Nervous System
q Acts to allow communication between cells/tissues of body.
q Like a computer or telephone network with a complex central control.
q Divided into 2 General Systems:
o Central Nervous System (CNS) – Command & Control rgion
§ Usually involve Brain & Spinal Cord
o Peripheral Nervous System (PNS)
q Composed of generalized nerve cells called Neurons
General Structure of a Neuron
q Neurons are composed of the following Structures
o Cell Body: contain majority of cell’s cytoplasm & organelles.
o Dendrites: Extensions which generally serve in the reception of nerve signals
o Axons: Long fibers used to transmit nervous impulses.
§ Often surrounded by Schwann Cells (PNS) or oligodendrocytes (CNS) which coat axons with a Myelin sheath
· Insulates axons and speeds nerve impulses as impulses jump from the Nodes of Ranvier.
§ End at Synaptic terminals. At the Synapses which are small spaces between adjacent nerve or effector cells.
q There are several forms of neurons: Some common types:
o Sensory Neurons: Carry sensory information from receptors to the CNS
o Motor Neurons: Carry nerve impulses to effector cells/tissues
o Interneurons: Integrate sensory & motor neurons.
Neural Function:
Neurons function through transmission of a propagating wave of electrochemical potential. This is produced by diffusion of Na+ and K+ ions across a neuron’s membranes.
q In resting membrane potential there is an excess of K+ on the inside of the cell, and an excess of Na+ on the outside.
q When stimulated, the cell allows the diffusion of ions through it’s membranes through gated ion channels. K+ , and Na+ This changes the potential across the membrane and is the basis of the electrochemical impulse. Once stimulated, this reaction propagates down the axon.
q Sodium and potassium concentrations are then restored through the sodium-potassium pump (see active transport systems)
q This can be accelerated as neural impulses can “jump” across the nodes of ranviers
q Nerve impulses continue along axons until they reach the synaptic terminals.
q Synaptic terminals end in close proximity to another neuron or an effector cell, and is separated by a region called a synapse.
q When the nerve impulse reaches the terminus, chemical messengers called neurotransmitters are released from synaptic vesicles into the synapse. These stimulate neuroreceptor proteins in the adjacent cell, and the signal is propagated to the next cell.
The synapse
Neurotransmitter function
The Brain
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