Chapter 14: Functional Organization of the Nervous System
I. Introduction
The nervous and endocrine systems together provide most of the control functions for the human body. In general, the nervous system controls the rapid activities of the body (muscle contractions, visceral events, etc.), whereas the endocrine system regulates principally the slower metabolic functions of the body.
II. Structural Organization of the Nervous System
The nervous system is divided into: (fig. 14.3)
1) the central nervous system (CNS) - the brain and spinal cord
2) the peripheral nervous system (PNS) - the 12 pairs of cranial nerves and 31 pairs of spinal nerves
Two broad categories of cells make up the nervous system:
1) Neurons - The basic structural and functional unit of the nervous system.
Approx. 100 billion located in the CNS.
Capable of generating and conducting an action potential.
Communicate with other cells by means of a chemical or electrical synapse.
2) Neuroglia - support/maintain neurons.
III. Neuron Structure (fig. 14.2)
A neuron has three main cellular parts:
1) Cell body or perikaryon
Cluster together in CNS: nuclei.
Cluster together in PNS: ganglia.
2) Dendrites
Conduct action potentials towards the neuron cell body.
Multiple dendrites may connect via synapses to the neuron cell body (up to 200,000).
3) Axon
A single axon conducts action potentials away from the neuron cell body.
An axon exits the neuron cell body via the axon hillock.
Often the single axon forms multiple collateral axons.
End as axon terminals which communicate with other cells by means of a chemical or electrical synapse.
A nerve is a bundle of axons.
IV. Classification of Neurons
A. Functional organization:
1) Sensory (afferent) neurons
Conduct action potentials from sensory receptors (Ch. 18) towards the CNS.
2) Motor (efferent) neurons
Conduct action potentials from CNS towards an effector organ.
a. somatic motor neurons - reflex and voluntary control of skeletal muscle
b. autonomic motor neurons - involuntary control of smooth and cardiac muscle
and glands.
Sympathetic and Parasympathetic branches
(Ch. 17).
3) Interneurons
Located within the CNS.
B. Structural organization: (fig. 14.4)
1) bipolar - usually interneurons
2) multipolar - usually motor neurons
3) pseudomultipolar - usually sensory neurons
V. Neuroglia
Six primary types of nervous cells support the activity of neurons:
1) Schwann cells - form a myelin sheath around all axons and some dendrites of the PNS. (fig. 14.5)
2) Oligodendrocytes - form a myelin sheath around some of the axons of the CNS.
(fig. 14.7)
Myelin = a lipoprotein.
Forms partial insulating cover around some neuron axons = myelin sheath.
Increases speed of conduction of the action potential.
Consistent gaps of the myelin sheath are called Nodes of Ranvier.
3) microglia - phagocytic cells that patrol the CNS for foreign particles.
4) astrocytes - contribute to the formation of the blood-brain barrier.
5) ependymal cells
6) satellite cells
VI. The Physiology of an Action Potential
The resting membrane potential present across the cell membrane of all cells is formed through the interaction of: (Ch. 5, pp. 122-125)
1) the action of the Na+/K+ pump
2) the selective permeability of the plasma membrane to Na+ and K+
3) the relative intracellular abundance of negatively charged ions
The resting membrane potential for many cells is approx. -65 mV.
An action potential involves localized inversion of the resting membrane potential to +40 mV followed by a return to the resting membrane potential of -65 mV. This localized event is conducted along the length of the neuron dendrite or axon.
An action potential consists of 3 distinct electrochemical events:
(figs. 14.11 - 14.13)
1. Depolarization = an inversion of resting membrane potential (-65 to +40 mV)
Occurs due to Na+ flooding into cell by facilitated diffusion (through voltage-gated* Na+ channel).
2. Hyperpolarization = an overshoot of resting membrane potential (+40 to -90 mV)
Occurs due to K+ flooding out of cell by facilitated diffusion (through voltage-gated* K+ channel).
*voltage-gated ion channels are cell membrane proteins which open and close in response to changes in membrane potential.
3. Repolarization - resting membrane potential reestablished at -65 mV.
The absolute and relative refractory periods ensure unidirectional conduction of action potentials (fig. 14.15)
An action potential is an all-or-none event.
Stimulus intensity is dependant on the number of action potentials. (fig. 14.14)
VII. The Conduction of Action Potentials (figs. 14.16 and 14.17)
a. Myelinated neurons:
The action potential “jumps” from one node of Ranvier to the next - saltatory conduction.
Results in rapid rate of conduction (100 m/s +).
b. Unmyelinated neurons:
The action potential conducted down entire length of axon.
Results in a slower rate of conduction (1 m/s).
VIII. The Synapse
A synapse is the functional connection between a neuron and another cell.
These connections can be:
1) Electrical - gap junctions; relatively rare
2) Chemical - eg. the neuromuscular synapse (cholinergic synapse)
Chemical synapses consists of: (fig. 14.21)
1) Axon or presynaptic terminal
Contains synaptic vesicles with a chemical called a neurotransmitter.
2) Synaptic cleft
3) Postsynaptic terminal
An example of a chemical synapse - the cholinergic synapse
A neurotransmitter called acetylcholine is released from cholinergic axon terminals.
This is the synapse of the neuromuscular junction.
After diffusing across the synaptic cleft acetylcholine binds to chemically-gated Na+ channels to initiate depolarization in the myofiber.
To initiate a complete action potential (all-or-nothing) several cholinergic impulses are required (EPSPs).
Acetylcholinesterase rapidly breaks down acetylcholine in the synaptic cleft.
IX. Synaptic Integration (figs. 14.26 and 14.29)
The action potentials of neurons within the CNS are the result of the integration of EPSPs and IPSPs:
1) EPSPs (excitatory post-synaptic potentials) - cause depolarization eg. acetylcholine
2) IPSPs (inhibitory post-synaptic potentials) - cause hyperpolarization eg. GABA, glycine