Biology A-level Topic 8: Grey Matter

What are the two regions of the human nervous system?

**Central Nervous System** (CNS) and the **Peripheral Nervous System** (PNS)

What are the two regions made up of?

==CNS==

*consists of brain and spinal cord*

* relay neurones
* intermediate neurones

==PNS==

* motor neurones
* sensory neurones

What does the CNS and PNS do (briefly)?

__PNS__

* **provides** sensory **info** to the CNS

__CNS__

* **process info** and acts as **command centre**

Feature of the CNS

The neurones its made up of **only have connections with other neurones**. (not receptors/effectors)

Feature of the PNS

* the neurones its made up from **connect the CNS to receptors and effectors.**

what are the three types of neurone?

sensory, relay, motor

compare the structures of each type of neurone

__**Sensory**__

* myelinated dendron
* short axon
* cell body in middle

__**Motor**__

* myelinated dendron
* long axon
* cell body at the top

__**Relay**__

* not myelinated (short and found in cns so not needed)
* more dendrites
* small cell body

what is a ==sensory neurone==?

transmit information from the **receptors to the CNS**

*(I’ve touched something lemme tell the brain)*

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* its myelinated therefor fast conducting.

what is a ==relay neurone==?

transmits **electrical impulse from sensory to motor**

what is a ==motor neurone==?

transmit information from the **CNS to effectors**

*(I, the brain, has got the info lemme tell my body to react)*

Motor neurone in the somatic (voluntary)

neurones are **fast conducting** - supplying **skeletal muscles**

Motor neurone in the autonomic (non-voluntary)

neurones are **slow conducting** - supply **smooth muscle** in the **organs, cardiac muscle**.

what two systems consist of the Peripheral nervous system?

**Somatic Nervous System** and the **Autonomic Nervous System**

What is the role of the ==somatic nervous system==?

**contact effectors** (they mostly **contact** the **skeletal muscles)** and is concerned with the **conscious/voluntary activity**

what is the role of the ==autonomic nervous system==

**controls unconscious activities** including **heart rate, smooth muscle and digestive system** *(internal)*

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*(i’m guessing how your internal body reacts to certain things, like if you scared obvs your heart rate will increase)*

what two other systems can autonomic nervous system be divided into?

**Sympathetic** and **Parasympathetic**

what is ==sympathetic==?

**preps body for activity**

known as the **fight or flight response** (*speeds up heart rate, stops digestive system like its prepping you*)

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(idk but think like sims, they always/chaotic on the move)

what is ==parasympathetic==?

preps body for **rest and digest, conserves energy**

you can start to **relax** now and continue with your bodily functions

what is antagonistic and how does it link to sympathetic and parasympathetic?

*one contracts the other relaxes*

as we are in sympathetic mode, parasympathetic mode is ditched

(cause if I were to see nanno walking into class i most definitely wont be relaxing and unbothered lmao)

what is the function of a neurone/nerve cells?

**carry** **electrical signals** in the nervous system.

what is a ==neurone==?

a **single** cell

what is a ==nerve cell==?

more **complex structure** containing a **bundle of the axons** **of many neurones** surrounded by a protective covering

what are the three main parts of a neurone?

* Cell Body
* Dendrites
* The Axon

What is the ==Cell Body?==

__Structure__


1. contains **nucleus** and **other organelles** (eg. mitochondria)

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__Function__


1. **carries out** all normal **functions of the cell** (ATP, protein synthesis etc)
2. ATP is needed for **active transport of ions in and out of the cell** in an **impulse**.

What is the ==Dendrite==?

__Structure__


1. **branches** of the neurone membrane

__Function__


1. **receives signals** from **other** neurones

What is the ==Axon==?

__Structure__


1. **single , long thin extension**
2. **vary** in length
3. surrounded by **lipid layer** (Myelin Sheath)

__Function__


1. carries **electrical signals away** from the **cell body** towards **synapse.**

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Name all the structures of a neurone (8)

* cell body
* nucleus
* dendrites
* axon
* myelin sheath
* schwann cell
* node of ranvier
* axon terminal

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what is the role of the myelin sheath and what is it produced by?

what is it also surrounded by?

