Control and Coordination#

Reflex Arc#

A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain. 不需要大脑思考就可以做出的反应（信号从receptor传递到effector的过程）：

1. receptor接收到stimulus
2. 经过sensory neurone传递信号
3. 进入CNS中的relay neurone
4. 经过motor neurone
5. effector做出反应

反射弧的例子：

• Knee-jerk reflex (removing the hand rapidly from a sharp or hot object)
• Blinking
• Focusing of the eye on an object
• Controlling how much light enters the eye

除了膝跳反应，其他的例子都和眼睛有关，比如：眨眼，对焦和瞳孔收缩

Reflex action的特征：

• automatic 自动的
• involuntary 无意识的
• innate 先天的 (the same stimulus always brings about the same response) 统一
• fast 快速的，无需由大脑思考
• limit damage to body 一般可以避免身体受到伤害

Endocrine System#

Hormones that secreted by endocrine gland travel in blood and reach the target cells.

• Small peptide hormones (ADH, insulin, glucagon) - water soluble, serve as cell signaling molecule
• Steroid hormones (testosterone, oestrogen, progesterone) - insoluble, they can pass through the phospholipid bilayer

Structure#

The structure of sensory neurone

• nucleus is in the cell body / soma
• long dendrites - receive impulses from other neurones
• short axon - conduct electrical impulses to axon terminal
• there are synaptic ends
• many mitochondria
• many ribosomes (the presence of Nissl’s granules)
• myelin sheath made of Schwann cells
• nodes of Ranvier

Sensory neurone的sensory end上有dendrites，来自dendrites的信号先汇聚成一个dendron，再经过nucleus所在的部分后，传递信号的部分叫作axon，最后由synaptic ends将信号传递给下一个神经元。

The structure of motor neurone

• Dendrites on the cell body
• nucleus is in the cell body
• many mitochondria
• many ribosomes
• one long axon
• synaptic knobs at end furthest from cell body
• myelin sheath made of Schwann cells
• nodes of Ranvier

The basic structure of sensory neurone is same as motor neurone, however they also have differences

Resting Potentials#

• resting potentials and action potentials
• depolarization, repolarization and hyperpolarization
• refractory period and saltatory conduction

神经元内部始终保持着负电荷，这个电荷差距被称作resting potential。

The resting potential is usually about -65mV

为了维持这些负电荷，neurone的cell surface membrane会将带正点的阳离子排出细胞之外：

• Sodium-potassium pump pumps 3 $Na^+$ ions out and 2 $K^+$ in against concentration gradient by using ATP 每泵一次正电荷减少一

  WARNING

  Sodium-potassium pump一直在工作，一直在将钠离子运出细胞，把potassium离子带进细胞。哪怕是在voltage-gated channels在开启的状态下。

• Then $K^+$ ions diffuse out and $Na^+$ diffuse in by facilitated diffusion through their protein channels 被泵出去的离子再通过diffusion回到细胞中

• More $K^+$ channels open than $Na^+$ channels, so more $K^+$ goes out than $Na^+$ in 有更多的钾离子会离开细胞，导致电荷不平衡

• Inside of membrane is more negative than outside, membrane is polarised

• The resting potential is about -65mV

• Voltage-gated channels are closed

• To a myelinated neurone, ion movement only occurs at nodes of Ranvier

Action Potential in a Stimulated Neurone#

The electrical impulses are changes in the distribution of electrical charge across the cell surface membrane. 细胞膜上的电荷分布时不均匀的，electrical impulse在传递时会像水波一样在神经元上激起一系列电荷变化。

This rapid change in potential difference across the membrane is action potential.

Action potential is caused by every rapid movement of sodium ions and potassium ions across voltage-gated channels in the membrane.

