biology unit 5 (biol5) nerves

7/30/2019 Biology unit 5 (BIOL5) nerves

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Nervous

system

peripheral

nervoussystem

motor nervous

system

voluntary

nervous

system

autonomic

nervous

system

sensorynervoussystem

central

nervoussystem

brain spinal cord

Effector an organ that responds to stimulation by nerve impulse resulting in a change or

response. Need communication often between receptor and effector (hormones/nerves)

Nervous organisation

Neurones:

Cell body contains nucleus and large amounts of rough endoplasmic reticulum which is

associated with the production of proteins and neurotransmitters.

Dendrons small extensions of the cell body which is subdivided into smaller branched

fibres, called dendrites, that carry nerve impulses towards the cell body.

An axon, a single long fibre that carries nerve impulses away from the cell body.

Schwann cells, which surround the axon, protecting it and providing electrical insulation.

They also carry out phagocytosis (the removal of cell debris) and play a part in nerve

regeneration. Schwann cells wrap themselves around the axon many times, so that layers of

their membranes build up around it.


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A myelin sheath, which forms a covering of the axon and is made up of the membranes of

the Schwann cells. These membranes are rich in a lipid known as myelin. Neurones with a

myelin sheath are called myelinated neurones. Some neurones lack a myelin sheath and are

called unmyelinated neurones. Myelinated neurones transmit nerve impulses faster.

Node of Ranvier, gaps between adjacent Schwann cells where there is no myelin sheath.

Sensory neurone: transmit nerve impulse from a receptor to an intermediate motor

neurone. They have one Dendron that carries the impulse towards the cell body and one

axon that carries it away from the cell body.

Motor neurone: transmit nerve impulses from an intermediate or sensory neurone to an

effector, such as a gland or muscle. They have a long axon and many short dendrites.

Intermediate neurone: transmit impulses between neurones, for example, from sensory to

motor neurones. They have numerous short processes.

EYE

1. Less cones more rods

2. Fovea max number of cones

3. The periphery is all rods

4. Blind spot has no rods or cones.

Sensory neurones make up optic nerve.

Rods contain rhodopsin which is

stimulated by low light intensities. Rods

have low acuity of vision because several

rods are connected to bipolar neuron so

the sensory neurones are stimulated over

a section of retina compared to the cones.

Rods and cones are the receptors in the eye.

Many rods are connected to one bipolar neurone called retinal convergence.


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Each rod cell is stimulated by low light intensities (black/grey) and therefore

collectively all the rods produce a generator potential above the threshold so that

impulses can be sent along the bipolar neurone.

Low light intensity.

Retinal convergence rod cells are attached to a single bipolar cell therefore there is

a much greater chance the threshold can be exceeded causing an action potential.

Low visual acuity.

3 types of cone cell. Each type has a different form of iodopsin which is stimulated by a

different high light intensity. Therefore cone stimulated by red, blue and green light. Cones

have a very high acuity of vision since most cones are connected to one bipolar neurone.

But cannot be stimulated in low light intensities.

High acuity of vision.

Fovea has the maximum number of cones.

Rod cells Cone cellsRod-shaped Cone-shaped

Greater numbers than cone cells Fewer numbers than rod cells

Distribution more at the periphery ofthe retina, absent at the fovea.

Fewer at the periphery of the retina,concentrated at the fovea.

Give poor visual acuity. Give good visual acuity.

Sensitive to low-intensity light. Not sensitive to low-intensity light.

2 electrodes read -65mv.

This is because themembrane is polarised

i.e. more positive ions are

outside the neurone

compared to the inside

(Axoplasm)

Because:

1. The Na+/K+ pump removes 3Na+ from the Axoplasm compared to the 2K+ that enter

the Axoplasm.

2. The membrane is naturally permeable to K+ therefore more K+ leave the Axoplasm

than Na+ entering. i.e. more K+ leave by diffusion than Na+ entering the Axoplasm.

B. if a stimulus is above the threshold the Na+

voltage gates open and Na+ diffuses into the

Axoplasm therefore the membrane is depolarised.

C. Na+ voltage gates close +40mv and K+ diffuse out

of the Axoplasm and the membrane becomes

repolarised.

D. membrane is hyperpolarised i.e. too many K+ has

left. The Na+/K+ensures resting potential is reached.

65mv


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Summation:

a,s and h are sensory neurones. E.g. a =

sight, s = thirst and h = no predators (using

the example of a mammal who has to get

water from a lake to survive)Action potentials must be propagated along

a, s and h so that collectively there is

sufficient transmitter substance being

released so that depolarisation occurs in T.

See retinal convergence (rods)

Filters out weak stimulus; e.g. a low

frequency of action potentials along T

doesnt generate sufficient transmitter

substance for neurone J to be

depolarised. But a high frequency does.

Inhibitory synapses:

As action potentials arrive at the end of T the transmitter substance causes K+ gates to be

open so K+ diffuses out of J and into the synapse and or Cl- gates open and Cl- to diffuse

into J. This causes Js membrane to be really hyperpolarised therefore no action

potentials.

The Pacinian corpuscle

Is a

mechanoreceptor found in

the dermis of skin

responds to mechanical

pressure.