* acts as an **electrical insulator** which **speeds rate** of electrical impulse.

**produced/surrounded by schwann cells** (layer of fatty substance-increases speed of impulse)

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*surrounded by protein/lipid covering*

what exactly is the definition of ==myelination==

a layer of **fatty substance** formed from **Schwann cells** wrapped **around** the **neurone**.

what is the ==synapse==?

**gap** between **neurones**

what is the ==reflex arc==?

an **unconscious, involuntary response** to a **stimulus.**

* performed by **components of** the **peripheral nervous system**
* after **action has been performed** **info** is **sent to the brain** so person **becomes aware** what just happened.

what are two examples of reflex arc?

* pupil reflex
* withdrawal reflex - body **withdraws away from a pain**ful stimulus.

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(*the pathway taken by nerve impulses or electrical signals to carry out these responses is called a reflex arc.*)

what is a ==stimulus==?

any **change in the external environment** (touching fire/a sharp knife etc)

give two benefits of the reflex action

* very quick and **rapid response** which **protects body from harm** (survival)
* does not have to be learnt, **effective from birth** (bebe grabbing your finger)

give two reasons why the reflex response is quick

* signals **do not need to go the the brain** and back (doesn’t go to brain hence why its ‘unconscious’)
* does **not involve many synapse**, normally slow down to transmit across neurones

what components are included in the reflex arc? (5)

* receptor
* sensory neurone
* relay neurone
* motor neurone
* effector organ

explain in order the reflex arc (5)

1. **Receptor** detects **stimulus**, generates **nerve impulse**
2. **Sensory neurone** carries **nerve impulse** to the **CNS** (Spinal Chord) along sensory pathway
3. Sensory neurone **synapse with relay neurone**
4. Relay neurone **synapse with motor neruone** and **leaves spinal chord**
5. Motor neurone **carries** **impulse producing a response** as it carries to **effector organ**

Role of the ==Receptor==

* **detects stimulus** and physical change
* turns this **into** an **electrical signal**

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*eg. pain receptors, thermoreceptors, mechanoreceptors, chemoreceptors*

Role of the ==Sensory neuron==

carries nerve impulse/electrical **signal to the CNS**

Role of the ==Relay/Intermediate neurone==

**receive info** from sensory, **process this quick** and decides **response** (should i move my hand or not)

Role of the ==Motor neurone==

carries **nerve impulse to the effector** organ

* *they have a* ***single large axon***

Role of ==Effector organ==

acts in **response to the stimulus**

* *almost always a* ***muscle fibre*** *or a* ***group of muscle fibre***

What is the amount of light entering our eyes controlled by?

the **reflex action**

In light change, what controls the size of the pupil?

**Muscles** of the **Iris**

What are the Iris muscles called?

**Circular**

**Radial**

Circular and Radial muscles are ==antagonistic==, what does this mean?

As **one contracts,** the **other relaxes**

What nervous system are the radial and circular muscles controlled by?

**autonomic** (unconscious, the body does it itself)

What type of autonomic reflex is the radial muscles controlled by?

**sympathetic** (fight or flight, pulls eye open to see in case there is anything spooky!)

What type of autonomic reflex is the circular muscles controlled by?

**parasympathetic** (we always usually in the light so your eye is ‘relaxed’ here)

What happens to the eye when a person is in dim/dark light?

pupil **dilates** *(expand*) to **allow more light to enter the eye**, any light is can get, so we can see a bit clearer.

What do Radial and Circular muscles do in dark light?

* **Radial muscles contract** - ‘**pulling**’ pupil **open** to allow **more light**
* **Circular muscles** are **relaxed** as its not their time to shine

What happens to the eye when a person is in white/bright light?

pupil **constricts** (*squeeze*) to **allow less light to enter the eye**, **protecting** receptors and **retina from damage**

What do Radial and Circular muscles do in bright light?

* **Radial Muscles relax** as its not their time to shine
* **Circular muscles contract** - ‘**squeezing**’ pupil to **allow less light** in (smaller hole of light coming through eye)

What is ==Resting Potential==?

state of nerve fibres when they are **not conducting/transmitting an impulse**

What is the resting potential of a neurone?

**-70mV**

What is the definition of ==potential difference==?

