Chapter 13 - Neuronal Communication

What are factors of the internal environment?

• blood glucose concentration

• internal temperature

• water potential

• cell pH

What are factors of the external environment?

• humidity

• external temperature

• light intensity

• new or sudden sound

What are the types of responses?

• animal

  • electrical (neurones)

  • chemical (hormones)

• plant

  • chemical (number of)

What is homeostasis?

• functions of organs to maintain a constant internal environment

Cell signalling

• nervous and hormonal systems

• one cell releases a chemical which effects a target cell

• can transfer signals locally (between neurones at synapses - neurotransmitter)

• can transfer signals across large distances (hormones)

How does plants coordination work?

• plants use hormones as no nervous system

What do neurones do?

• transmit electrical impulses

• provide responses to changes in internal & external environment

Features of sensory neurones

• cell body

  • nucleus surrounded by cytoplasm

  • lots of endoplasmic reticulum and mitochondria (produce neurotransmitters)

• one dendron

  • carries impulse to cell body

• one axon

  • carries impulse away from cell body

Features of relay neurones

• many short axons

  • carries impulse away from cell body

• many short dendrons

• carries impulse to cell body

Features of motor neurones

• one long axon

  • away from cell body

• many short dendrites

  • to cell body

Role of neurones

• sensory

  • receptor —> relay/motor/brain

• relay

  • between neurones

• motor

  • relay/sensory —> effector

Electrical impulse pathway

receptor → sensory neurone → relay neurone → motor neurone → effector

Myelin sheath

• covers axons of some neurones

• many layers of plasma membrane

• produced from Schwann cells

  • grow around the axon and lay a double layer of phospholipid bilayer

• insulating layer

• increases transmission of electrical impulses (from 1mps to 100mps)

Node of Ranvier

• small gap between myelin sheaths

• 2-3μm

• occur every 1-3mm

• allows the impulse to be transmitted faster as the impulse will jump from one node to the next - saltatory conduction

How do sensory receptors work?

• specific to a single type of stimulus

• act as a transducer

  • detect stimuli e.g. light, heat, sound, pressure

  • convert a stimulus into a nervous impulse (generator potential)

Types of sensory receptor

• mechanoreceptor

  • stimulus - pressure & movement

  • receptor example - pacinian corpuscle

  • sense organ receptor - skin

• chemoreceptor

  • stimulus - chemicals

  • receptor example - olfactory receptor

  • sense organ receptor - nose

• thermoreceptor

  • stimulus - heat

  • receptor example - end-bulbs of Krause

  • sense organ receptor - tongue

• photoreceptor

  • stimulus - light

  • receptor example - cone cell

  • sense organ receptor - eye

What are the features of the Pacinian corpuscle?

• detect mechanical pressure

• deep in skin

• most abundant in fingers and soles of feet

• found within joints

• end of sensory neurone in centre

• surrounded by layers of connective tissue separated by layers of gel

What do sodium ion channels do in the Pacinian corpuscle?

• found in the membrane of the neurone

• transport sodium ions across the membrane

• sensory neurone in Pacinian corpuscle has a stretch-mediated sodium channel

• when they change shape, permeability to sodium changes

How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse?

1. Stretch mediated Na⁺ channels are too narrow and neurone in PC has resting potential

2. When pressure is applied, the PC changes shape & stretches membrane around neurone

3. Sodium ion channels widen & sodium can diffuse in

4. Ions change the potential of the membrane (depolarise it) which results in a generator potential

5. Generator potential creates an action potential (nerve impulse)

Resting potential

• -70mV

• outside of membrane is more positively charged than the inside of the axon

• sodium potassium pump

  • 3 Na⁺ are actively transported out of the axon

  • 2 K⁺ are actively transported in

• more sodium ions outside the membrane and more potassium inside the cytoplasm

• sodium ions diffuse back in down the electrochemical gradient

• potassium ions diffuse out

• gated sodium ion channels

  • mostly closed

  • prevents movement

  • more positive ions outside than inside

• creates resting potential

Action potential

• when a stimulus is detected

• +40mV

• known as depolarisation

  • negative to positive

• repolarisation happens again

  • positive to negative

1. neurone has resting potential, non voltage gated potassium ions are open, sodium voltage gated ion channels are closed

2. energy from stimulus opens vg Na⁺ channels, so Na⁺ diffuses into the axon

3. causes more channels to open (positive feedback)

4. at 40mV, voltage gated sodium ion channels close and voltage gated potassium ions open

5. potassium ions diffuse out and the axon becomes more negative on the outside

6. initially, the inside becomes more negative as lots of potassium ions diffuse out (hyperpolarisation) before returning to resting potential

How is a nervous impulse initiated?

