Chapter 3 - Neurons

neurons

nerve cells - receive stimuli + transport information

cell body

cytoplasm mass that contains the nucleus

contains DNA

directs metabolism

dendrites

projections that carry impulses to the cell body

receive impulses from neighbouring neurons

if enough input - the cell can generate an output

axon

projections that carry impulses away from the cell body

tube-like structure = axon cylinder

axon terminal = branches at end that connect to dendrites

myelin sheath

white fatty casing on the axon

produced by Schwann cells

increases the speed of impulses - saltatory conduction

insulates the axon

neurilemma

membrane covering myelin sheath of neurons in PNS

assists in the repair of neurons

nodes of ranvier

gaps in the myelin sheath

functional types of neurons

sensory - carry messages from receptors in sense organs to the CNS

motor - neurons carry messages from the CNS to effectors - muscles or glands

interneurons - located in CNS and link between sensory and motor neurons

structural types of neurons

multipolar - one axon and multiple dendrites extending from cell body - most interneurons in brain and spinal cords and motor neurons

bipolar neurons - one axon and one dendrite - e.g. sensory neurons in the eyes, ears and nose

unipolar neurons - one extension, the axon - found in insects

pseudounipolar - single axon from cell body - separates into two extensions

nerve impulses

electrochemical change that travels along a nerve fibre

involves a change in electrical voltage

bought about by change in chemicals - conc. of ions

voltage

potential energy between two points

current

flow of electrical charge - in the body it refers to the flow of ions

potential difference / membrane potential

when positive and negative charges are separated, they have the potential to come together and release energy - the potential difference

measured in volts or millivolts

membrane potential

the difference in the ion concentration between intracellular and extracellular environments

resting membrane potential is -70mV

membrane is polarised when at rest

ion concentration in the cellular environment

intracellular - low conc. of sodium and chloride ions, high conc. of potassium and large negative anions

extracellular - high conc. of sodium and chloride

sodium = Na+

potassium = K+

chloride = Cl-

cell membrane permeability (channels)

highly permeable to K+ and Cl-

slightly permeable to Na+

impermeable to large anions - the large anions repel Cl- (- and - repel)

K+ tends to diffuse out of the cell, neuron usually negatively charged

ligand-gated channels = channels open in response to chemical change in the form of a neurotransmitter

voltage-gated channel = channels open in response to changes in ion concentration

all channels are selective

sodium-potassium pump

maintains the cellular ion conc.

active process against the concentration gradient = requires ATP

ejects 3 Na+ (into extracellular) and transports in 2 K+ (into intracellular)

net reduction of positive ions inside the cell — increases negativity so is against the conc. gradient

steps of the sodium-potassium pump

1. sodium potassium pump binds three sodium ions and one molecule of ATP

2. split ATP in ADP and one phosphate to provide energy (phosphate binds to channel and ADP leaves) - sodium ions driven through channel

3. sodium ions are released to the outside of the membrane - new shape of the channel allows two potassium ions to bind

   1. release of the phosphate allows channel to revert to original form, releases potassium ions on inside of the membrane

how is membrane potential maintained

1. sodium potassium pump

2. difference in the number of leakage channels for sodium and potassium leakage channels

3. membrane being impermeable to large negative ions

results in intracellular fluid being less positively charged than the extracellular fluid

action potential

1. depolarisation = the sudden increase in membrane potential. This occurs if the level of stimulation exceeds about 15mV (threshold). When a neuron is stimulated by a neurotransmitter or sensory receptor some sodium channels open (ligand-gated or mechanical-gated). This causes more sodium to move into the cell. If the stimulus is strong enough to increase the potential to -55mV, then voltage-gated sodium channels open. Increases to roughly +40mV

2. repolarisation = sodium channels close, stopping influx of sodium ions. Voltage-gated potassium channels open to increase K+ ions flow out of the cell. This makes the intracellular environment more negative than the outside

3. hyperpolarisation = the potassium channels remain open longer than needed, this results in the membrane potential dropping lower than the resting membrane potential

4. refractory period = once sodium channels have been opened they become inactivated and are unresponsive to stimulus. For a brief period after being stimulated the membrane is unable to undergo another action potential - lasts from when the threshold reaches —55mV to resting membrane potential -70mV

all or none response

size of response is not related to strength of stimulus

once membrane potential of —55mV is reached the action potential process occurs the same no matter the size of the stimulus

conduction along unmyelinated fibres

depolarisation in one area of the membrane causes a movement of sodium ions into the adjacent areas, this movement stimulates the opening of the voltage-gated sodium channels in the next part of the membrane

this process repeats itself along the whole length of the membrane

stimulation usually occurs at the end of a fibre so the impulse travels one way

nerve impulse is prevented from going backwards along the fibre by the refractory period

transmission along myelinated fibres

the myelin sheath insulates the nerve fibre from extracellular fluid

this means that ions can only flow in and out of the cell at the nodes of Ranvier. The action potential jumps from one node of Ranvier to the next, this is known as saltatory conduction

this allows conduction at much faster speeds - 140m/s compared to 2m/s

how is the strength of a stimulus determined

a strong stimulus causes depolarisation of more nerve fibres than a weak stimulus

a strong stimulus produces more nerve impulses in a given time than a weak stimulus - more frequent stimulation

transmission across the synapse steps

1. when the nerve impulse reaches the axon terminal it activates the voltage-gated calcium ion channels

2. higher conc. of calcium ions in the extracellular fluid they flow into the cell at the presynaptic axon terminal

3. this causes synaptic vesicles to fuse with the membrane releasing neurotransmitters via exocytosis

4. neurotransmitters diffuse across the gap and attach to receptors on the membrane of the next neuron

5. this stimulates ligand-gated protein channels to open, which allows the influx of sodium ions and initiates an action potential in the post-synaptic membrane

thermoreceptors

located in the skin and the hypothalamus

detect changes in temperature

thermoreceptors in skin detect external temp changes

hypothalamus thermoreceptors detect core temp changes

osmoreceptors

located in the hypothalamus

detect changes in osmotic pressure (concentrations of dissolved substances in blood plasma)

chemoreceptors

located in nose, mouth and internal

detects changes in any chemicals

odours, taste, pH, CO2 and O2 levels

touch receptors/ mechanoreceptors

located in skin

sensitive to touch, receptors deep in skin respond to pressure

pain receptors - nociceptors

located in most organs (not in brain), skin, mucous membranes

stimulated by damage to tissues, or excessive heat/chemical stimuli

reflex

rapid, automatic response to a change in internal or external environment

4 properties of a reflex

a stimulus is required to trigger the reflex

reflexes are involuntary - occurring without conscious though

rapid - only small number of neurons are involved

stereotyped - occurring the same way each time it happens

reflex arc

the receptor reacts to the change in the internal or external environment

sensory neuron carries impulses from receptor to spinal cord or brain

at least one synapse is involved

motor neuron carries nerve impulses to the effector

effector receives nerve impulse and carries out the appropriate response

learnt vs innate reflexes

innate = present from birth e.g. suckling, chewing

acquired = learnt e.g. balancing while riding a bike, catching a ball

endocrine vs nervous system

• endocrine system has a slower response while nervous system has a quicker response

• ES has hormones travelling through the bloodstream whilst NS has nerve impulses travelling along nerve fibres

• ES is longer lasting NS is shorter lasting

• ES affects one organ NS can affect many organs

• ES has a general/ widespread response NS has a local/ specific response

• SIMILARITY = shared substances e.g. noradrenaline, adh and dopamine are both hormones and neurotransmitters