**Difference of electrical charge/potential between two points**. In this case **inside and outside the axon** of a neurone.

What do we mean when we say a neurone has a negative resting potential?

**Inside** of the neuron is **negatively charged** **compared to** the **outside** of the neurone (outside and inside of axon)

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*due to uneven distribution of charges*

The neuron has to be ==polarised== to be -70mV, what does this mean?

That if **one side is positive,** the **other** must be **negative.**

What two components are used in the resting potential?

* Sodium Potassium Pump
* Potassium Gated Ion Channel

What is the role of the sodium potassium pump?

proteins in the cell membrane that **maintain diffusion gradients** for K+ and Na+ **across the membrane**

What type of transport does the sodium potassium pump use? What does it need?

**Active transport,** **ATP** is needed, lots of **mitochondria**

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*therefore this involves hydrolysis of ATP into ADP +Pi*

What are features of the potassium gated ion channel?

* specific to **K+**
* Mainly **open** and there **are more of these** than Na/K+ pump therefore **membrane is more permeable to potassium.**
* Diffuses K+ **faster**.

what two forces are involved in the movement of K+ ions through the K+ channel? what is this force called?

==ELECTROCHEMICAL==


1. __*concentration gradient*__

**How much more K+** there is on the **outside or inside compared to Na+**.


2. __*electrical gradient*__

Is the **charge more positive on the outside or the inside**? the **difference in charges** that drives the movement of K+

Explain the process of resting potential *add picture*

* **Low Na+ and High K+** on the **outside** compared to inside.
* **Sodium Potassium pump** combats this by using **active transport and ATP** to transport **3 Na+** to the **outside** and **2 K+** to the **inside**.
* This **creates potential difference** (more negative as there’s 2 K+ instead of equally 3) and **high concentration of K+** in the **inside**
* High concentration gradient of 2 K+ is on the inside therefore they **go through the potassium channel outside to even this out.**
* Eventually **less 2 K+ inside** so it **goes back down** (through **facilitated diffusion**)
* **Continuous movement of K+** as it reaches **equilibrium/electrochemical gradient of -70mV**
* Ending with **high K+ and low Na+** on the **inside** compared to the outside.

Why at the end of resting potential, the potential -70mV?

* Outside=positive, Inside=negative
* **always higher Na+ at the outside** as there’s **no channel** for that to move around like K+
* There is **no Net movement** (one moves, the other moves)

Why are proteins needed for the resting potential?

membrane is **not permeable** enough

what is an ==action potential==?

a large **change in the voltage** across the **membrane**

what happens when a stimulus (changing environment) is received by a receptor?

causes **temporary reversal** of charges - **depolarisation**

axon membrane goes from **-70mV to +40mV**

what is the definition of ==depolarisation==

**loss of difference of charges** between inside and outside of axon membrane due to **change in permeability and sodium**

what are the three steps of action potential?

stimulus

depolarisation

repolarisation

Action potential/ depolarisation described (5)

1. threshold reached due to **stimulus/increase pearmeability**

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2. **threshold of -55mV** causes **voltage dependent sodium channels** to **open** (norm closed in resting potential)

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3. diffusion of **sodium (Na+) by facilitated diffusion into** the **axon** along **electrochemical gradient** - *they are + and therefore diffuse into the - axon, there are also more k+ inside*

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4. this diffusion of na+ triggers **reversal of potential difference** so becomes **less negative inside +40mV**

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5. action potential is **established**

Action potential/ repolarisation described (3)

1. action potential established at +40mV causes **na+ channels to CLOSE**
2. voltage dependent **k+ channels open**, and many more continue to open **due to electrical gradient** - *too positive inside the axon*
3. causes **repolarisation reducing charge** and making axon **inside more negative**

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Action potential/ hyperpolarisation (6)

1. repolarisation does too much and causes **overshoot of electrochemical gradient**
2. becomes **highly negative** due to many **k+ diffusing out**
3. this state reached **-90mV** (hyperpolarisation)
4. **k+ channels** now **close**
5. **sodium/potassium pump restores**
6. resting potential re-established

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what is the ==refractory period==?

where the nerves are **unable to generate any new action potentials**

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why is the refractory period important?

