8: GREY MATTER

What is a neurone?

A single highly specialised cell in the nervous system

What is a nerve?

A complex structure consisting of a bundle of the axons of many neurones, surrounded by a protective covering

What are dendrites?

The fine branched extension of a nerve cell that transmits impulses towards the cell body

What are Schwann cells?

A type of cell that wraps around the axons of neurones of the peripheral nervous system and forms insulating fatty myelin sheaths around them

What are the nodes of Ranvier?

microscopic gaps in the myelin sheath

What is the function of the sensory neurone?

To carry impulses from a receptor towards the CNS

What is the structure of a sensory neurone?

Cell body is a ganglion - sticking out from the axon

Long dendrites

Short axon

What is the function of a relay neurone?

to carry impulses from one part of the CNS to another

What is the structure of a relay neurone?

Central cell body

Short dendrites

Long axon

What is the structure of a motor neurone?

Cell body at one end

Short dendrites

Long axon

What is the function of a motor neurone?

carry impulses from CNS to effector (muscles or glands)

What is a reflex arc?

A rapid involuntary response to an external stimulus.

Direct connection between sensory and motor neuron without connection to brain

Outline the order of the reflex arc

Receptor
Sensory neurone
Sensory neurone enters spinal cord through dorsal route
Sensory synapses with relay
Relay synapses with motor neurone
Motor neurone leaves spinal cord through ventral route
Effector

What is the sympathetic nervous system?

the division of the autonomic nervous system that arouses the body, mobilizing its energy in stressful situations

What is the parasympathetic nervous system?

the division of the autonomic nervous system that calms the body, conserving its energy

What is the autonomic nervous system?

Division of the peripheral nervous system that regulates events that are involuntary, such as the activity of the smooth and cardiac muscles and glands.

What is the peripheral nervous system?

The nervous system outside the spinal cord and the brain - so the sensory and motor neurons that connect the central nervous system to the rest of the body

What happens to the pupil in dim light?

Dilates

Radial muscles contract

Circular muscles relax . sympathetic nervous response

What happens to the pupil in bright light

Pupil constricts

Radial muscles relax

Circular muscles contract. parasymphatic nervous response

What controls circular muscles?

Parasympathetic nervous system

What controls radial muscles?

Sympathetic nervous system

What are the names of the antagonistic muscles in the iris that can control the size of the pupil?

Radial muscles

Circular muscles

What is the definition of resting potential?

The difference in electric charge between the inside and outside of a neuron's cell membrane when the neurone is not involved in the passage of an impulse

What is the value for resting potential?

-70 mV

When a neurone is at resting potential, what is it said to be? Compare the electrical charge between the inside and outside of an axon?

Polarised, -ve inside +ve outside

How is resting potential created and maintained?

Sodium-Potassium creates uneven distribution of ions across cell surface membrane by pumping 3Na+ out of the cell and 2K+ into the cell

Concentration gradient is established. K+ ions diffuse out of the cell via facilitated diffusion

Relative to outside of the cell, the inside is negative - an electrical gradient in established

K+ ions diffuse out of the cell due to the concentration gradient generated by the pump but diffuse back into the cell due to the electrical gradient

When the potential difference across the membranes is -70 mv there is an electrochemical equilibrium resulting in no net movement of K+ ions

T/F: The cell surface membrane is impermeable to Na+ ions, unless voltage gated channels for Na+ are open

True

What is an action potential?

when the potential difference across the membrane is locally reversed - making the inside of the axon positive and the outside of the axon negative

What are the 4 stages of an action potential that wold be visible on a graph showing time against potential difference

Resting Potential

Depolarisation

Repolarisation

Hyperpolarisation

(back to resting potential)

Define Depolarisation

a temporary reversal of potential across the membrane of a neurone such that the inside becomes more positive than the outside

Outline the series of changes that occur during depolarisation

Axon is stimulated

Change in potential difference across membrane causes Na+ voltage-dependent gated channels to open

As Na+ enters the axon, depolarisation increases - causing more Na+ gates to rapidly open, allowing more Na+ ions in (positive feedback)

Potential difference increases to +40 mV

Define 'Repolarisation'

Restoring the potential difference across the membrane so that inside the axon is more negative than outside the axon

Outline the changes that occur in repolarisation.

Voltage-dependent K+ gates open after the Na+ ones shut

They leave the axon - diffusing down the concentration gradient but also attracted to the negative charge outside the cell

The potential difference drops down to -90mV, causing the neurone to be hyperpolarised

Define hyperpolarisation

The potential difference of the neurone is more negative than it's resting potential

How is resting potential restored once a neurone has been hyperpolarised?

The voltage-dependant K+ gates close and K+ diffuses back into the axon

How does hyperpolarisation inhibit further action potentials?

