L2 - Neurons

Question 1

What are neurons

Answer 1

• Nerve cells
• 100 billion neurons in the human nervous system
• 80% of which are located in the brain
• By transmitting signals electrically and chemically, these neurons provide the nervous system with its primary means of communication
• size can vary from less then a millimetre to up to a metre

Question 2

Structure of a typical neuron

Answer 2

(See notes)
- dendrite
- nucleus
- cell body
- axon
- myelin sheath
- Schwann cell
- axon terminal
- node of ranvier

Question 3

Parts of neuron in detail

Answer 3

• cell body (or soma) includes nucleus, which contains the genetic material of the cell, dendrites hang out from the cell body & carry nerve impulses from neighbouring neurons towards the cell body
• The axon carries the impulses away from the cell body down the length of the neuron it is covered in a fatty layer of myelin sheath that protects the axon and speeds up electrical transmission of the impulse
• If the myelin sheath was continuous this would have the reverse effect and slow down the electrical impulse. So it is segmented by gaps called nodes of Ranvier which speed up the impulse transmission by forcing it to ‘jump’ across the gaps along the axon
• at the end of the axon are terminal buttons (axon terminals) that communicate with the next neuron in the chain across the synapse
• impulses start at the dendrite and travel across neuron to axon terminal

Question 4

3 types of neurons

Answer 4

• sensory neurons
• motor neurons
• relay neurons

Question 5

Sensory neurons

Answer 5

• these carry messages from the PNS to the CNS, has long dendrites and short axons
• have long dendrites as it allows them to pick up more information
• found in receptors such as the eyes, ears, tongue and skin, & carry nerve impulses to the spinal cord and brain
• when they reach the brain, they are translated into ‘sensations’, such as vision, hearing, taste and touch
• not all sensory neurons reach the brain, as some neurons stop at the spinal cord, for reflex actions
  see diagram for structure

Question 6

Relay neuron

Answer 6

• these connect the sensory neurons to the motor or other relay neurons, has short dendrites & axons as it just communicates info and decides if action should be taken - if so communicates to motor neurons
• are found between sensory input and motor output/response
• Relay neurons are found in the brain and spinal cord and allow sensory and motor neurons to communicate
  see notes for diagram

Question 7

Motor neuron

Answer 7

• connect the CNS to effectors such as muscles and glands, has short dendrites and long axons
• tells muscles/effectors to carry out an action
• Motor neurons are found in the central nervous system (CNS) and control muscle movements
• When motor neurons are stimulated they release neurotransmitters that bind to the receptors on muscles to trigger a response, which lead to movement
  see notes for diagram

Question 8

How do neurons work?

Answer 8

• all three neurons consist of similar parts
• The dendrites receive signals from other neurons/ sensory receptor cells, are typically connected to the cell body, which is often referred to as the ‘control centre’ of the neuron, due to nucleus
• The axon is a long slender fibre that carries nerve impulses, in the form of an electrical signal known as action potential, away from the cell body towards the axon terminals, where the neuron ends
• Most axons are surrounded by a myelin sheath (except for relay neurons) which insulates the axon so that the electrical impulses travel faster along the axon
• The axon terminal connects the neuron to other neurons (or directly to organs), using a process called synaptic transmission.

Question 9

What neuron is only found in brain/spinal cord/visual system?

Answer 9

Relay neuron