Produces a

generator potential by acting

as a tranducer.

Single sensory

neurone surrounded by layers of connective tissue to make up the capsule.

When pressure is exerted it becomes squeezed out of shape.

This deforms the sensory nerve ending inside it, causing sodium channels in the

membrane to open up.

a

s

h

T

T J


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An influx of sodium ions into the sensory nerve ending causes its membrane to

depolarise and create a generator potential.

The greater the pressure applied to the Pacinian corpuscle. The greater the

deformation and the more stretch-mediated sodium channels open up and the

greater the depolarisation.

Once the generator potential reaches a certain threshold level an action potential is

generated and transmitted along the axon.

Below the threshold level, only local depolarisation occurs and this is insufficient to

create an action potential.

This has the effect of cutting out minor mechanical stimuli.

The threshold level is exceeded when summation of generator potentials occurs and

triggers an action potential.

Greater pressure upon the Pacinian corpuscle results in an increase in the frequency

of the action potentials produced.

The brain differentiates by using the frequency of action potentials.

Speed of nerve impulses:

Myelin sheath

o Allows saltatory conduction which increases speed of conductance.

Diameter of axon

o The greater the diameter of an axon, the faster the speed of conductance.

This is due to less leakage of ions from a large axon (leakage makes

membrane potentials harder to maintain)

Temperature

o

Affects rate of diffusion of ions.o Higher the temperature the faster the nerve impulse.

o Respiration makes ATP which is used in active transport involves enzymes.

Refractory period:

It ensures that an action potential is propagated in one direction

o An action potential can only pass from an active region to a resting region.

This is because an action potential cannot be propagated in a region that is

refractory, which means that it can only move in a forward direction. This

prevents the action potential from spreading out in both directions, which itwould do otherwise.

It produces discrete impulses

o Due to the refractory period, a new action potential cannot be formed

immediately behind the first one. This ensures that action potentials are

separated from one another.

Limits the number of action potentials

o As action potentials are separated from one another this limits the number of

action potentials that can pass along an axon in a given time.

All or nothing principle: nerve impulses are described as all or nothing responses. There is a

certain level of stimulus known as the threshold values, which triggers an action potential.


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Below the threshold value, no action potential and therefore no impulse is generated. If a

stimulus is above the threshold value it will still only generate one action potential this is

the all part. Frequency of action potential allows differentiation or different neurones will

have different threshold values.

Synapses:

Structure:

Neurotransmitters e.g. acetylcholine

Synaptic cleft

Presynaptic neurone

Postsynaptic neurone

Synaptic knob

Synaptic vesicles

Functions:

They allow; a single impulse along one neurone to be transmitted to a number of

different neurones at a synapse. This allows a single stimulus to create a number of

simultaneous responses.

A number of impulses to be combined at a synapse. This allows stimuli from different

receptors to interact in order to produce a single response.

A chemical is made only in the presynaptic neurone and not in the post synaptic

neurone.

The neurotransmitter is stored in synaptic vesicles and released into the synapse

when an action potential reaches the synaptic knob.

When released, the neurotransmitter binds with the receptor molecules and sets up anew action potential in the postsynaptic neurone.


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Features of the synapse:

Unidirectionality

o

Synapses can only pass impulses in one direction. Summation

o Spatial

o Temporal

o (see above)

Inhibition

o On the postsynaptic membrane of some synapses, the protein channels

carrying chloride ions can be made to open

o This leads to an inward diffusion of Cl- ions, making the inside of the

postsynaptic membrane even more negative than when it is at resting

potential.

o This is called hyperpolarisation and makes it less likely that a new action

potential will be created

o For this reason these synapses are called inhibitory synapses.

Transmission acetylcholine (see text book)

stimulus

e.g you see a

friend waving.

Receptors

light receptors(photorecptors)

in your eyes

detect the wave

CNS

CNS processesinformation and

decides what to

do about it

Effector

muscle cells arestimulated by

the motor

neurones

Response

muscles contract

to make your

arm wave.


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Receptors

Mechanoreceptors

Parcinian corpuscle

o

Detect mechanical stimulio Connected to a sensory neurone

o Wrapped in loads of connective tissue called lamellae

o Once stimulated causes deformation of stretch-mediated sodium channels in

the sensory neurones cell membrane.

o The sodium ion channels open and sodium ions diffuse into the cell creating a

generator potential.

o If the generator potential reaches the threshold it triggers an action potential.

Photoreceptors

Light receptor Eye

Light enters through pupil controlled by iris

Light focused by lens onto retina

The fovea is an area where there are loads of light receptors

Nerve impulses from the photoreceptor cells are carried from the retina to the brain

by the optic nerve, which is a bundle of neurones/

Where the optic nerve leaves the eye is called the blind spot no receptors.

Rods are more sensitive but cones let you see more detail

Rods

o Sensitive to light

o Many rods join to one neurone so many weak generator potentials combine to

reach the threshold and trigger an action potential.

o Low visual acuity

Cones

o Less sensitive

o One cone joins one neurone

o Takes more light to meet threshold to make an action potential

o High visual acuity

biology unit 5 (biol5) nerves

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