Nervous System

Development of Control Systems

• Response to stimuli (MRSGREN) essential for survival• Single celled organisms versus multi-cellular organisms • Differentiation/specialisation• Need coordination• Nervous versus endocrine• Both work together• e.g. rabbit running from fox – what systems involved?

• Sense organs detect change (receptors)• Effectors respond• Distance between two• Complex organisms have many sensors and

effectors• CNS – every sensor and effector has at least 1 link• Locomotion led to brain in anterior • Spinal cord links brain to rest of body

Nervous System3 functions:1. Collect information about external environment

2. Process and integrate this information (often in relation to previous experience)

3. Act upon information by coordinating the organisms activities.

Neurones

Types of neurone

sensory neurone

relay neurone (interneurone)

motor neurone

Structure

• Cell body with nucleus • Dendrites transmit impulses to cell body• Impulses leave via axon• Myelin sheath composed of Schwann cells• Neurons packed and wrapped to form nerves• May contain sensory, effector or mixture

Nerve Structure

Multiple Sclerosis

• Schwann cells make up myelin sheath• 70% lipid, 30% protein• Provides insulation and allows rapid

conduction of electrical signals• Gradual degradation of myelin sheath • Demyelinated axons (plaques) • Cannot conduct impulses

Effects of MS

• Affects optic nerve, cerebellum, cervical spinal cord, ventricles in brain

• Symptoms – weakness of limbs, pins and needles, numbness, blurred vision and eye pain.

• Relapse and remission cycle• No cure• More women (3:2)• Temperate climate

Resting Potential

• Neurons have an electrical potential(voltage) across the cell membrane, i.e. membrane is POLARISED.

• The inside of the cell is more negativethan the outside

• This is called the Resting Membrane Potential = 70mV

Resting Potential

What causes this?

What causes this?

• Concentrations of K+ and COO- ions are high inside the neuron

• Concentrations of Na+ and Cl- are high outside the neuron

• Membrane is more permeable to K+ than Na+• Concentration of K+ inside 20x greater so K+ ions

rapidly diffuse out until equilibrium reached• This results in the inside being more negative than

outside

Cont’d

• Difference in concentration of ions maintained by active transport against concentration gradient

• Sodium/potassium CATION pumps transport Na+ out and K+ in

• Requires ATP

Cation Pumps

Action Potential

• Explanation of Action Potential

Action Potential

• Stimulation can reverse the charge on a neuron (-70 to +40mV)

• Membrane DEPOLARISED• If stimulus exceeds certain value (THRESHOLD)

an action potential results• Rapid reversal of the resting membrane potential that travels down the axon

Cont’d

• Above the threshold value size of action potential remains the same

• ALL or NOTHING RESPONSE• Size of action potential remains constant as it

passes along neurone

Action Potential

Ion Movement

• Action potential result of sudden influx of Na+ ions

• Due to increased permeability of membrane• Cation pump maintains high levels of Na+• Influx of Na+ depolarises membrane• Positive feedback• K+ moves in opposite direction• Continues until membrane repolarised• VOLTAGE GATED CHANNELS

Action potential animation• Action potential propagation animation

Refractory Period

• After an action potential, outward movement of K+ quickly restores resting potential

• But for 1ms after action potential influx of Na+ is prevented

• Called REFRACTORY PERIOD• A new AP cannot be generated during this

time

Importance of Refractory Period• Action potential can only be propagated in the

region which is not refractory• Impulse moves in forward direction

• By the end of the refractory period the action potential has passed further down the nerve so a second action potential is separated from 1st by 1ms

• Limits frequency of impulses along nerve

Refractory Cont’d

• Has two parts:

• ABSOLUTE refractory period – 1ms• No new impulse can be propagated

• RELATIVE refractory period – 5ms• New impulses only propagated if stimulus is more

intense than normal threshold

Transmission of Impulse

• Action potential moves rapidly along neurone

• 0.5 m/s to 100 m/s

• 2 factors affect speed• Diameter of axon• Myelin sheath

Saltatory Conduction

• Myelin sheath not continuous• Nodes of RANVIER every 1mm• Action potential can only form in

unmyelinated areas so jump from node to node

Nervous Impulse Animation

• Nervous Impulse