate control each subunit • H O for HO bonding2 with cystein Aquaporin 1 erythrocyte 尿崩病 Aquaporin2 kidney cells resorb water from urine; mutation → diabetes insipidus → large volume urine

3 Trans-epithelial transport Import of molecules on the lumen side of intestinal Aquaporins are membrane water channels that play critical epithelial cells and their export on the blood facing sides roles in controlling the water contents of cells. Water crosses the hydrophobic membrane either by simple diffusion or through a facilitative transport mechanism mediated by these specialized proteins. These protein channels are widely distributed in all kingdoms of life, including bacteria, plants, and mammals. Important in osmotic regulation, acting to prevent bursting of the cells whenever there are changes of the exterior salt concentration.

Transcellular transport of glucose from the intestinal lumen into the blood Parietal cells acidify the stomach contents while maintaining a neutral Glucose + normal saline → co-transport for energy supply cytolic pH 1 3 Acidification of the stomach lumen by parietal cells in the gastric lining

2 Basolateral Na+/ K+ ATPase generates Na+ gradient that 3 drives the Symporter

1 P-class Cholera toxin 4 activated by Cl- 2

4 Voltage-gate ion channels and the propagation of action potential in nerve cells Typical morphology of two types of mammalian neurons

Neuron

Action potential

neurotransmitter

100m/sec

• Cell body: • Axon: 軸突 – “Nutrition center.” • Axon terminal – Cell bodies within CNS clustered into nuclei, and in PNS • Dendrites: 樹突 in ganglia. • Myelin: 髓鞘 • Dendrites: • Neuron: 神經元 – Provide receptive area. • Node of Ranvier: 郎氏結 – Transmit electrical impulses to cell body. • Schwann cell: 許旺細胞 • Axon: • Synapse: 胞突 – Conducts impulses away from cell body. • Depolarization: 去極化 – Axoplasmic flow: • Repolarization: 再極化 • Proteins and other molecules are transported by rhythmic contractions to nerve endings. – Axonal transport: • Employs microtubules for transport. • May occur in orthograde or retrograde direction.

5 Action potential

An action potential is triggered by a depolarization of the plasma membrane – that is, by a shift in the membrane potential to a less negative value

Electrical vs Chemical Synapses

Neurotransmitters Receptors ELECTRICAL (gap junction) CHEMICAL (synapse) 1. Ligand gated ion channels • 3.5 nm pre-post distance • 20-40 nm pre-post distance 2. G-protein coupled receptors • Cytoplasmic continuity (連續性) • No Cytoplasmic continuity • Gap-junction channels • Synaptic cleft (Ca2+ depend • Ion current neurotransmitter release) • No synaptic delay • Presynaptic vesicles and Synaptic vesicle: postsynaptic receptors • Bidirectional direction of • Chemical transmitter Storage of neurotransmitter. transmission • Smooth and cardiac muscles • 3 ms synaptic delay Low pH of vesicle lumen powers entry of • Unidirectional direction of neuritransmitter into lumen by H+/protein antipoter transmission • Functional connection between a neuron and another neuron (or effector cell such as muscle, gland)

6 Chemical synapse

Gap junction

Depolarization of the plasma membrane due to open of gated Na+ channel All cells maintain a resting membrane potential (RMP): – Potential voltage difference across membrane. • Largely the result of negatively charged organic molecules within the cell. • Limited diffusion of positively charged inorganic ions. – Permeability of cell membrane: • Electrochemical gradients of Na+ and K+. •Na+/K+ ATPase pump.

Depolarization: Potential difference reduced (become more positive).

Repolarization: Return to resting membrane potential (become more negative).

