Anatomy- Chapter 11- Functional Organization of Nervous Tissue
Functions of the Nervous System
- Sensory input: sensory receptors monitor numerous external and internal stimuli. Some stimuli result
in sensations we are aware of.
- Integration: the brain and spinal cord are the major organs for processing sensory input and initiating
responses. The input may produce an immediate response, may be stored as memory or may be
- Control of muscles and glands: the nervous system controls the major movements of the body
through the control of skeletal muscle. The nervous system controls the secretions from many glands.
- Homeostasis: the regulatory and coordinating activities of the nervous system are necessary for
- Mental Activity: The brain is the center of mental activities, including consciousness, thinking,
memory and emotions.
Divisions of the nervous system
- The central nervous system consists of the brain and the spinal cord.
- the peripheral nervous system consists of sensory receptors, nerves, ganglia and plexuses.
Cells of the Nervous System
- each neuron cell body contains a single relatively large and centrally located nucleus with a prominent
- Nissl substance, abundant rough ER, is the primary site of protein synthesis.
- Many dendrite surfaces have small extensions called dendritic spines, where axons of other neurons
form synapses with the dendrites.
- When stimulated, dendrites generate small electric currents, which are conducted to the neuron cell
- Presynaptic terminals in axons release neurotransmitters which are chemicals that cross the the
synapse to stimulate or inhibit the postsynaptic cell.
- Action potentials are generated at the trigger zone, which consists of the axon hillock and the part of
the axon nearest the cell body.
- Axon transport mechanisms can move cytoskeletal proteins, organelles and vesicles containing
neurohormones to be secreted down the axon to the presynaptic terminals.
- Provides a way for infections and harmful substances to make their way from the periphery to the
Types of Neurons
- Sensory neurons conduct action potentials towards the CNS.
- Motor neurons conduct action potentials away from the CNS toward muscles or glands.
- Interneurons conduct action potentials from one neuron to another within the CNS.
- Multipolar neurons have many dendrites and a single axon. Most of the neurons within the CNS and
motor neurons are multipolar.
- Bipolar neurons have two processes: one dendrite and one axon. The dendrite is often specialized to
receive the stimulus, and the axon conducts action potentials to the CNS.
- Bipolar neurons are located in some sensory organs, such as in the retina of the eye and in the nasal
- Unipolar neurons have a single process extending from the cell body. It divides into two branches a
short distance from the cell body. - One branch extends to the CNS, and the other extends to the periphery and has dendritelike sensory
- The two branches function as a single axon.
- Most sensory neurons are unipolar.
Glial Cells of the CNS
- Astrocytes: glial cells that are star-shaped because of cytoplasmic processes that extend from the cell
- These extensions widen and spread out to form foot processes, which cover the surfaces of blood
vessels, neurons and the pia mater.
- Astrocytes have an extensive cytoskeleton of microfilaments, which enables them to form a supporting
framework for blood vessels and neurons.
- They play a role in regulating the extracellular composition of brain fluid.
- They release chemicals that promote the formation of tight junctions between the endothelial cells of
- The endothelial cells with their tight junctions form the blood brain barrier, which determines what
substances can pass from the blood into the nervous tissue of the brain and spinal cord.
- It protects neurons from toxic substances in the blood, allows the exchange of nutrients and waste
products between neurons and the blood, and prevents fluctuations in the composition of the blood
from affecting the functions of the brain.
- Almost all injuries to CNS tissue induce reactive astrocytosis, in which astrocytes participate in walling
off the injury site and limiting the spread of inflammation to the surrounding healthy tissue.
- Reactive scar-forming astrocytes also limit the regeneration of the axons of injured neurons.
- Other functions incl