Voltage-gated channels

• contain proteins that can transport ions
• change shape to open / close when voltage changes
• Each type is specific for a certain type of ions 只针对与某一种ion

Ligand-gated channels

• contain proteins that can transport ions
• change shape to open when ligand bind with it, and vice versa 当ligand与channel结合时，channel会保持打开，反之亦然。
• Each type is specific for a certain type of ions 只针对与某一种ion

The transmission of an action potential in a myelinated neurone:

其中步骤1~4是单个node of Ranvier中发生的事情，7~9是多个nodes配合的过程。

1. $Na^+$ voltage-gated channels are stimulated to open. $Na^+$ enters the axon by facilitated diffusion down an electrochemical gradient, causing depolarization. This cause the potential difference becomes less negative

   Axon部分的细胞膜整体呈负电荷，所以当带正电的$Na^+$进入axon时，会“中和”axon的电负性，丧失电负性的过程被称为depolarization。

2. Then $Na^+$ voltage-gated channels close 进入一些$Na^+$后有直接保持关闭

3. $K^+$ voltage-gated channels open, and $K^+$ ions move out of the cell by facilitated diffusion down an electrochemical gradient, causing repolarization. The potential difference becomes more negative

   这个部分和上面的步骤一类似，只不过会有更多的带正电的$K^+$离开axon，带走负电荷使axon的电负性重新恢复，重新恢复的过程被称为repolarization。

4. Hyperpolarization occurs

5. Myelin sheath insulates axon and prevents ions movement

6. Action potential (depolarization) only occur at nodes of Ranvier

7. Action potentials jump from node to node, this is saltatory conduction 跳跃式传导

8. Local circuits set up between nodes, cause $Na^+$ channels to open and trigger next depolarization at next node of Ranvier

9. The transmission of action potential is one-way transmission due to refractory period 不应期

   node of Ranvier在传递一个信号后，会进入一段“冷却时间”，使它不会再接收信号，也就是说它无法将信号传递给一个刚激活的node（之前将信号传递给当前node的node）。Action potentials are discrete events, they do not merge into one another

   • The length of the refractory period limits the maximum frequency of action potentials (control the frequency) 可以简单类比为“有冷却时间导致无法一直立刻传递新的信号”
   • Action potentials can only travel in one direction along the neurone

The role of sodium ion channel:

• the voltage-gated sodium ion channel changes shape due to the voltage change
• sodium ions diffuse into the cell down the concentration gradient
• the channel closes when the potential reaches +30mV

Depolarization triggers more $Na^+$ voltage-gated channels to open so that more sodium ions enter. There is more depolarisation.

This is an example of positive feedback: a small depolarization leads to a greater and greater depolarization.

当depolarisation发生到一定的程度，超过threshold potential时，再发生action potential。这个要么发生、要么不发生的话，这个被称为all-or-nothing law.

Facts about action potential:

• each action potential has the same size / amplitude 大小一样

• the strength of a stimulus is indicated by frequency of action potentials and the number of stimulated neurone

  信号强度取决于频率和参与传递的神经元数量

• the nature of the stimulus is deduced from the position of the sensory neurone by the brain

• axons with larger diameter transmit impulses faster than thin ones.

  This is because they have a greater surface area over which diffusion of ions can occur, which increases the rate of diffusion and reduces resistance

• Myelinated axon transmits action potentials faster. Speed is increased by 50 times due to saltatory conduction

• Elongated axon helps transmission

NOTE

The ways to increase the speed of transmission:

• Myelin sheath / Schwann cell
• … insulates axon / dendron
• … impermeable to Na⁺ / K⁺
• Depolarisation only at nodes of Ranvier
• … local circuits - action potentials “jump” from node to node
• Saltatory conduction
• Axons with large diameter / giant axon
• Reduce resistance
• Elongated axon / dendron / neurone

Receptors and Synpase#

Receptor (又被成为chemoreceptors) serves as energy transducer, it initiates a receptor potential, if the potential reaches the potential threshold, the action potential will be passed along the nerve (all-or-nothing law).

