Chapter 28
Nervous System
I.
Nervous System Structure and Function
1. Two
major systems are responsible for coordinating the functions of the animal body:
the endocrine system and the nervous system.
2.
Neurons are nervous cells that transmit information by electrical and chemical
signals.
3. A
neuron contains 3 major parts:
(1)
Cell body (soma): consists of
the nucleus and organelles
(2)
Dendrite: a neuron fiber that
receives signals from its tip inward, toward the rest of the neuron
(3)
Axon: a neuron extension that
conducts signals to another neuron or to an effector cells
4. The
nervous system has two main anatomical divisions:
(1)
The central nervous system
(CNS): contains the brain and the spinal cord
(2)
The peripheral nervous system
(PNS): consists of neurons that carry information into and out of the CNS
5. A
nervous system has three interconnected functions:
(1)
Sensory input: is the
conduction of signals from sensory receptors
(2)
Integration: is the analysis
and interpretation of the sensory signals and the formation of appropriate
responses in CNS
(3)
Motor output: is the
conduction of signals from the integration centers through the PNS to effector
cells
6.
Three functional types of neurons:
(1)
Sensory neurons: transfer
signals from sensory receptors into the CNS
(2)
Interneurons: integrate data
and pass appropriate signals to other interneurons or to motor neurons
(3)
Motor neurons: transfer
signals from the CNS to effector cells
7.
Reflex is an automatic reaction to a stimulus, mediated by the spinal cord or
lower brain.
8.
Synapse is a junction between two neurons, where electrical and chemical signals
are transmitted.
9.
Neurons require supporting cells glia (=glial cells)
which nourish neurons, insulate the axons
of neurons, or help maintain homeostasis.
10.
Myelin sheath is a series of Schwann cells surround the axon of a neuron. Each
pair of cells in the sheath is separated by a space called a node of Ranvior.
II.
Nerve Signals and Their
Transmission
1.
Membrane potential is the
charge difference between a
neuronꞌs cytoplasm and extracellular fluid due to the
differential distribution of ions.
2.
Resting
membrane
potential is the
voltage (about -70 mV) across the plasma membrane of a resting neuron.
(1)
Resting
membrane
potential is
generated by the concentration gradients of ions from sodium-potassium (Na+-K+)
pumps using energy from ATP to actively move Na+ out of the neuron
and K+ in.
(2)
The membrane of a resting
neuron is polarized with the inside more negatively charged than the outside.
3.
Action potential (AP) is a
change in membrane voltage that transmits a nerve signal along an axon.
(1)
Resting state (polarization): voltage-gated
Na+ and K+ ion channels are closed, and resting potential is
maintained by ungated channels.
(2)
Depolarization: a stimulus
opens Na+ ion channels. Membrane polarity becomes the reverse of resting
state.
4. The
action potential propagates along the axon.
5.
Synaptic cleft is a narrow
gap between the presynaptic and postsynaptic terminals in a chemical synapse.
6.
Neurotransmitter is a
chemical messenger that carries information from a transmitting neuron to a
receiving cell, for example
acetylcholine
(ACh).
(1)
These neurotransmitters
are produced in synaptic vesicles of the sending neuronꞌs synaptic
(presynaptic) terminals.
(2)
The action potential
arrives at the
presynaptic terminal and causes synaptic vesicles to fuse with the
plasma membrane of the sending neuron.
(3)
The fused vesicles release
their neurotransmitters by exocytosis into the synaptic cleft.
(4)
The released
neurotransmitters bind to receptors on Na+ ion channel proteins in
the receiving
neuronꞌs plasm
(postsynaptic) membrane. This binding opens chemically gated Na+
ion channels, and a new action potential starts.
(5)
Neurotransmitters are
degraded by enzymes or transported back into the sending neuron, and the Na+
ion channels close.
7. Chemical synapses enable complex information to be processed.
(1) A neuron may receive information via neurotransmitters from hundreds of other neurons.
(2) Excitatory postsynaptic potential (EPSP): Neurotransmitters that open Na+ ion channels may trigger action potentials in the receiving cell.
(3) Inhibitory postsynaptic potential (IPSP): Neurotransmitters that open K+ or Cl- ion channels may decrease the tendency to develop action potentials in the receiving cell.
(4) Synaptic summation represents the net effect by adding many excitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP) together.
8.
Many neurotransmitters are
small, nitrogen-containing organic molecules, such as acetylcholine.