2. The structure and function of sensory, relay and motor neurons

what are neurons?

nerve cells in the brain, spinal chord, PNS etc.

approx how many neurons in the nervous system?

100 billion

what is the function of neurons?

to recieve information and transmit it to other neurons- this is done both electrically and chemically

3 typical components of a neuron

• cell body

• dendrites

• axon

size of a neuron

4 micrometers-100 micrometers

length- few mm- 1 meter

direction of electrochemical messages in a neuron

dendrites conduct message into cell body, then axon conducs message away from cell body

dendrites—>cell body —> axon

what is myelin and what is its function?

fatty insulative substance surrounding the axon cable.

it helps speed up the rate at which nerve impulses are transmitted along the axon

what is a synapse?

an area where 2 neurons come close enough that they are able to pass chemical signals from one cell to another

process of synaptic transmission

1. action potential (electrical impulse) passes through the axon of a neuron to the synaptic terminal/axon terminal, which excites pre-synaptic neuron

2. This causes calcium to be released

3. calcium causes vesicles to fuse with synaptic membrane and release neurotransmitters into synaptic cleft

4. neurotransmitters cross synaptic cleft

5. neurotransmitters bind to receptors on the post-synaptic neuron

6. this triggers a signal in the post-synaptic neuron and can cause an action to occur

name the 2 effects that neurotransmitters can have

• excitatory effects

• inhibitory effects

excitation

• neuron is more likely to fire

• ‘on’ switch

• neuron becomes more positively charged

  • e.g. glutamate

inhibition

• neuron is less likely to fire

• ‘off’ switch

• neurons becomes more negatively charged

  • e.g seretonin

summation/net effect

overall sum of excitatory and inhibitory on a post-synaptic neuron

what makes an action potential more likely?

if net effect on post-synaptic neuron is inhibitory less likely

net effect is excitatory more likely

what are the 3 types of neurons?

sensory, motor, relay

structure of sensory neruons

long dendrites and short axons

where are sensory neruons located

PNS

purpose of sensory neruons

respond to stimulation in sensory receptors, found in various locations in the body e.g. eyes, ears, tongue, skin

send signals to brain and spinal cord (CNS) about sensory experience using an electrical impulse

there are sensory neurons for all senses

which direcion do sensory neurons move impulses?

towards the CNS. (They are known as afferent neurons- meaning moving towards a central organ/point)

where are the cell body and axons located in the body for sensory neurons

outside the spinal chord, in the torso, arms and legs

how does info move through a neuron?

1. information enters the neuron through the dendrites

2. information sent through dendrites to cell body (control centre)

3. information is sent through the axon until it reaches the axon terminals

electrical impulses can only flow in ONE direction through each neuron

sructure of motor neurons

long axon and short dendrites

where are motor neurons located

PNS

purpose of motor neurons

send messages from CNS to muscles, organs and glands (effectors)

information about the response required to certain stimuli is sent through motor neurons as electrical impulses

what direction do motor neurons move impulses?

move info away from CNS (called efferent neurons- meaning conducting outwards or away from a central point)

what are axon terminals called if they are attached to a muscle

motor end plates

relay neurons/interneurons structure

smaller neurons. short dendrites and axons

where are relay neurons located

CNS (brain and spinal cord)

purpose of relay neurons

linking sensory and motor neurons- they can send signals to other relay neurons or link sensory and motor neurons

what is the myelin sheath

an electrically insulative layer of myelin wrapped around the axon of a neuron. Myelin is rich in lipid

purpose of the myelin sheath

increase impulse speed

what are Schwann cells?

specialised cells which produce a tightly wrapped myelin sheath around the axon of a neuron

what is the neurilemma

the outermost layer which covers the myelin

what are the nodes of Ranvier?

small gaps between the myelin on the axon

function of nodes of ranvier

helps the electrical impulse jump from section to section to increase the speed of the electrical impulse

what are axon terminals?

the structure found at the end of the axon, which allows electrical impulses to be passed from one neuron to the next without the neurons physically touching.

They are short extensions that terminate in tiny knobs, containing chemicals called neurotransmitters