感受器的例子：

• rod / cone cells in retina - sense sight
• taste buds on tongue - sense taste 多个感搜不同味道的细胞组成在一起，形成一个taste bud

关于receptors是如何产生action potential的：

1. the receptors response to stimuli
2. the receptors are energy transducer
3. the stimuli cause the sodium ion channels to open
4. sodium ions enter the cell
5. depolarize of the membrane occurs
6. the receptors generate the receptor potential
7. if the receptor potential greater than the threshold, then action potential generated (describe all-or-nothing principle)
8. $\uparrow$ stimuli $\uparrow$ frequency of action potential

上面的步骤仅限于发生最开始的action potential，要将信号传递到下一个neurone时，需要通过neurotransmitter来连接synaptic gap：

1. action potential opens the calcium ion channel protein
2. calcium ions enter the cell
3. cause the movement of vesicles containing neurotransmitter (toward the presynaptic membrane)
4. the vesicles fuse with the menbrane
5. neurotransmitters diffuse to postsynaptic membrane
6. bind with the receptors (ligand-gated channel protein) and stimulate the next action potential

The examples of neurotransmitter:

• noradrenaline 去甲肾上腺素
• acetylcholine [acetyl-cho-line] 或者一般简写为ACh
• dopamine
• glutamic acid

在书中一般讨论的是一种使用ACh作为neurotransmitter的synpase，被称为cholinrgic synpase. 这种synpase中有一种酶，叫作acetylcholinesterase [actyl-cholin-ester-ase]，用于hydrolyse水解ACh，是ACh可以被recycle。

The role of acetylcholinesterase in a synpase:

• breaks down acetylcholine

• so ACh leaves the binding site (of the receptor)

• depolarisation stops in postsynpatic membrane\

• … action potential also stops

• ACh is recycled

The roles of synapse in the nervous system:

• one-way transmission
• interconnection of nerve pathways
• involved in memory and learning

How Muscles Contract#

一个sacromere是肌肉收缩的最基本的单位。

当sacrolemma接收到来自motor neurone的信号时：

1. The depolarization of sacrolemma spreads down T-tubules. Calcium ions diffuse out of sarcoplasmic reticulum (the release of calcium ions)

2. Calcium ion binds to troponin. Troponin changes and moves tropomyosin. Myosin head binding sites on action filaments are exposed.

3. Myosin heads bind to these sites, forming cross bridges with action filaments.

4. Myosin heads tilt 倾斜, releasing APT and Pi. Power stroke 动力冲程 occurs, and sarcomere is shortened

5. Myosin head has ATPase (breakdown ATP molecule)

6. New ATP binds to myosin head, myosin head detaches from actin (cross bridges break)

7. Myosin head hydrolyses ATP and tilts back to original position

8. The binding and moving process is repeated

   如果ATP足够、calcium ion足够，那么步骤3~7会重复

9. Sarcomere shortens

10. When stimulation stops, calcium ions are pumped back into the sarcoplasmic reticulum

以上的移动方式被称为sliding filament model.

Typical Questions#

NOTE

The role of tropomyosin:

• tropomyosin uncovers the myosin binding site on actin
• changes shape when calcium ion binded
• allows myosin to form cross-bridge

NOTE

The role of myosin:

• ATP hydrolysis
• causes myosin head to tilt
• forms cross-link with actin
• myosin head returns to previous position, when ADP and Pi detach
• power stroke occurs
• new ATP binds
• myosin head detaches from actin

NOTE

当ATP过少时会发生的事情：

• no attachment of myosin heads
• so no energy transfered to myosin
• so no cross-bridges
• so no power stroke
• so no recovery of power stroke
• 同时，ATP过少的话也无法将calcium ions回收进sarcoplasmic reticulum

ATP在sliding filament model中的precise function:

• ATP binds to myosin head
• ATP will be hydrolysed by ATPase
• … causes the head detaches from actin
• and then the head tilts hack into original position

Antagonist#

Antagonist 拮抗肌 - 在同一关节两侧，作用完全相反的肌肉。当一块收缩时，另一块必须放松，才能完成顺畅的动作

Each skeletal muscle in the body has an antagonist, for example:

• the triceps 三头肌 is the antagonist of the biceps 二头肌

When one of the two muscles contracts, another muscle relaxes, lengthening sarcomere.

