Interactions Unit

							          Nervous System, part I
    Consider behavioral responses:
What body systems might be involved in
creating a response to a predator?




Fig. 45.34 (7th Ed.)
Animal Defenses against Predators

    A response to a predator requires:

Sense the predator – sight, scent, etc.
Identify what it is – nervous system
Decide how to respond – nervous system

Respond – nervous system, muscle system, others

          First focus – nervous system!
                   Chapter 44

          2 cell types: neuron and glia

                                                  2
                 Neural Signal
 Function: transmit signal quickly but transiently
       Compare to hormones, which transmit signal
slowly but persistently
       Receive sensory information
       Sense organs and peripheral nervous system

Interpret/integrate information
   central nervous system - brain & spinal cord

Generate motor responses
  motor neurons to muscles & glands

Need structures to respond to, create, and propagate
signals quickly
      3 Types of Neurons

                Sensory neurons
                  Afferent neurons, receive
                sensory input

                Interneurons (association)
                 Connect neurons together
                 Located within central
                nervous system

                Motorneurons
                  Efferent neurons, project
                onto muscle or gland cell

Fig. 44.1
         Structure of the Neuron
Basic Component of the Nervous System:
                                  cell body contains nucleus
                                      general cellular functions
                                  dendrites e.g. pain receptors
                                    receive input from other cells
                                  axon
                                    conducts stimulus to next cell

                                  Schwann cells- present near
                                  myelinated neurons
                                   increase transmission speed via
                                  saltatory conduction

                                  nodes of Ranvier: unmyelinated
                                  regions of myelinated axons

             Fig. 44.3
                Axon Structure
How is a myelin sheath formed?

Schwann cell or oligodendrocyte: flat and pancake-like
  Wraps itself around the axon of a neuron as it grows

 Multiple layers of plasma membrane provide electrical
insulation




            Fig. 44.4
               Nervous System
Remember....

   Direction of net flow of ions across a membrane
depends upon two factors:

      concentration gradient
         AND                   =   electrochemical
      electrical gradient               gradient

Neurons “work” by using concentration gradients to
create electrical charges
Resting membrane potential is -70 mV
At rest, a neuron has a net negative charge as a
result of:                   see Figure 44.6
- Sodium-potassium pumps
- Leaky potassium channels

The ion concentrations:        Part of Table 44.1
               Inside (mM)        Outside (mM)
Sodium             10-15               140
Potassium          150                 5-10
Chloride           5-10                110
Anionic proteins high                  low



                                                    8
Instead of Fig. 44.5




                       9
   Resting membrane potential is -70 mV
                        Some Terminology

Depolarization - makes inside of cell less negative by allowing
cation (sodium) entry

Hyperpolarization - makes membrane potential more
negative

Summation effect - intermediate potential resulting
from several neural inputs

Graded potentials - amplitude of membrane potential
changes depending upon magnitude of stimulus
               Two Types of Ion Channels

                        chemical/ligand-gated ion channel
                      Opens in response to binding of specific
                      molecules to the channel protein
                      Ligands for ligand-gated channels in the
                      nervous system are often called
                      neurotransmitters (NTs) or drugs

                         voltage-gated ion channel
                         Opens when threshold potential
                         (-55 mV) is reached

                         Main types: Na+, K+ and Ca2+

Fig. 45.8,-10, 7th ed. (Fig. 44.6-7)
               Potentials
 When a particular level of depolarization is
    reached, a nerve impulse results:
     threshold potential of ~-55 mV


Action potential is caused by several actions:
     1. Voltage-gated ion channels open
          (usually sodium, -55 mV)
     The channels change conformation at this
voltage to allow sodium ions to cross the cell
            membrane (exit cell)

2. Then other voltage-gated ion channels open
          (potassium ions enter cell)
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