Neurones Flashcards
Describe the structure of a Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissues, separated by viscous gel and contained by a capsule.
Stretch-mediated Na+ channels on plasma membrane
Capillary runs along base layer of tissue
What stimulus does a Pacinian corpuscle respond to ?
Pressure deforms the membrane causing stretch-mediated Na+ ion channels to open
Na+ diffuses into the membrane, when threshold is met an action potential is generated.
This action potential moves along the sensory neurone
What features do all neurones have ?
Cell Body - Contains organelles + High proportion of RER
Dendrons - Branch into dendrites which carry impulses towards the cell body
Axon - Long, unbranched fibres carry the impulse away from the cell body
Structure of sensory neurone
Cell body in the middle of the axon, long axon
Transmits impulses from receptors to CNS
Refer to GoodNotes
Structure + Function of a relay neurone
Transmits impulses between neurones
**Refer to Goodnotes*
Structure + Function of motor neurone
Transmits impulses from relay neurones in the CNS to effectors. The cell body is at the end of the axon
Refer to GoodNotes
Describe the additional features of a myelinated neurone
Schwann cells - warp around the axon
Myelin Sheath - made from myelin-rich membranes of Schwann cells
Nodes of Ranvier - Very short gaps between Schwann cells where there is no myelin sheath
Explain why myelinated axons can conduct impulses faster than unmyelinated axons
Impulses can jump from one node of ranvier to another due to saltatory conduction . Depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along the whole axon length
Where are myelinated and non-myelinated neurones found in the body ?
Myelinated - Most neurones in central & peripheral nervous systems e.g. those involved in spinal reflex
Non-myelinated - Group C nerve fibres involved in transmitting secondary pain
How is resting potential established ?
Membrane is more permeable to K+ than Na+
Sodium-Potassium pump actively transports 3Na+ out of cell & 2K+ into cell
Establishes electrochemical gradient: Cell contents more negative than extra cellular environment (-70mV)
What happens during depolarisation ?
A stimulus causes Na+ channels to open so Na+ ions diffuse down the electrochemical gradient
P.d. Across membrane becomes more positive
If membrane reaches threshold potential (-50mV) Voltage-gates Na+ channels open.
Significant influx of Na+ ions reveres p.d. to +40mV
What happens during repolarisation ?
Voltage-gated Na+ channels close and voltage-gated K+ channels open
Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
P.d. Across membrane becomes more negative
What happens during hyperpolarisation ?
Too many K+ ions diffuse out, the p.d. Becomes more negative than resting potential
Refractory period: no stimulus is large enough to raise membrane potential to threshold
Voltage-gated K+ channels close & sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period
No action potential can be generated in hyper polarised sections of membrane
- Ensures impulse travels in one direction
- Ensures impulses are discrete events
Why is the frequency of impulse transmission significant ?
Enables organism to distinguish size of stimulus although all action potentials have same magnitude
Larger stimuli result in higher frequency of transmission since they overcome hyper polarisation more quickly