Venus Fly Trap#

这种植物通常位于潮湿的环境（wetlands），因为昆虫可以提供土壤中匮乏的mineral ions：

• low mineral ions in the content of soil
• insects provide mineral ions for growth

The specialised lead id divided into 2 lobe either side of a midrib, each lobe has 3 sensory hairs.

• when the trap is open, the lobes are convex 凸的 in shape
• when the trap is closed, the lobes are concave 凹的 in shape

当一个昆虫接触到sensory hair：

1. mechanical energy converted to electrical in sensory hair cell

2. calcium ion channels in the cells at the base of the hair open

3. calcium ions flow in and generate a receptor potential

4. an action potential occurs when:

   • two of the sensory hairs are stimulated at the same time
   • OR one hair is stimulate twice within ~35 seconds

   Then the depolarisation spreads over lobe

5. the cells at the base of the trap change shape, and the trap closes

   sealing the trap requires ongoing activation of the sensory hairs, which occurs when the prey continues to move around inside the close trap

   1. hydrogen ions pumped out of cells into cell walls
   2. cross-links in the cell wall broken
   3. calcium pectate 果胶钙 of middle lamella dissolves
   4. cell wall loosens
   5. calcium ions enter hinge cells
   6. water enters hinge cells by osmosis
   7. cells become turgid
   8. lobs change from convex to concave
   9. trap shuts quickly in 0.3s, elastic tension released

6. further stimulation of the sensory hairs causes calcium ions to enter gland cells, where they stimulate the exocytosis of vesicles containing digestive enzymes

The trap then stays closed for up to a week to allow the prey to be digested and the nutrients from is to be absorbed by the plant.

The adaptation of Venus fly trap

• stimulation of a single hair dose not trigger closure
  • at least two hairs must be touched OR one hair touched twice within in 35 seconds
  • to prevent the closure from the physical stimuli from rains of debris falls into the trap
• Gaps between stiff hairs allow very small insects to crawl out
  • no energy wasted on digesting a very small meal

Auxin#

Auxin participates in cell elongation via acid growth hypothesis

Plants make several chemicals known as auxin, of which the principal one is IAA

Auxin:

• is synthesised in the growing tips of shoots of roots

• stimulates cell elongation

The expansins 膨胀素 break bonds between cellulose microfibrils of the plant cell wall.

The acid growth hypothesis of auxin:

• auxin stimulates proton pumps in the cell surface membrane
• hydrogen ions pumped into cell wall by active transport
• pH of cell wall decreases and pH-dependent enzymes activated
• expansins break the bonds between cellulose microfibrils
• the cell wall becomes more elastic
• water enters the cell wall, so the cell wall expands

Gibberellin#

Gibberellin（由$GA_1$激活的$GA$）参与：

• Stem elongation - cause breakdown of DELLA protein and transcription of growth genes

• Gibberellin synthesis and dwarfness caused by a lack of active gibberellin

• Seed germination 种子发芽

• The major sites of gibberellin production are young roots and leaves

  Gibberellin主要产生在新长的根和叶中

关于GA是如何stimulate stem elongation的：

1. without gibberellin, transcription factor is attached to DELLA protein, which prevents the activation and transcription of genes
2. GA binds to receptor to cause the destruction of DELLA protein
3. PIF (transcription factor) binds to the length of DNA promoter, then RNA polymerase binds to the promoter to start transcription of growth genes
4. Growth genes are expressed, causing:
   • cell division
   • cell elongation
   • increase in internode length

Interaction with auxin and acid growth:

1. expansins are stiumulated
2. cell wall loosens
3. water moves into cells by osmosis
4. cell expands

Explain the control of gibberellin synthesis and why lack of it leads to dwarfness:

1. Gibberellin synthesis is controlled by gene Le/le

2. Dominant allele Le gives functional enzymes which converts inactive GA to active GA

3. Tall plants have dominant allele Le with a genotype of LeLe of Lele to produce active GA which causes stem elongation

4. Dwarf plants only have recessive allele le with a genotype of lele and have no active GA

5. GA1 is the active form of gibberellin