13.2 neurons

role of neurons

to transmit electrical impulses rapidly around the body so that the organism can respond to changes in its internal and external environment

cell body in a neuron

this contains the nucleus surrounded by the cytoplasm. Within the cytoplasm there are large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters - chemicals which pass signals from one neuron to the next

dendrons

these are short extensions which come from the cell body. These extentions divide into smaller and smaller branches known as dendrites. They are responsible for transmitting electrical impulses towards the cell b ody

axons

these are singular, elongated nerve fibres that transmit impulses away from the cell body. These fibres can be very long for example those that transmit impulses from the tips of toes and fingers to the spinal chord. The fibre is cylindrical in shape consisting of a very narrow region of cytoplasm surrounded by a plasma membrane

sensory neurons function + structure

these neurons transmit impulses from a sensory receptor cell to a relay neuron, motor neuron or the brain. They have one dendron, which carries the impulse to the cell body, and they have one axon, which carries the impulse away from the cell body

relay neurons function and structure

these neurons transmit impulses between neurons. For example, between sensory neurons and motor neurons. They have many short axons and dendrons

motor neurons

these neurons transmit impulses from a relay neuron or a sensory neuron to an effector, such as a muscle or a gland. They have one long axon and many short dendrites

electrical impulses follow the pathway:

receptor → sensory neuron → relay neuron → motor neuron → effector cell

myelinated neurons

• the axons of some neurons are covered in a myelin sheath, made by many layers of plasma membrane

• Schwann cells produce these layers of membranes by growing round the axon many times

• the myelin sheath acts as an insulating layer and allows these myelinated neurons to conduct the electrical impulses at a much faster speed than the unmyelinated neurons