biology: topic 8

carry impulses from receptors to the central nervous system

conducts impulses from the central nervous system to the effectors

transmit impulses from sensory neurones to motor neurones

located within the central nervous system

contains nucleus and cell organelles within cytoplasm

conduct impulses toward the cell body, collected from other neurones

transmit impulse away from the cell body

aka myelin sheath

made up of schwann cels wrapped around the axon

the change in environment

detects the stimulus

eg photoreceptors, thermoreceptors, chemoreceptors

muscles or glands that carry out the response

what happens in response to the stimuli

stimulus → receptor → sensory neurone → spine → brain → spine → motor neurone → effector → response

skips the spine and brain, instead goes through the relay neurone

autonomic nervous system

• radial muscles

contract to dilate

sympathetic reflex

• circular muscles

contract to contract pupil

parasympathetic reflex

• high light levels hit the photoreceptors in the retina

• causes nerve impulses to pass along the optic nerve

• sends an impulse to nerve sites within the CNS (including coordinating cells in the midbrain)

• impulses sent along parasympathetic motor neurones to the circular muscles

• radial muscles relax to constrict the pupil and reduce the light entering the eye

• low light levels detected by photoreceptors in the retina

• impulses sent down sensory neurone in the optic nerve in the midbrain

• impulses sent along sympathetic motor neurones to radial muscles

• contract to widen the pupil

-70mV

due to the ion distribution

more X- ions inside, X+ outside

sodium-potassium pumps

K+ → cell

cell → Na+

work against the concentration gradient, requiring energy from ATP

chlorine ions move out of the cell to balance the charge, though not actively BY the cell

1. Na+/K+ pump creates concentration gradients across the membrane

2. K+ diffuse outside of the cell down the K+ concentration gradient, making the outside of the membrane positive and inside negative to create a potential difference

3. the potential difference will pull K+ back into the cell

4. at -70mV, the two gradients counteract each other and there’s no net movement of K+

1. as it becomes less negative, voltage gates Na+ channels open and Na+ flows into the axon to depolarise the membrane

2. at +40mV, voltage-dependent Na+ channels close, voltage-dependent K+ channels open

3. K+ leave the axon, repolarising the membrane of the neurone and charge the outside

4. the membrane becomes hyperpolarised as it takes time for the channels to shut (-90mV)

5. K+ diffuse back until resting potential is restored

1. part of the membrane becomes depolarised at the site of the action potential

2. local electrical current is created as Na+ flow between the depolarised part of the membrane and adjacent region

3. depolarisation spreads to the adjacent region

4. nearby Na+ gates open to trigger another action potential

5. repeated along the membrane to cause a wave of depolarisation

due to hyperpolarisation at the end of an action potential, there is a refractory period

a new action potential cannot be generated as there’s too great a difference in charge (-90mV instead of -70mV

this ensures an impulse only travels in one direction

1. depolarised by an action potential

2. channel membranes open, increase membrane permeability to Ca2+

3. Ca2+ concentration is greater outside, so diffuses across the membrane into the cytoplasm

4. increased Ca2+ concentration causes synaptic vesicles to fuse with presynaptic membrane

5. neurotransmitter is released into the sunaptic cleft by exocytosis

1. neurotransmitter diffuses across the synaptic cleft and reaches the postsynaptic membrane

2. binds to complementary shaped receptor

3. receptor changes shape to open cation channels, making the membrane permeable to Na+

4. this flow causes depolarisation, the extent of which depends on the amount of neurotransmitter reaching the membrane and number of receptors on it

• some neurotransmitters are actively taken up and reused by the presynaptic membrane

• others rapidly diffuse away from the synaptic cleft

• some are taken up by other cells or broken down by enzymes so can no longer bind to receptors

the wider the diameter, the faster the impulse travels

• due to myelination with schwann cells, there are gaps along the axon called nodes of ranvier

• depolarisation can only occur at these places

• the impulse jumps from one node to the next

• this is much quicker than depolarising along the whole membrane

no

the stimulus must be at or above the threshold level to generate an action potential

• as long as it is at or above, the size of impulse generated is the exact same regardless of stimulus size

• frequency of impulses

• number of neurones in a nerve conducting impulse

eg strong stimulus → high frequency and many neurones

• control of nerve pathways, allowing flexibility of response

• integration of information from different neurones to allow a coordinated response

• type of synapse

• number of impulses received

• excitatory synapse

help stimulate an action potential

• inhibitory synapse

make it less likely for a postsynaptic membrane to depolarise

a postsynaptic cell can have both types of synapse, generation depends on the balance of the synapses at any one time.

