Chapter 3 - Neurones

typical neurone transmission pathway - transmits electrical and chemical signals

synapse

junction between the axon terminal of one neurone with dendrite of another adjacent neurone

dendrites VS axon

dendrites carry nerve impulses TOWARDS cell body

axons carry nerve impulses AWAY from cell body

types of neurone - multipolar

• one axon and multiple dendrites extending from cell body

• structure for most motor and interneurones

type of neurone - bipolar

• one axon and one dendrite extending from cell body

• structure occurs in eye, ear and nose

type or neurone - unipolar

• one extension from cell body which is the axon

• structure typically found in invertebrates

type of neurone - pseudounipolar

• one axon extending from cell body

• most sensory neurones are this type

generation of action potential

1) resting potential

2) depolarisation

3) repolarisation

4) hyperpolarisation

5) recovery

resting potential

• membrane potential maintained at -70mV

• sodium potassium pump pumps 3 sodium ions out axon while somultaneously pumping 2 potassium ions into axon, by active transport

• both voltage gated sodium and potassium ion channels CLOSED

depolarisation

• when stimulus strong enougn, membrane potential reach threshold value of 55mV, and initiate action potential

• voltage gated sodium ion channels open, sodium ions diffuse into axon down conc gradient

• membrane potential become more positive

repolarisation

• when membrane potential reach 30mV, membrane repolarise

• voltage gated sodium ion channels close

• voltage gated potassium ion channels open, potassium ions diffuse out of axon into extracellular fluid, down conc gradient

• membrane potential become more negative

hyperpolarisation

• voltage gated potassium ion channel close

• membrane potential more negative than resting potential

recovery

• sodium potassium pump pumps 3 sodium ions out of axon, while simultaneously pumping 2 potassium ions into axon, by active transport

• voltage gated sodium and potassium channels CLOSED

how is resting potential maintained?

• sodium potassium pump

• cell membrane HIGHYL permeable to K+ and Cl- ions, SLIGHTLY permeable to Na+ and IMPERMEABLE to large negatively charged organic ions

what type of response are action potentials

• ALL OR NOTHING

• size of nerve impulse is always same, regardless of stimulus

• BUT stronger stimulus stimulate greater freq of nerve impulse THUS MORE RECEPTORS ACTIVATED

why nerve cell membrance ONLY relay signals in ONE direction

when action potential is generated, section of membrane behind the action potential is in its refractory period, thus membrane potential not at -70mv(resting potential)

transmission of impulses along MYELINATED axon - saltatory conduction

• cause action potentials to jump from one node of ranvier to another

• local current flow distance is longer

trnamsission of impulses along UNMYELINATED axon - continuous propagation

• depolarization of one area on membrance causes a local current flow between neighbouring areas on the membrane immediately adjacent to the site of the original stimulus

• each action potential generates another action potential just infront of it

• local current flow distance is shorter

role or myelin sheath

fatty layer wrapping around axon to insulate the axon to speed up nerve impulse transmission

WHY local current flow distance longer but nerve impulse transmission faster?

allows electrical signals to jump farther along axon before needing to regenerate at another ion channel

synaptic transmission

receptor

• detects stimuli

• similar receptor cells are grouped together in a sense organ

• provides SOMATIC and SPECIAL sensation

• somatic sensation - originate from receptors found in more than 1 location in body, EG: touch, pain

• special sensation - originate from receptors restricted to a particular area of the body. EG: sight, taste

reflexes

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

• mostly coordinated by spinal cord

4 properties of reflexes

1) stimulus is required, not spontaneous

2) reflex is involuntary, occur without consious thought

3) response is rapid, only small number of neurones involved

4) response is stereotyped, occurs in same way every time

purpose of reflexes

rapid and involuntary to prevent further or long term injury to body

innate vs acquired reflexes

• innate: present from birth, determined genetically

• acquired reflexes: learnt through constant repetition