By increasing the size of the stimulus that would be needed to reach the action potentials threshold stimulus size

Outline Impulse Propagation

Part of the membrane becomes depolarised at the site of an action potential

local currents are set tup - Na+ moves into the adjacent polarised region - causing a change in the potential difference across this part of the membrane

A second action potential is triggered at the neighbouring site to the original

At the site of the first action potential, Na+ channels close and K+ channels open - repolarising this part of the membrane

Site of first action potential becomes hyperpolarised

3rd action potential is triggered by the second

Simultaneously, resting potential is triggered at the first site

What is the 'refractory period'? how long and until when does it last for?

a period of time (5 ms) following stimulation during which a nerve or muscle is unresponsive to further stimulation.

It lasts until all the voltage-dependent K+ and Na+ channels have shut

What does the refractory period ensure?

that the impulse moves along an axon in one direction and isn't propogated the way it came , since the impulse can only move to a region where the membrane is polarized

What is meant by the phrase 'action-potentials have an all-or-nothing effect'

action potentials are all the same size regardless of the strength of the stimulus

Define 'Threshold Stimulus' and state the value needed for it.

The minimum stimulus needed to open Na+channels and thereby achieve an action potential

-50 / -55

What 2 factors do the strength of a stimulus effect?

The number of neurones in a nerve that are conducting impulses

The frequency of impulses

Generally, how does axon diameter affect the speed of nervous conduction?

The wider the axon the faster the impulse travels (less resistance, like a wire)

Define Saltatory Conduction?

The propagation of action potentials along myelinated axons from one node of ranvier to the next.

Outline Salatory Conduction

Myelin sheath acts as an electrical insulator, preventing the flow of axons across the axon and thereby stopping action potentials

At nodes of ranvier there is no myelination so the surface membrane is exposed. These are the site of the voltage-dependant gates. Action potentials can occur here.

ions flow across the membrane at one node during depolarisation, setting up a circuit which reduces the potential difference of the membrane at the next node thereby triggering an action potential there

This way, action potentials effectively jump from one node to the next

Define 'Impulse Velocity'

The speed at which an impulse is travelling

T/F: Unmyelinated neurones have higher impulse velocity than myelinated neurones

False

Myelinated neurones have higher impulse velocities than unmyelinated neurones

Name a common neurotransmitter

acetylcholine

Outline what occurs to cause the release of neurotransmitter into the synaptic cleft

Depolarisation of a pre-synaptic knob causes calcium ion channels to open

Calcium ions diffuse across the membrane into the cytoplasm, causing vesicles containing neurotransmitter to fuse with the pre-synaptic membrane and release their contents into the synaptic cleft via exocytosis

How may a neurotransmitter cause an action poteintial in a post-synaptic membrane?

Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors embedded on the post-synaptic membrane

This causes a conformational change in the shape of the protein, causing cation channels to open and Na+ diffuses into the cell

What is a cation?

A positively charged ion

How is acetylcholine (ACh) removed from the synaptic cleft?

Enzyme acetylcholinase breaks down ACh at the post-synaptic membrane, presenting it from binding to receptors.

Some of the broken down products are reabsorbed by the pre-synaptic membrane and are re-used

What are the general ways that neurotransmitters are 'inactivated'. 3

Pre-synaptic membrane actively takes them up via reuptake molecules

Other nerves of the nervous system take them up

Others rapidly diffuse away from the synaptic cleft

What are the roles of the synapse? 2

To control nerve pathways, allowing for flexibility of response

To integrate info from different neurones - allowing for a co-ordinated response

What determines if an action potential is generated in a neurone?

The balance / overall effect of excitatory and inhibitory synapses

What 2 factors affect whether a post-synaptic membrane is depolarised or not?

The type of synapse

The number of impulses recieved

What are excitatory synapses?

Synapes that make the post-synaptic membrane more permeable to Na+ ions and are more likely to generate an action potential

Define summation and explain why it occurs?

The additive effect of several impulses to collectively generate an action potential as often, one impulse is not enough to generate enough neurotransmitter to depolarise the post-synaptic membrane

Define spatial summation

Impulses from several different neurones to produce an action potential in the post synaptic neurone

Define temporal summation

Several impulses from one neurone producing an action potential in the post-synaptic neurone

What are inhibitory synapses and how do they, generally, achieve their function?

Synapses which make it less likely that a new action potential will be created on the post-synaptic neurone by making the inside of the membrane hyperpolarised and therefore harder to depolarise as more excitatory synapses are needed to depolarise the membrane

Outline how inhibitory synapses prevent action potentials occuring

The neurotransmitters for these cell open up Cl ion and K ion channels

These ions move down their concentration gradients, Cl- moves into the cell whilst K+ leaves the cell

The membrane becomes hyperpolarised (more negative) making depolarisation less likely

How does hormonal control and nervous control differ in terms of transmission of information?

NERVOUS: electrical transmission by nerve impulses and chemical transmission at synapses

HORMONAL: chemical transmission through the blood

How do nervous control and hormonal control differ in terms of the way in which target areas are stimulated?

NERVOUS: Action potential is carried by neurones with connections to specific cells

HORMONAL: blood carries the hormone to all cells, but only target cells can respond to it

How do nervous control and hormonal control differ in terms of where their responses occur?