• make the membrane more permeable to Na+

• a single synapse does not depolarise the membrane enough for an action potential

• several impulses arriving within a short amount of time will do, however

  • this happens either through spatial summation (many from diff. neurones) or temporal summation (lots from the same neurone)

open Cl- and K+ ion channels, allowing the ions to move down their concentration gradients

• produces hyperpolarisation of -90mV

• action potential is NOT generated as it can’t in a hyperpolarised area

endothelial cells of capillaries are more tightly packed together

• forms blood brain barrier

• aimed to protect it from changes in ionic composition and toxic molecules

• problems occur with an imbalance in chemicalc crossing the barrier

• dopamine released by neurones in the midbrain and is involved in movement

• these neurones’ axons extend to the spinal cord, brainstem and frontal cortex

the dopamine-releasing neurones die, so little dopamine is released into the motor cortex

• resulting in a loss of motor control

• and symptoms such as:

  • muscle stiffness and tremors

  • slowness of movement

  • poor balance and walking problems

• slow the loss of dopamine by protecting dopamine secreting neurones

• treat symptoms with L-DOPA drugs

• dopamine agonists (trigger the same neural pathway)

• gene therapy (does not always accept or retain the new gene)

• deep brain stimluation

  • electrodes placed into the brain and connected to a battery pack in the chest that applies a voltage to trigger the neural pathway

schizophrenia

• hallucinations, delusions

antagonist drugs that block dopamine binding sites on postsynaptic receptors, NOT stimulating them

• can cause side effects of symptoms of parkinson’s

• NOT parkinson’s itself as the neural cells are still alive

neurotransmitter that plays a role in determining mood

the neurones that secrete it are found in the brain stem

• axons extend into the cortex, spinal cord and cerebellum

linked to depression, along with noradrenaline

fewer nerve impulses than normal are transmitted around the brain, so lower levels of neurotransmitter released

• molecules needed for seratonin synthesis are present in only low concentrations

• seratonin binding sites are more numerous to compensate for the low levels of the molecules

• monoamine oxidase inhibitors (MAOIs)

enzymes that break down neurotransmitters are inhibited, maintaining seratonin levels

(rarely used now)

• selective seratonin reuptake inhibitors (SSRIs)

inhibits reuptake of seratonin from synaptic clefts

maintain higher levels of seratonin, increasing the rate of nerve impulses

there may be a gene known to increase susceptibility that may be triggered by environmental factors

→ twin studies

→ epigenetic causes

chemicals with similar molecular structure to a particular neurotransmitter is likely to bind to the same receptor site

• from this it could stimulate the postsynaptic neurone

• the chemicals may also prevent the release of a neurotransmitter, block or open ion channels or inhibit the breakdown of enzymes

MDMA impacts thinking, mood and memory

• increases seratonin concentration in the synaptic cleft by binding to the molecules in the presynaptic membrane

  • prevents the reuptake of seratonin into the membrane

• euphoria and enhanced senses

• clouded thinking and agitation

• sweating

• fatigue

• rapid heart rate

• insomnia and depression

  • as cells cannot meet the seratonin demand that MDMA increases

neurotransmitter that binds to postsynaptic neurone to change their shape, allowing sodium ions so diffuse in via the newly opened sodium ion channel

eg IAA

responsible for phototropisms, geotropisms and growth responses

produce in low concenrations, then transported to produce the response

• root tip → inhibits elongation

• shoot tip → promotes elongation

moves towards shaded side

promoted elongation of cells on shaded side

curves towards the light

positively phototropic

promotes elongation of cells

auxin moves down with the pull of gravity

promotes elongation of cells downward

negatively geotropic

auxin moved to the shaded side

inhibits elongation

root moves away from the light

negatively phototropic

auxin moves away from the gravitational pull

inhibiting elongation

root grows down

positively geotropic

absorb red and far-red light

consists of a protein component, bonded to a non protein light absorbing pigment molecule

phytochrome red (660nm)