Hyperpolarization: More negative than RMP

7 Voltage-gate Na+ channel is very sensitive channel For refractory peroid

Voltage sensing Conformational changes underlying voltage sensing in AP are large and involve movements of arginine residues through the membrane, near the protein lipid interface Voltage gate sodium channel

Voltage gated K+ channel (delayed K+ channel) Action potentials are propagated unidirectionally without diminution

The “ball-and-chain” model of rapid inactivation for a voltage-gated K+ channel

8 All voltage-gate ion channels have similar structures

Nerve cell can conduct many action potentials in the absence of ATP

For voltage-gated channel

Movement of the channel-inactivating segment into the open pore blocks ion flow

No N-terminal ball

Ball and chain model for inactivation of voltage-gated K+ channel Ball and Chain model for rapid inactivation of voltage-gated K Channels

9 Refractory Periods

Action potential is generated when the membrane is locally During the time interval between the opening of the Na+ channel activation gate and the opening of the inactivation gate, a Na+ depolarized by ˜20 mV: channel CANNOT be stimulated. – This is the ABSOLUTE REFRACTORY PERIOD. Na+ goes in, K+ goes out – A Na+ channel cannot be involved in another AP until the inactivation gate has been reset. – This being said, can you determine why an AP is said to be unidirectional.

For this process to occur, the voltage-gated channels should be: (a) highly selective c) voltage sensitive (b) very fast d) have a mechanism for rapid inactivation

10 Formation and structure of a myelin sheath in the peripheral nervous system

Cross link

Glial cells are also called neuroglia or supporting cells. Glial cells CNS glial cells Oligodendrocytes: – - form myelin sheaths around several axons. • 100 billion neurons – occur mostly postnatally. Nodes of Ranvier: • 10x more glial cells! - - unmyelinated areas of the axon located between adjacent oligodendrocytes (or Schwann cells) • Glial cells: - - action potential takes place here

- - Support neurons (physically, and nutrients) PNS Supporting Cells Schwann cells: - - Cover neurons with myelin – - form myelin sheath around one axon

- - Clean up debris – Nodes of Ranvier: – - unmyelinated areas of the axon located between - “housewives?” and … who is in “control?!” adjacent Schwann cells (or oligodendrocytes) – - action potential takes place here

11 Action potentials jump from node to node in myelinate axons Continuous vs. saltatory conduction

Neurotransmitter and receptor and transport proteins in signal MS (multiple sclerosis) transmission at synapses

Neurotransmitter Demyelinating disease packaged → synaptic vesicles → axon terminus → presynaptic cell→ release → bond to receptor → removed by enzyme or transport

12 Exocytosis of synaptic vesicle Cycling of nuerotransmitters and of synaptic vesicles in axon terminals 1. Action potential

+2 2. Influx of Ca triggers release of H+/protein antiport neurotransmitter

13 Multiple proteins Synaptic-vesicle and plasma- Neurotransmitter release participate in docking and membrane proteins important for fusion of synaptic vesicles vesicle docking and fusion Undocked synaptic vesicle

Docked synaptic vesicle Cluster of protein molecules in membrane of synaptic vesicle “Omega” Cluster of Docked synaptic vesicle protein in figures presynaptic membrane

Entry of Fusion pore calcium widens, Molecules of Presynaptic opens membrane neurotransmitter membrane fusion pore vesicle fuses begin to leave with presynaptic terminal button membrane

Signal transmission at nerve terminals When action potential reaches nerve terminal, signal has to be Stimulation → threshold → action potential → pre-synaptic transmitted to target cell (neurone or muscle) neuron → release neurotransmitter → post-synaptic neuron Signal transmission takes place at synapses receptor → response (excitatory or inhibitor) At synapse, pre and post synaptic cells separated from each other by synaptic cleft (~ 20 nm) Electrical signal cannot cross cleft ⇒ converted into chemical signal - neurotransmitter MOIVE-1: AP → NT release Neurotransmitter molecules packaged in membrane bound Moive: synapse synaptic vesicles within nerve terminals When action potential reaches nerve terminal, voltage-gated Ca2+ channels activated in nerve terminal ⇒ Ca2+ influx down electrochemical gradient ⇒ fusion of vesicles with plasma membrane ⇒ neurotransmitters released into synaptic cleft by exocytosis