NERVOUS CONTROL: response is local ie - a specific gland or muscle cell

HORMONAL CONTROL: response is quite widespread - ie in growth and development

How do nervous control and hormonal control differ in terms of the length of the changes they occur?

NERVOUS: short term changes ie - muscle contraction

HORMONAL CONTROL: long term changes ie - growth

T/F: Hormones are fast acting whilst nerve impulses are slow acting

False!

Hormones are slow acting

Nerve impulses are fast acting

What secretes hormones into the blood?

Endocrine glands

What are hormones?

A chemical messenger, transported in the blood that influence the development, growth or sex of an animal

Where are hormones made?

Golgi apparatus found within endocrine gland cells

In what 2 forms are hormones made?

An inactive form

OR

packaged within a secretory vesicle that fuses with the cell membrane and expels its contents via exocytosis

How do peptide hormones act on their target cells?

Bind to receptors on the cell membrane, triggering a secondary messenger, which is a transcription factor.

This causes a change in the proteins made.

How do steroid hormones act on their target cells?

They bind directly to receptors.

The hormone-receptor complex acts as a transcription factor

What is a transcription factor?

a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence.

T/F: The cell membrane is permeable to proteins but impermeable to lipids

False!

impermeable to proteins as they are too large

permeable to lipids

What do plants use to co-ordinate growth, development and responses to their environment as they don't have nervous systems?

Plant growth regulators ie - auxins for growth

How did Charles Darwin find that 'some influence' is transmitted from the shoot tip to its lower parts?

Cutting of the tip of the plant stopped it from bending towards the light.

Replacing the tip makes the plant restart growing towards the light

What did Boysen-Jensen discover when agar jelly containing auxin was put directly on top of a plant without a shoot tip in a dark room?

It would grow vertically upwards

What did Boysen-Jensen discover when agar jelly containing auxin were put on one side of a plant missing its tip?

The plant would curve away from the side receiving the chemical messenger

Give an example of an auxin name.

Indoleacetic acid (IAA)

What model explains why plant shoots to grow towards light?

Cholodny-Went Model

Outline the Cholodny-Went Model

Growth curvature occurs due to the unequal distribution of auxin.

auxin produced in shoot top and difffuses down

Though the amount of auxin produced on the shaded and unshaded side are equal, auxin accumulates on the shaded side of the plant - causing cell elongation there

As a result, the plant grows towards light

How do auxins cause cell elongation?

bind to receptors, triggering secondary signal molecules that activates a transcription factor

This stimulates the activity of proton pumps that move H+ ions out of the cytoplasm and into the cell wall

The low pH activates proteins 'expansin' which disrupt the hydrogen bonds holding together cellulose microfibrils and hemicelluloses

Cell wall 'loosens' there is a slippage of polysaccharides relative to each other and bonds reform in new places, allowing the cell to expand

Acidification of the cell wall increases the potential difference across the cell, enhancing the uptake of ions into the cell

The movement of these ions cause the uptake of water into the cell via osmosis

The cell swells - causing elongation

What type of hormones are auxins?

peptide hormones

What is the conjuctivita?

Thin protective covering of epithelial cells; preventing the cornea against damage from friction

What is the cornea?

Transparent, curved layer at the front of the eye which bends light

What is the sclera?

The white, outer layer / tough supporting wall of the eye

What is the choroid and where is it found??

black pigmented network of blood vessels used to supply the eye with nutrients and remove waste

Found between the retina and the schlera

What is the ciliary body?

cilliary muscles + suspensory ligaments

These muscles control the shape of the lens

What is the iris?

The pigmented muscular structure between the cornea and the lens. It consists of the circular and radial muscles - they control the amount of light entering the eye

What is the pupil?

the hole in the middle of the iris that light passes through

What is the lens?

The transparent, flexible, curved structure behind the iris that focuses incoming light rays onto the retina using its refractive properties

What is the retina?

The light-sensitive layer of photoreceptors (rods and cones) + sensory neurones that trigger nerve impulses that pass via the optic nerve to the brain

What is the blind spot?

the point at which the optic nerve leaves the eye, creating a "blind" spot because no receptor cells are located there

What is the vitreous humor?

the transparent jellylike tissue filling the eyeball behind the lens.

maintains eye shape
suspens the lens, ensuring it isn't damaged

What is the aqueous humor?

the clear fluid filling the space in the front of the eyeball between the lens and the cornea.

It is secreted from the ciliary body

How do our eyes focus light coming from a near object?

Light rays from nearby objects diverge as they approach the eye

The lens is round and fat so that more refraction occurs for the light rays to be focused more precisely on the retina

For this to occur, the suspensory ligaments are slack and the ciliary muscles contract

How do our eyes focus light coming for a far object

Light rays from a far away object are nearly parallel as they approach the eye

Less refraction is needed to focus the light so the lens is flat


For this, the suspensory ligaments are taut/contracted and the ciliary muscles are relaxed

T/F: Cones work in bright light and dim light

False, rods work in bright light and dim light, cones work in bright light only

What type of vision do cones allow?

Colour vision in bright light