biology topic 8

central nervous system

brain, spinal cord (optic nerve)

peripheral nervous system

nerves outside the CNS

afferent nervous system

sensory, transmits impulses to CNS from internal organs or from those generated by external stimuli

efferent nervous system

motor, transmits impulses from CNS to organs, muscles or glands

somatic nervous system

voluntary responses, stimulate skeletal muscle

autonomic nervous system

involuntary responses - smooth muscle, cardiac muscle and glands

parasympathetic nervous system

rest and digest - decreased heart rate, pupil constriction, bladder contraction

sympathetic nervous system

fight or flight - increased heart rate, pupil dilation, bladder relaxation

neurons

• generate and carry nerve impulses

• in CNS - relay neurons

• in PNS - sensory and motor neurons

neuroglia (glial cells)

• provide structural and metabolic support for neurons

• in CNS - astrocytes, microglia, oligodendrocytes

• in PNS - Schwann cells

multipolar neuron

many dendrites branching from cell body, long axon

bipolar neuron

dendron and axon arising from cell body

unipolar neuron

dendron and axon arising from stem from cell body

motor neurons

• transmit nerve impulses to muscles or glands from CNS

• cell body located in CNS

• axon extends out of CNS

• multipolar

sensory neuron

• carry nerve impulses from sensory cells to CNS

• cell body and dendron outside CNS

• cell body found in dorsal root ganglia

• unipolar

relay neuron

• connect sensory neurons with motor neurons

• mostly found in CNS

• bipolar

Schwann cells

• formed layered covering called myelin sheath

• protects neurons form damage

• speeds up transmission of nerve impulses

nodes of Ranvier

small gaps between Schwann cells

nerves

• a bundle of nerve fibres in the PNS

• include blood vessels

• surrounded by similar fluid to cerebrospinal fluid

reflex

an immediate involuntary response to a stimulus

coordination pathway

stimulus - receptor - coordination - effector - response

coordination pathway in a reflex arc

stimulus - receptor - sensory neuron - relay neuron - motor neuron - effector - response

reflex arc

a neural pathway that control a reflex

what is the advantage of reflex arc pathways?

they produce rapid responses, important for protection and survival

pupil dilation

in dim light, radial muscles contract and circular muscles relax

pupil constriction

in bright light, circular muscles contract and radial muscles relax

consensual reflex

a reflex that is observed on both sides of the body when one side has been stimulated e.g. pupil dilation

why is pupil dilation a consensual reflex?

sensory impulses to the optic nerve stimulate motor impulses in both the right and left oculomotor nerves

what is a nerve impulse?

an action potential propagated in one direction along an axon as a wave of depolarisation

what is an action potential?

a rapid reversal of potential difference across a neuron membrane due to the movement of sodium and potassium ions

resting potential of a neuron

-70mV

how is resting potential maintained?

• Na+/K+ pump creates a concentration gradient by actively pumping 3 Na+ out of the cell and 2 K+ into cell

• K+ diffuse out through K+ channels along concentration gradient

• Na+ cant diffuse in as Na+ channels are closed

time taken for an action potential

1-3ms

action potential - depolarisation

• stimulus leads to change in shape of voltage-gated Na+ channels, opening them

• Na+ diffuses into axon along electrochemical gradient leading to depolarisation of the membrane

• to +40mV

action potential - repolarisation

• voltage gated Na+ channels close

• voltage gated K+ channels open, K+ ions diffuse into axon

action potential - hyperpolarisation

with voltage gated K+ channels open, membrane is more permeable to K+ ions in resting axon, leading to hyperpolarisation - down to -90mV

action potential - return to resting potential

• voltage-gated K+ close

• non voltage gated K+ channels still open so K+ now diffuses back into axon

• Na+/K+ pump helps restore ion concentrations

refractory period

the time after and action potential when the membrane cant react to a new stimulus - 4-8ms

why is there a refractory period?

• creates an upper limit to the frequency of nerve impulses (<100 APs per sec)

• ensures that nerve impulses only propagate in one direction

action potential threshold

stimulus must cause depolarisation above -50- -55 ,V to generate an action potential

all or nothing response

stimulus either triggers full action potential or not at all

impulses strength

stronger stimuli trigger a higher frequency of action potentials

propagation of an action potential along an axon

a local electrical current is formed as the membrane has the opposite electrical charge to the adjacent resting membrane

axon diameter effect on impulse conduction speed

the greater the diameter of the axon, the faster the speed of conduction

myelination effect on impulse conduction speed

myelinated axons conduct nerve impulses faster than non-myelinated fibres

temperature effect on impulse conduction speed

increase in temperature, increase in speed

saltatory conduction

nerve impulses jump from node of Ranvier to node as they can’t form where myelin is present

synapse

a tiny gap across which a nerve impulse passes from one neuron to another

synaptic transmission 1

1. nerve impulse is propagated towards presynaptic membrane

2. Ca2+ channels in presynaptic membrane open and Ca2+ from tissue fluid diffuses into presynaptic bulb

3. vesicles containing neurotransmitter move to and fuse with presynaptic membrane

