Neurons

• •Cell body (soma) includes nucleus – contains genetic material of the cell

  •Dendrites – branch like structures protrude from cell body, carry nerve impulses from neighbouring neurons towards cell body

  •Axon – carries impulses away from the cell body down the length of the neuron

  •Axon covered in fatty layer of myelin sheath – protects axon & speeds up electrical transmission of the impulse

  •segmented by gaps called nodes of Ranvier

  •At the end of the axon are terminal buttons that communicate with the next neuron in the chain across a synapse
• Types of neuron


  Sensory Neurons - Carry nerves impulses from sensory receptors to the brain and spinal cord.. Relay neurons - The most common type of neuron in the CNS. They allow sensory and motor neurons to communicate with each other. Motor neurons - Form synapses with muscles and control their contractions
• •Sensory neurons

  -Carry messages from PNS (peripheral nervous system) to the CNS, they have long dendrites & short axons

  •Relay neurons

  -Connect sensory neurons to the motor or other relay neurons, they have short dendrites & short axons

  •Motor neurons

  -These connect the CNS to effectors such as muscles and glands, they have short dendrites & long axons
• Sensory neurons

  •Carry nerve impulses from sensory receptors to spinal cord and brain.

  •Sensory neurons convert sensory info from these sensory receptors into neural impulses.

  •When these impulses reach the brain, they are translated into sensations of, for example, visual input, heat, pain, so the person can react appropriately.

  •Not all sensory info travels as far as the brain. Some neurons terminate in the spinal cord. This allows reflex actions to occur quickly without the delay of sending impulses to the brain.
• Relay neurons

  •Most neurons are neither sensory nor motor, but lie somewhere between the sensory input and the motor output.

  •Relay neurons allow sensory and motor neurons to communicate with each other.

  •They lie wholly within the brain and spinal cord.
• Motor neurons

  •Motor neurons are located in the CNS and project their axons outside the CNS and directly or indirectly control muscles.

  •They form synapses with muscles and control their contractions.

  •When stimulated, the motor neuron releases neurotransmitters that bind to receptors on the muscle and triggers a response which leads to muscle movement.

  •The strength of the muscle contraction depends on the rate of firing of the axons of motor neurons that control it.

  •Muscle relaxation is caused by inhibition of the motor neuron.
• firing of a neuron - electric transmission

  •When neuron is in a resting state it is negatively charged compared to outside

  •When a neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second causing at action potential to occur

  •Creates an electrical impulse that travels down the axon towards the end of the neuron
• Reflex arc

  •Receptors – receptors in the skin detect a stimulus (change in temperature)

  •Sensory neuron (PNS) – sends electrical impulses to  relay neurons which are located in the spinal cord (CNS)

  •Relay neurons – connect sensory neurons to motor neurons.

  •Motor neuron – sends electrical impulses to an effector

  •Effector produces a response (muscle contracts to move hand away)
• Example of a reflex

  •A stimulus, such as a hammer, hits the knee.

  •This is detected by the sense organs in the PNS, which convey a message along a sensory neuron.

  •The message reaches the 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 and therefore causes the knee to move or jerk.