• a stimulus causes a sudden influx of sodium ions

• this causes opening of voltage gated sodium ion channels further along the axon

• action potential is propagated in the same way further along

Refractory period

• short period of time after an impulse when the axon cannot be excited again

Saltory Conduction

• myelinated axons transfer electrical impulses faster than non-myelinated

• depolarisation can only occur at nodes of ranvier

• action potential jumps between nodes

What affects the speed of impulses?

• axon diameter

  • the bigger the diameter, the faster the impulse is transmitted

  • less resistance to flow of ions in cytoplasm

• temperature

  • higher the temperature, the faster the impulse

  • only occurs up to 40 degrees as it will cause proteins to denature

• myelination

  • myelin sheath causes faster impulses

All or nothing principle

• threshold value will trigger a response

• if reached, an action potential will be created

• if not reached, nothing will happen

• larger the stimulus, the more frequently the action potentials are generated but the size of the action potential doesn’t change

Synapse structure

• synaptic cleft

• presynaptic neurone

• postsynaptic neurone

• vesicles

  • contain acetylcholine

• neurotransmitter receptors

Neurotransmitters

• excitatory

  • result in depolarisation of postsynaptic neurone

  • e.g. acetylcholine

• inhibitory

  • result in hyperpolarisation of the postsynaptic membrane

  • prevents an action potential being triggered

  • e.g. GABA

Synaptic transmission

1. action potential reaches the end of the presynaptic neurone and causes calcium ion channels to open. calcium ions enter

2. this causes vesicles to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft

3. ACh fuses with receptors on the postsynaptic membrane, causing sodium ion channels to open. sodium ions diffuse into the postsynaptic neurone

4. influx of sodium ions generates an action potential and the postsynaptic neurone is depolarised

5. AChase hydrolyses ACh into choline and ethanoic acid which diffuse across the synaptic cleft

6. ATP is used to turn this back into ACh

Role of synapses

• ensure impulses are unidirectional

• allow and impulse to be transmitted from 1 neurone to multiple neurones

• or multiple neurones to 1 neurone

What is summation and how does it work?

• build up of neurotransmitters if a single impulse is not enough to trigger an action potential

• spatial

  • multiple presynaptic neurones and 1 postsynaptic neurone

  • builds up a high enough level of neurotransmitters to trigger an action potential

• temporal

  • single presynaptic neurone releases a neurotransmitter several times over a short period

  • builds up until the quantity is sufficient to trigger an action potential

How is the nervous system organised?

• central nervous system

  • brain and spinal cord

• peripheral nervous system

  • neurones

  • somatic nervous system

    • under conscious control

    • voluntary

  • autonomic nervous system

    • under subconscious control

    • involuntary

    • sympathetic

      • fight or flight responses

      • outcome increases activity

    • parasympathetic

      • relaxing responses

      • outcome decreases activity

Cerebrum

• controls voluntary actions

• learning, memory, personality and conscious thought

• highly convoluted

  • increases surface area

• split into left and right hemispheres

  • each one controls half of the body

  • left controls right and vice versa

• reasoning and decision-making take place in the frontal and prefrontal lobe of the cerebral cortex (outer layer)

Cerebellum

• unconscious functions such as posture, balance and non-voluntary movement

• coordinates movement

Medulla Oblongta

• automatic control

• heart rate and breathing rate

• reflex activities

Hypothalamus

• regulatory centre for temperature and water balance

• controls behaviour patterns

• monitors blood plasma composition

• blood glucose and water concentration

• produces hormones

Pituitary gland

• stores and releases hormones

• anterior pituitary

  • six hormones including FSH

• posterior pituitary

  • stores and releases hormones produced by the hypothalamus

Structure of the brain

Structure of the cerebrum

Reflex arc

receptor → sensory neurone → relay neurone → motor neurone → effector