* **limits** number of impulse transmissions to **prevent over reaction and overwhelmed**


* ac travels in **one direction** \*

what is a ==threshold==? what does it do?

**-55mV**

* if not reached/surpassed, there will be **no ac (all or nothing)**
* **prevents** use from overreacting or getting action potential **when not needed** (not always in active state)

what are 4 factors that affect the propagation of action potential?

* absence of myelin
* presence of myelin
* diameter
* temperature

absence of myelin (3)

* found in **short neurones**
* as waves move down, **earlier parts start to depolarise**
* happens **all the way down towards the terinal end**

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(axon is in **refractory**)

presence of myelin (5)

* found in **longer neurones**
* **increases** transmission through **saltatory conduction**
* **nodes of ranvier** are gaps where **depolarisation** only **occurs**
* one node has **ions flowing**, causes **reduction of potential difference in next** node
* action potential **jumps** from one node to another

why is myelinated axon more effective

* **saltatory conduction** (jumps from one node to another)
* **energy efficient** as it uses less ATP for sodium potassium pump

diameter on propagation of action potential

bigger axon diameter - **faster impulse**

*due to* ***less resistance***

temperature on propagation of action potential

higher temp - **ions diffuse faster**

*if temp to high it can* ***denature carrier proteins***

what is a ==synapse==?

where neurons **communicate with one another** using **neurotransmitters** at junctions

what are two different types of neurotransmitters and their purpose?

**excitatory** - causes postsynaptic to **depolarise** /action potential *triggered*

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**inhibitory** - causes postsynaptic to **hyperpolarise** /action potential *not triggered*

what are the six parts of the synapse area

* synaptic cleft
* presynaptic
* postsynaptic
* synaptic knob
* neurotransmitters receptors
* synaptic vesicles

==synaptic cleft==

**intercellular space** between **axon terminal** and **dendrite** that **contains enzymes** that break down neurotransmitters

==presynaptic neurone==

where the **action potential has come from** (the og)

==post synaptic neurone==

where the **action potential/impulse is recieved**

==synaptic knob==

**swollen end** of axon that **contains mitochondria** and **SER** to **synthesise** neurotransmitters and **make vesicles** (20-30nm)

==synaptic vesicle/neurotransmitter receptor==

**receptor molecule** which neurotransmitter **binds to** in postsynaptic membrane

the process of synaptic transmission of any cell (6) \*picture needed

1. **incoming action potential** causes **depolarisation of knob**
2. this causes **voltage dependent calcium channel to open** allowing **ca2+** to diffuse into through electrochemical gradient
3. this causes **synaptic vesicles to fuse with presynaptic membrane**
4. neurotransmitter **released by exocytosis into the synaptic cleft**
5. neurotransmitter **binds to receptor on sodium ion channels** causing it to **open and release na+** in (action potential begins) this only **happens when threshold is reached**
6. neurotransmitter **taken up across presynaptic membrane** and **closes sodium channels** preventing more and new action potentials

what is a cholinergic synapse?

containing **acetylcholine**

how is acetylcholine recycled?

* **acetylcholinerase** **hydrolyses** into choline and acetate
* these **diffuse back into presynaptic bulb**

what is the ATP from the mitochondria and the SER in the presynaptic knob used for?

ATP - **recombine** acetate and choline

SER - **make** acetylcholine

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*vesicles store this for future*

why is it important for the acetylcholine to be broken down and taken across presynaptic bub

otherwise **channels will stay open** -continuous action potential

another term to use for presynaptic and postsynaptic neuron

pre synaptic - sensory

post synaptic - relay

what are the three roles of the synapse?

* control of nerve pathways, allowing flexible response
* integration of info from diff neurons for coordination
* ensure impulse go in one direction

what two main factors affect the likelihood that the post synaptic membrane will depolarise?

* type of synapse
* number of impulses received

how does the excitatory synapse accept neurotransmitters?

(same steps as depolarisation)

* cell undergoes small amount of depolarisation but not enough to cause an action potential

how does the inhibitory synapse accept neurotransmitters? (4)

* causes hyperpolarisation
* synapse opens chloride and potassium channels
* chloride in and potassium out
* greater potential difference