STRUCTURE/FUNCTION OF SENSORY, RELAY + MOTOR NEURONS

Neurones

  • The nerve cells are called neurones
  • The average human brain has about 100 billion neurons

Sensory Neurons

  • Carry signals from sensory receptors (sight/smell) to the spinal cord + brain
  • They carry signals from the PNS to the CNS

Relay Neurons

  • Allow sensory + motor neurones to communicate
  • Transfers messages from sensory neurons to other interconnecting neurons/motor neurons

Motor Neurons

  • Carry signals from the CNS to muscles/glands + control contractions
  • Length of fibres = short dendrites + short or long axons
  • Length of fibres = short dendtrites + long axons
  • Length of fibres = long dendrites + short axons

Functions of the different parts of a neurone

  • Dendrite:
  • Receives the nerve impulses or signal from adjacent neurons
  • Cell Body:
  • Contains the nucleus which contains genetic material
  • Axon:
  • Carries impulse away from the cell body
  • Myelin Sheath:
  • Covers the axon + speeds up electrical transmission + protects the axon
  • Nodes of Ranvier:
  • Gaps in the myelin sheath - helps the impulse move faste as it jumps across gaps on the axon
  • Axon terminals:
  • Communicate with the next neurone across a synapse

Neurotransmitters

  • Chemicals that are released from a synaptic vesicle into the synapse by neurons
  • They affect the transfer of an impulse to another nerve or muscle
  • There neurotransmitters are 'taken back up' into the terminal buttons of neurons through the process of reuptake

Synaptic Transmission

1 - An electrical impulse travels along the axon of the transmitting neuron
2 - This triggers the nerve-ending of the pre-synaptic neuron to release chemical messengers called neurotransmitters from the synaptic vesicles
3 - These chemicals diffuse across the synapse (the gap) which bind with receptor molecules on the membrane of the next neuron (postsynaptic neuron)
4 - The receptor molecules on the post-synaptic neuron bind only the the specific chemicals releases from the first neuron. This stimulates the post-synaptic neuron to transmit the electrical impulse
5 - Re-uptake: the neurotransmitter is reabsorbed in the vesicles of the pre-synaptic neuron after it has performed its function of transmitting a neural impulse and stored for later release

Excitatory Effects

Inhibitory Effects

  • The creation of a negative charge on the pre-synaptic cell
  • E.g. serotonin
  • This makes it less likely for an impulse to continue down to the postsynaptic cell
  • Calming effect on the mind, induces sleep
  • The creation of a positive charge on the pre-synaptic cell
  • E.g. dopamine/adrenaline
  • This neurotransmitter then lands on the receptor to end up on the post synaptic cell + make it more likely to fire
  • 'On switch' for the nervous system

The Knee-jerk reflex

  • In a reflex, like the knee-jerk reflex, a stimulus, such as hammer hitting the knee, is detected by sense organs in the peripheral nervous system (PNS), which conveys a message along a sensory neuron
  • The message reaches the central nervous system (CNS) where it connects with a relay neuron
  • This then transfers the message to a motor neuron
  • This then carries the message to an effector such as a muscle, which causes the muscle to contract + the knee to move/jerk