14 Synapses Conversion of electrical signal into chemical signal presynaptic resting nerve terminal activated nerve terminal nerve terminal presynaptic voltage-gated nerve terminal action Ca2+ channel voltage-gated potential (closed) Ca2+ channel neurotransmitter (open) postsynaptic synaptic membrane synaptic vesicle cleft dendrite of Ca2+ postsynaptic neurone synaptic neurotransmitter neurotransmitter presynaptic cleft receptor released membrane

neurotransmitter synaptic vesicles receptor

postsynaptic Adapted from ECB Fig 12-39 cell 2 μm Fig 12-40 ECB from Adapted

Conversion of chemical signal to electrical signal Conversion of chemical signal to electrical signal Neurotransmitter diffuses across synaptic cleft and binds to activated nerve terminal active synapse neurotransmitter receptors concentrated on postsynaptic membrane of target cell nerve terminal Binding of neurotransmitter to receptors ⇒ change in membrane potential If membrane potential depolarises above threshold ⇒ action potential neurotransmitter Neurotransmitter rapidly removed from synaptic cleft by enzyme neurotransmitter bound to receptor in synaptic cleft degradation or reuptake into terminal ∴ when presynaptic Fig 12-41 ECB from Adapted cell stops firing ⇒ postsynaptic cells stop firing neurotransmitter- gated ion channel change in Neurotransmitter receptors of various types but most commonly (receptor) membrane potential transmitter (ligand)-gated ion channels ⇒ rapid response - ions milliseconds postsynaptic cell

15 Excitatory and inhibitory synapses Signaling at synapse id terminated by degradation or Excitatory neurotransmitters cause postsynaptic cell to fire reuptake of neurotransmitter action potentials

Inhibitory neurotransmitters prevent postsynaptic cell from 1. degradation firing i.e. acetyocholine Excitatory neurotransmitters (eg acetylcholine, glutamine) act on ion channel receptors selective for Na+ and Ca2+ hydrolyzed by acetyocholineaterase Neurotransmitter binding to receptor ⇒ channel opening ⇒ Na+ influx (may other + ion) ⇒ depolarisation of postsynaptic membrane ⇒ threshold ⇒ action potential 2. reuptake Inhibitory neurotransmitters (eg γ-aminobutyric acid (GABA) i.e.transport into axon terminals by Na+/linked - and glycine) act on Cl channels symport transporters for GABA, norepinephrine, Neurotransmitter binding to receptor ⇒ channel opening ⇒ Cl- dopamine, and serotonin influx ⇒ prevents depolarisation of postsynaptic membrane ⇒ no action potential 3. Diffusion away

Neurotransmitter Removal Cocaine

Inhibited norepinephrine, serotonin, dopamine transporter Why did we want to remove ACh Anti-depressant drugs- fluoxetine (Prozac)→ block serotonin uptake from the neuro- muscular junction? Tricyclic antidepressant desipramine → norepinephrine uptake How was ACh removed from the NMJ? NTs are removed from the synaptic cleft via: – Enzymatic degradation – Diffusion – Reuptake

16 Synaptic vesicles in the axon terminal near the region where neurotransmitter release Neuromuscular Junction When a nerve impulse reaches the neuromuscular junction: 1. Voltage-regulated calcium channels in the axon membrane open and allow Ca2+ to enter the axon 2. Ca2+ inside the axon terminal causes some of the synaptic vesicles to fuse with the axon membrane and release ACh into the synaptic cleft (exocytosis) 3. ACh diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma 4. Binding of ACh to receptors on the sarcolemma initiates an action potential in the muscle ACh is quickly destroyed by acetylcholinesterase

Sequential activation of gated ion channels at a neurotransmuscular Neuromuscular Junction junction

17 A model for the structure of the acetylcholine receptor

The acetylcholine receptors at the neuromuscular junction are transmitter-gated cation channels

Incoming signals must reach the threshold potential to trigger an action potential in post synaptic cells

MAP: microtuble associate protein Synaptotagmin in presynaptic

18 Sensory (afferent) – transmit impulses toward the CNS Motor (efferent) – carry impulses away from the CNS Interneurons (association neurons) – lie between sensory and motor pathways and shuttle signals through CNS pathways

19