4. neurotransmitter released by exocytosis

synaptic transmission 2

1. neurotransmitter diffuses across synaptic cleft

2. binding of neurotransmitter to receptor changes shape and stimulates opening of Na+ channels

3. Na+ diffuses into postsynaptic neuron

4. causes depolarisation - if above threshold, an AP is propagated

5. neurotransmitter is broken down and can no longer bind to receptor

6. products diffuse back to presynaptic neuron and taken up by endocytosis

7. used to resynthesise the neurotransmitter

EPSP

excitatory postsynaptic potential - depolarisation of postsynaptic neuron caused by entry of Na+

importance of synapses

• unidirectionality - one direction only

• fatigue - rate-limiting neurotransmitter recycling prevents overstimulation

• filter - must meet threshold

• network - convergence and divergence allow formation of neural networks

excitatory synapses

stimulate Na+ channels to open causing EPSP and transmission of a nerve impulse

inhibitory synapses

stimulate K+ or Cl- channels to open causing hyperpolarisation, blocking transmission of a nerve impulse

summation

when several impulses arrive at a synapse causing an EPSP above threshold

spatial summation

impulses from several different neurons

temporal summation

several impulses from the same neuron

acetylcholine

most common neurotransmitter - in brain and at neuromuscular junctions

neuromuscular junction

• specialised synapse

• Na+ influx across sarcolemma in response to acetylcholine causes depolarisation of the motor end plate

• causes release of Ca2+ from the sarcoplasmic reticulum, initiating muscle contraction

comparing nervous system with endocrine - NS

• electrical and chemical transmission

• transported along neurons

• fast acting response

• short term response

• often very localised

comparing nervous system with endocrine - ES

• chemical signals

• transported through the blood

• slow acting

• longer term response

• often widespread

hormone secondary messenger

when a hormone binds to a cell surface receptor and stimulates a secondary messenger which then activates or inhibits enzyme pathways

tropism

growth response of a plant to a directinal stimulus

phototropism

shoots towards light, roots none or away

geotropism

shoots away from gravity, roots towards

hydrotropism

shoots none, roots towards water

thigmotropism

shoots towards a support, roots away

plant growth substances

chemicals produced in a plant at low concentration and transported to the part of the plant where they produce a response

IAA

growth substance responsible for phototropism, called auxin at GCSE

effect of low IAA concentration

root growth stimulated, shoot growth not affected

effect of high IAA concentration

shoot growth stimulated, root growth inhibited

IAA and phototropism

• produced in shoot tip

• diffuses down shoot

• destroyed by light

• accumulates on shaded side causing cell elongation, which bends plant towards light

cell elongation in plants

• auxin loosens cell walls

• protons are pumped across the cell membrane

• activated an enzyme that allows cellulose molecules to move further apart

receptor cells

cells that are sensitive to a particular stimulus

sense organs

organs that contain groups of receptor cells

primary receptors

neurons with dendrites sensitive to a particular stimulus

secondary receptors

one (or more) specialised cells which are sensitive to a particular stimulus and which synapse with a normal sensory neuron

receptor cell functioning

a stimulus causes either hyperpolarisation or depolarisation of the receptor cell membrane resulting in a generator potential

generator potential

a change in the potential difference across a receptor cell membrane

cornea

clear covering of the eye that helps to refract and focus light

lens

transparent structure behind pupil that refracts and focuses light

pupil

hole in centre of iris that controls the amount of light entering the eye

iris

pigmented tissue, sphincter that controls in amount of light entering the eye

sclera

white outer layer of eye that is a protective layer

conjunctiva

covering over the cornea that protects it

suspensory ligaments

ligaments connecting the ciliary muscle and lens - involved in controlling accommodation

ciliary muscle

ring of muscle connected to lens via suspensory ligaments - control accommodation

retina

inner layer of eye containing rods and cones that respond to light stimulus

optic nerve

sensory nerve that carries nerve impulses from the retina to the brain

blind spot

no light sensitive cells where optic nerve leaves the eye

fovea

most light sensitive part of the retina - contains only cone cells which help with sharp central vision

choroid

black layer behind retina that prevents internal reflection of light

vitreous humour

clear gel that fills eye which helps to keep the retina in place

order of cells in retina

optic nerve, ganglion cells, bipolar cells, photoreceptors - order light hits them

rod cells

• dim light vision

• 120 million rods per eye

• outer segment contains up to 1000 vesicles containing rhodopsin

cone cells

• colour vision - three different wavelengths of light

• 6 million cones per eye

• outer segment contains infoldings of membrane containing iodopsin

components of rhodopsin

opsin (protein) and retinal(pigment)

what happens to retinal in light?

changes shape from cis retinal to trans retinal - cant bind to opsin - bleaching - ATP required to convert back to cis retinal

what is bleaching in the eye?

the breakdown of rhodopsin

activation of rod cells

1. cis retinal converted to trans retinal which no longer binds to opsin

2. transducin active

3. Na+ channel closed

4. Na+ pumped out but cant diffuse in

5. inside cell becomes hyperpolarised (-80mV) —> generator potential