3.8 NERVOUS SYSTEM

Stimulus

Change in internal or external environment

temp light ph

Receptor

Detects the stimulus

Convert to nerve impulse (tranducer)

Sensory neurone

Carries impulse from receptor → central nervous system

Enter spinal cord via dorsal root

Relay neurone

Found in grey matter of central nervous

Connect sensory → motor

Motor neurone

Carrie impulse from CNS → effector

Leave via ventral root

Effector

Muscle or gland

Nervous system vs Hormonal system

Nervous

• Fast

• Short lasting

• Electrical impulse

• Specific pathway

Hormonal

• Slow

• Lon lasting

• Chemical

• Transported in blood

Nervous system

Brain and spinal cod

Peripheral nervous system

All nerves outside of central

Spinal cord structure

• Central canal - contain cerebrospinal fluid

• Grey matter - cell bodies synapses

• White matter - myelinated neurones

• Dorsal root - sensory neurone in

• Ventral root - motor neurone out

• Relay neurone - inside grey matter

• Meninges - protective layer

Reflex arc definition

A rapid automatic involuntary response to a stimulus

Why is reflex arc fast

• Short pathway

• Few synapses → reduced delay

• Myelinated neurones -. faster conduction

Nerve net structure

• No brain or CNS

• neurones arranged in network

• Impulse travel many directions

• Simple response only

Mamma and hydra comparison

Hydra (cnidaria)

• Nerve net

• No central control

• Few receptors and effectors

• Slow simple responses

Mammal

• CNS and PNS

• Brain coordinates

• High specialised

• Fast complex

Motor neurone structure

Dendrite

Cell Body

Nucleus

Axon

Myelin sheath

Schwann cells

Nodes of ranvier

Axon terminals

Synaptic end bulbs

Dendrites functions

Receive impulse form other neurones

Increase surface area for connections

Cell body centron and nucelus

Organalles

Integrate incoming signals

Control cell activity and genetic material

Axon

Long fibre carrying impulses away from cell body

Myelin sheath and schwann cells

fatty insulating layer speed up transmission

Saltatory conduction

Nodes of ranvier

Gaps in myelin sheath

sites of ion exchange

Axon temrinals

End branch of axon

Form connections with effectors or other neurones

Synaptic end bulbs

Contain neurotransmitters

Release chemicals across synapse

Nerve impulses

Action potential rapid change in electrical potential across a neurone membrane by ion movement

Resting potential -70mV

Inside neurone is negative relative to outside

Maintained by active transport + ion leakage

Sodium potassium pump

3 sodium ions out and 2 potassium ions in

per atp hydrolysed

• Create a net negative charge inside the axon

Why is it negative inside

Na + higher outside

K+ leaks out through channels

Large negative proteins ATP^4- , anions stay inside

Action potential

1. Stimulus reach threshold

If stimulus strong enough → threshold reached

2. Depolarisation

Voltage gated sodium channels open na flood into axon and membrane becomes positive +40

3. Repolarisation

Na channel open and potassium channel opens k leave axon and membrane becomes negative

4. Hyperpolarisation

Too much potassium leaves more negative than resting potential

Refractory period

Time when neurone cannot fire another action potential

• Restore balance of sodium potassium imp

• Ensure one way transmission

• Prevent continuous firing

All or nothing law

If stimulus reaches threshold a full action potential is generated in not no action potential occurs

• Strength of stimulus does NOT change size of action potential

Saltatory conduction

Occur in myelinated neurones

Impulse jumps between nodes of ranvier

Only nodes depolarise

• fast transmission

• Less ion exchange

Oscilloscope trace

1. Resting potential -70mV

2. Sharp rise depolarisation

3. Peak +40mV

4. Fall - repolarisation

5. Dip - hyperepolarisation

6. Return to resting

Facotrs effecting speed of nerve impulse

1. Temperature

Higher → faster conduction more kinetic energy faster diffusion of ions and enzyme activity

2. Axon diameter

Larger → faster impulse - less resistance to ion flow more easity

3. Myelination

vertebrates only electrical insulator ion chanels only nodes of ranvier saltatory conduction impulse jumps nodes to nodes

Synapse

Junction between two neurones where a nerve impulse in transmitted chemically

Structure of synapse

Presynaptic membrane

Synaptic vesicle

Synaptic cleft

Postsynaptic membrane

Receptors

Ca²⁺ channel

Synaptic transmission

Action potential arrive at presynaptic membrane

Calcium channel open enter knob

Vesicles fuse with membrane excosytosis

Neurotransmitter acetylcholine released

Diffuse across cleft

Bind to receptors on postsynaptic membrane

Open na chanels

New action potential generated

Preventing continuous impulses

Prevent overstimulation ensue signals are separated

Mechanisms preventing overstimulus

1. Acteylcholinesterase

Break down acetyl choline → ethanoic acid + choline

Stops continuous stimulation

2. Reabsorption

Neurotransmitter reabsorbed into presynaptic neurone

3. Calcium removal - actively pumped out synaptic knob

Organophosphates on synapses

Inhibit cholinesterase

Acetylcholine is NOT broken down

Continuous stimulation of postsynaptic neurone

• Overstimulation → muscle spasms → paralysis → death

Psychoactive drugs on synaps

Mimicking neurotransmitters (agonists)

Blocking receptors (antiagonists)

Increase / decrease neurotransmitter release

Explanation for no myeli sheath

No saltatory conduction

Local circuits

Speed of conduction reduced does not reach destination

Medical treatment for no myelin sheat

Remyelinate axon

Stem cells

Ways hydra differ to veterrate

Respond ot limited stimuli

Impulse pass all directions

Small num of effectors

No CNS

Action potential carried more than one direction

How do organophosphates increase activity

Mimic transmitters bind to receptors

Prevent breakdown of transmitter

Reduce threshold for excitation on post synaptic

Decrease activity drugs

• Prevent synthesis neurotransmitter

• Block calcium ions

• No exocytosis

• Block receptors bind to them on post

• Na gated channles change shape

Psychoactive drugs

Cannabis drugs beta blockers

toxic effects of organophosphates

• Prevent breakdown of acetyl cholinesterase

• Remain bound to receptros

• Action potential continued to be generated

• Sustained contraction of muscles

• Uncontrolled contraction intercostal muscles breathing

Meninges layers

3 layers (outside → inside):

• Dura mater – tough, strong outer layer

• Arachnoid mater – middle layer

• Pia mater – delicate inner layer that sticks closely to the brain/spinal cord