sensory systems

Question 1

describe resting membrane potential?

Answer 1

The concentration of Na+ is much higher outside the cell membrane
K+ concentration is higher inside the cell membrane.

The fixed anions that are negatively charged make the inside of the cell membrane relatively negatively charged compared with the outside of the cell membrane.

Question 2

what is the sodium-potassium pump?

Answer 2

The Na+ K+ pump helps to maintain concentration gradients and therefore resting membrane potential by actively pumping three Na+ ions out of the neuron and two K+ ions into the neuron.

Question 3

name the three gated channels?

Answer 3

ligand-gated receptor
mechanically regulated channel
voltage-gated channel

Question 4

explain what a ligand-gated receptor?

Answer 4

Ligand gated channels open in response to a chemical or neurotransmitter binding with their receptors on the cell membrane. Some leakage of NA+ and K-

Question 5

explain what a mechanically regulated channel?

Answer 5

Mechanically gated channels open in response to direct stimulation of a receptor, eg deformation of a mechanoreceptor in the skin that is sensitive to touch.

Question 6

explain what a voltage-gated channel is?

Answer 6

Voltage gated channels are opened or closed in response to changes in the membrane potential ie changes in the electrical charge across the membrane

Question 7

explain how mechanoreceptors work?

Answer 7

Resting state: all gated Na+ and K+ channels are closed and no ions move through these channels.

A stimulus stretches or deforms the mechanoreceptor and this opens a type of Na+ gated channel in the mechanoreceptor membrane.

Because of the concentration gradient of Na+ and the negative charge within the cell, Na+ flows through these channels into the cell.

This depolarises the cell as the inside becomes less negative.

If this reaches a critical level of around -55%mV, then an action potential will be triggered.

Question 8

explain the action potential graph?

Answer 8

Depolarisation reaches threshold voltage gated channels open

Na+ flows into cell causing the steep rise on the graph to a peak of +30mV

Na+ gates close K+ gates open- K+ flows out of the cell

Depolarisation reverses

K+ continues to flow out of the cell causing a slight hyperpolarisation before returning to resting membrane potential.

Question 9

what is spatial summation?

Answer 9

Several graded potentials are generated at different areas of the neuron at the same time

Question 10

what is temporal summation?

Answer 10

Rapid firing of graded potentials – the resting membrane potential doesn’t have opportunity to return to resting state and gradually builds until the threshold level is reached.

Question 11

explain AP propagation without myelin?

Answer 11

Once the action potential is generated it is propagated along the axon to the axon terminal. Each section of the axon depolarises the next area of membrane opening voltage gated channels and generating another action potential.

The hyperpolarisation ensures that the action potential moves in one direction down the axon

Question 12

explain AP propagation with myelination?

Answer 12

If the axon is myelinated then the depolarisation jumps from one node of Ranvier to another because the myelin sheath of the Schwann cell acts as an insulator and doesn’t conduct current. This is called saltatory conduction.
Because the electrical signal is jumping from node to node transmission is much faster; therefore myelinated cells therefore conduct impulses more quickly.

Question 13

explain transmission across a synapse?

Answer 13

When the impulse reaches the axon terminal it causes voltage gated calcium channels open.

Calcium rushes into the synaptic bouton causing vesicles to bind to the cell membrane
These vesicles then release neurotransmitters into the synaptic cleft and they bind to the receptors in the next neurons causing ligand gated channels to open. This results in ions flowing into the neuron.

Question 14

explain receptor stimulation?

Answer 14

Sensory receptors receive different types of energy and if there is sufficient firing of the receptor this will create an action potential converting this energy into an electrical signal that travels the length of the sensory nerve and into the spinal cord.

Question 15

what is meant by a receptive field?

Answer 15

Each receptor will only respond to a specific type of stimulus within a specific area

Question 16

what is meant by sensitivities/ threshold levels?

Answer 16

the strength of stimulus required to create an action potential varies.

Question 17

what do sensory receptors also have ability to do?

Answer 17

adapt to a constant unchanging stimulus ie accommodate to a new level of stimulation and they do this by decreasing the level of sensitivity.

Question 18

name the touch receptors, which axons are large in diameter and myelinated?

Answer 18

Hair follicle receptors
Meissner’s corpuscles
Merkel’s disks
Pacinian corpuscles
Ruffini endings

Question 19

explain free nerve endings?

Answer 19

located in the dermis and subcutaneous tissue. They are the terminal branches of non-myelinated C fibres and small diameter myelinated A delta fibres, these are nociceptors that are sensitive to damage or mechanical or thermal stimuli that could cause damage.

Question 20

explain Hair follicle receptors ?

Answer 20

Sensitive to touch so when a hair is bent the deformation of the follicle activates the surrounding receptor.
Low threshold rapidly adapting

Question 21

Meissner corpuscles ?

Answer 21

Sensitive to light pressure and fine discriminative touch. Located beneath the epidermis of glabrous skin that is non hairy skin (fingers, palm of the hands and sole of the feet)
Low threshold rapidly adapting

Question 22

Merkel disks ?

Answer 22

Sensitive to pressure and low frequency vibration. Located in the skin below the epidermis. Small receptive fields.
Low threshold, slowly adapting

Question 23

Pacinian corpuscles ?

Answer 23

Sensitive to high frequency vibration. Located deep in the dermis layer of hairy & glabrous skin.
Low threshold, rapidly adapting

Question 24

Ruffini endings ?

Answer 24

Sensitive to stretching of the skin. are located in the dermis layer of hairy and glabrous skin.
Low threshold slowly adapting

Question 25

explain the order of the neurons for the ascending tract for touch?

Answer 25

1st order neuron: sensory neuron- medulla
2nd order neuron: medulla crosses midline tocontralateral thalamus
3rd order neuronthalamus to somatosensory cortex

Question 26

explain the order of the Spinothalamic Tract, for pain and temperature?

Answer 26

1st order neuron: sensory neuron- spinal cord

2nd order neuron: spinal cord- crosses midline and ascends to thalamus3rd order neuron: thalamus to somatosensory cortex