Nervous System—Part 1

The Nervous System: Fast Communication System with Central and Peripheral Components

• If the endocrine system is our slow communication system—on the order of minutes or even hours—the nervous system is our fast communication system.
  • Example: when you touch a hot stove, you immediately pull your hand away.
• The nervous system is divided into two subsystems:
  • Central Nervous System (CNS)
    • Brain and spinal cord
    • Protected by bone
    • Where all the information is processed (hence the protection)
  • Peripheral Nervous System (PNS)
    • Nerves that collect sensory information and convey this info to the CNS
    • Nerves that leave the CNS and tell muscles to contract

Figure 48.14, page 978, Campbell's Biology, 5th Edition

Nervous System Operation: A Three-Step Process

• Sensory input: nerves from the PNS send information to the CNS.
• Integration: the information is “processed” in the CNS; a response is sent back via the PNS.
• Motor output: nerves that are part of the PNS send a response to the appropriate muscles.

Figure 48.1, page 971, Campbell's Biology, 5th Edition

The Knee-jerk Reflex: The Simplest System

• This kind of reflex arc is the most basic form of action in the nervous system.
• Requires three nerve cells
  • Sensory neuron (PNS)
  • Interneuron (CNS)
  • Motor neuron (PNS)
• Information is processed in the spinal cord, not the brain, so the response is much more rapid.
  • Analogy: in a battle, you don't always confer with the general; your local sergeant or lieutenant may make decisions.
  • You don't need the expertise of the general and the response is much quicker.

Figure 48.4, page 963, Campbell's Biology, 5th Edition

Neuron: Basic Component of the Nervous System

• Neuron is another name for a nerve cell.
• In the brain there are around 100 billion ± 50 billion.
• Information moves one way (left to right in our diagram).
• Information is electrical, so a wave of electrical excitation (depolarization) moves in that one direction.

Unknown source

Anatomy of a Neuron

• Cell body: contains nucleus and other organelles.
• Dendrites: receive signals from other neurons.
  • From Greek dendron, meaning “tree,” since there are many of them.
  • They increase the surface area of the cell body for more input.
• Axon hillock: electrical excitation travels from the dendrites to the cell body to the axon hillock; this electrical excitation then moves along the surface of the neuron.
• Axon: long cable along which the electrical excitation travels; there is only one axon per neuron.
• Synaptic terminal: end of the axon
  • Can branch, so there are hundreds or thousands of these.
  • Relays the signal to a target cell (two possibilities):
    • Another neuron
    • A muscle
  • It relays its message to the target cell by releasing chemical messengers called neurotransmitters.
• Myelin sheath: insulating layer around the axon
  • Made of Schwann cells (in the PNS)
• Nodes of Ranvier: gaps between Schwann cells on the axon
  • Allows for faster signal transmission down the axon.

Figure 48.2(a), page 962, Campbell's Biology, 5th Edition