Pr + red light → Pfr

phytochrome far red (730nm)

Pfr + far red light → Pr

Pfr

hence overnight it reverts to Pr

• seed germination

• stem elongation

• leaf expansion

• chlorophyll formation

• flowering

when exposed to far red light, Pfr converts to Pr and germination is inhibited

red light triggers germination

if flashed with f.r light, germination is inhibited

if flashed again, germination is re-triggered, proving that the effects are reversible

relative day/night length and environmental cue determining time of flowering

• the Pr:Pfr ratio in plant allows it to internally determine the length of days and nights

• short days give enough time for Pfr → Pr

eg strawberries

associated with the summer

when there is darkness less than 12 hours

reqiure Pfr to flower, therefore not enough time for it to convert to Pr

eg poinsettias

requires uninterrupted darkness greater than 12 hours to give enough time for all Pfr → Pr

Pfr inhibits flowering

• shoots undergo greening once the shoot breaks through the soil into sunlight

• once in the light, phytochromes promote development of primary leaves and pigment

• need Pfr for chlorophyll production

each activated phytochrome interacts with other proteins, causing either binding to the protein or disrupting binding of a protein complex

short day plants

no flowering

germination

long day plants

chlorophyll formation

it is a signal protein that acts as a transcription factors to enable the usual transciption pathway

neurone cell bodies

neurone fibres

• controls higher functions

• thinking, feeling, seeing and learning

• mainly grey matter

• folded cortex to give a large surface area

• divided into lobes

joined at the centre with a band of axons called the corpus callosum

• emotional response, planning ahead, reasoning and decision making

• the ‘conscious’ area of the brain

• last to be fully developed

• primary motor cortex, controlling body movements via motor neurones passing through the hindbrain and spinal cord

• auditory information

  • near to the ears

• visual information

• input from the eyes to deal with vision, shape recognition, colour and perspective

  • at the back of the brain

• memory recognition

• ability to calculate

  • sense of movement and orientation

• controls the autonomic nervous system

• thermoregulation

• right in the centre of the brain

• monitors:

  • blood chemistry

  • hormone secretions of the pituitary gland

  • basic drives → thirst, hunger, aggression and reproductive behaviour

• larger structure attached to hypothalamus

• routes all incoming sensory information to the correct parts of the brain

• lays down long term memory

  • underneath the hypothalamus

• coordinates smooth motor movements

  • uses info from muscles and ears for posture and balance

• the most primitive part of the brain

• controls reflex centres:

  • heart rate

  • blood pressure

  • sneezing

  • digestive muscles

• maintains basic life responses even where major areas of the brain are damaged

• bottom of the skull, down the back of the neck

• will not be considered ‘dead’ until the medulla is no longer functioning

• producing frozen pictures of the brain to identify structures to detect brain disease

• monitor tissues over the course of an illness

1. narrow beam X-rays rotate around the patient

2. the strength of the beam varies depending on the density of the tissue it is passing through

3. X-rays are detected to produce an image

• diagnosis of tumors, brain injuries, strokes and infections

• MRIs have better resolutions than CT scans so more detailed images of the brain can be produced

1. magnetic fields and radio waves detect soft tissue

2. in a magnetic field, nuclei of atoms line up with the direction of the magnetic field

3. H atoms are monitored due to the high water content in the tissues and they line up with the magnetic field

4. energy absorbed by the H ions is detected and analysed by the computer to produce an image

• makes it possible to study human activities

• can also be used to follow the sequence of events over a short period of time

1. increased neural activity results in an increase in O2 absorption from the blood, reducing the signal received by the computer

2. the less signal absorbed, the higher activity in that area

3. different ares of the brain light up on the image when they are active

• evaluate the structures and functions of tissues and organs

• diagnosis of cancers, heart disease, brain disorders

• monitors spread of cancers and observe the effect of treatment

1. patient injected with a radiotracer (short half life isotopes incorporated into glucose or water that will bind to receptors)

2. as it decays it emits positrons

3. when a particular area is active, there is increased blood flow, so more radiotracers are present in that area

4. release of gamma rays as they collide with positrons that are converted into an image on the computer