Neurons: The Nerve Cells

Our Goals Today

• Identify and give functions for each of the following: dendrite, cell body, axon, axoplasm, and axomembrane

• Differentiate among sensory, motor, and interneurons with respect to structure and function

• Relate the structure of a myelinated nerve fibre to the speed of impulse conduction, with reference to myelin sheath, Schwann cell, node of Ranvier, and saltatory transmission

• Describe the structure of a reflex arc (receptor, sensory neuron, interneuron, motor neuron, and effector) and relate its structureto how it functions

Neuron• Dendrite: conducts impulses towards a cell

body• Cell Body: contains the nucleus (maintains the

cell)• Axon: conducts impulses away from a cell body• Axomembrane: cell membrane surrounding the

axon• Axoplasm: cytoplasm inside the axon• Within each neuron the impulse travels in the

following sequence:• Dendrite → Cell Body → Axon

Neuron

Types of Neurons

• Sensory Neurons• Association or

Interneurons• Motor Neurons

Sensory Neurons

• Long dendrite, short axon• Cell body and dendrite are located

outside the spinal cord• Conduct impulses to the spinal cord

Motor Neurons

• Short dendrite , long axon• Long axon between the cell body and

an effector (muscle or gland)• Axon is located outside of the spinal

cord• Dendrite and cell body are located

within the spinal cord

Interneurons

• Interconnect nerve cells– Sensory neurons with motor neurons

• Entirely in the CNS

Nerves• Grouping of nerve cells (neurons)• Mixed nerves: dendrites of sensory neurons and

axons of motors neurons located in the same nerve• May contain hundreds of long fibers• Covered with a myelin sheath

– Fatty tissue– Composed of Schwann cells wrapped around each

nerve fiber– Areas between Schwann cells are called nodes of

Ranvier

• Myelin sheath has two functions– Prevents cross-communication between neurons– Allows impulses to travel faster

Nerves

Sensory Nerves

• Connected through a dorsal root

• Sensory neurons - cell bodies are located in the dorsal root ganglion

Motor Nerves• Connected through a

ventral root• Motor neurons - cell bodies

are located in the spinal cord

• Cell bodies of motor neurons along with interneurons makes inner part of the spinal cord dark in colour (grey matter)

Spinal Nerves: connected to the CNS at either the dorsal or ventral side of the spinal cord (mixed nerves)

White Matter: outer regions of the spinal cord

Reflex Arc• No thought required, protective mechanism• Involves all 3 types of neurons1) Receptor (eg. Pain receptor) detects a stimuli

that exceeds the threshold and initiates an impulse

2) Impulse travels through a sensory neuron 3) To an interneuron 4) To a motor neuron5) Axon of the motor neuron sends the impulse to

the effector– Muscle (to cause movement)– Gland (cause a secretion)

• Synapse: connections between 2 nerve cells• Brain will be alerted by other interneurons that

reflex took place

Reflexive Arc

Remember Our Goals...

• Identify and give functions for each of the following: dendrite, cell body, axon, axoplasm, and axomembrane

• Differentiate among sensory, motor, and interneurons with respect to structure and function

• Relate the structure of a myelinated nerve fibre to the speed of impulse conduction, with reference to myelin sheath, Schwann cell, node of Ranvier, and saltatory transmission

• Describe the structure of a reflex arc (receptor, sensory neuron, interneuron, motor neuron, and effector) and relate its structureto how it functions