Transcript Chapter 7 Part 1 Nervous Tissue

Nervous Tissue
Fundamentals
of the Nervous
System and
"Biology gives you a brain. Life turns itNervous
into a mind."
Tissue
-Jeffrey Eugenides
Nervous System
• The master controlling and communicating
system of the body
• Functions
– Sensory input – monitoring stimuli
– Integration – interpretation of sensory input
– Motor output – response to stimuli
Nervous System
Figure 11.1
Organization of the Nervous
System
• Central nervous system (CNS)
– Brain and spinal cord
– Integration and command center
• Peripheral nervous system (PNS)
– Paired spinal and cranial nerves
– Carries messages to and from the spinal cord
and brain
Peripheral Nervous System (PNS):
Two Functional Divisions
• Sensory (afferent) division
– Sensory afferent fibers – carry impulses from
skin, skeletal muscles, and joints to the brain
– Visceral afferent fibers – transmit impulses
from visceral organs to the brain
• Motor (efferent) division
– Transmits impulses from the CNS to effector
organs
Motor Division: Two Main Parts
• Somatic nervous system
– Conscious control of skeletal muscles
• Autonomic nervous system (ANS)
– Regulates smooth muscle, cardiac muscle,
and glands
– Divisions – sympathetic and parasympathetic
The Cells of the Nervous
System
• The human nervous system is comprised of
two kinds of cells:
– Neurons: excitable cells that transmit electrical
signals
– Glia: Supporting cells – cells that surround and
wrap neurons
• The human brain contains approximately 100
billion individual neurons.
• Behavior depends upon the communication
between neurons.
Fig. 2-1, p. 30
Supporting Cells: Neuroglia
• The supporting cells (neuroglia or glial
cells):
– Provide a supportive scaffolding for neurons
– Segregate and insulate neurons
– Guide young neurons to the proper
connections
– Promote health and growth
Supporting Cells: Neuroglia
• Glial cells make up 90 percent of the
brain's cells. Glial cells are nerve cells that
don't carry nerve impulses.
• The various glial (meaning "glue") cells
perform
many
important
functions,
including: digestion of parts of dead
neurons,
manufacturing
myelin
for
neurons, providing physical and nutritional
support for neurons,
Astrocytes
• Most abundant, versatile, and highly
branched glial cells
• They cling to neurons and their synaptic
endings, and cover capillaries
Astrocytes
• Functionally, they:
– Support and brace neurons
– Anchor neurons to their nutrient supplies
– Guide migration of young neurons
– Control the chemical environment
Astrocytes
Figure 11.3a
Microglia and Ependymal Cells
• Microglia – small, ovoid cells with spiny
processes
– Phagocytes that monitor the health of neurons
• Ependymal cells – range in shape from
squamous to columnar (Ciliated)
– They line the central cavities of the brain and spinal column
– Their apical surfaces are covered in a layer of cilia, which
circulate CSF around the central nervous system. Their
apical surfaces are also covered with microvilli, which
absorb CSF. Ependymal cells are a type of Glial cell and are
also CSF producing cells
Microglia and Ependymal Cells
Figure 11.3b, c
Oligodendrocytes, Schwann Cells, and
Satellite Cells
• Oligodendrocytes – branched cells that
wrap CNS nerve fibers
• Schwann cells (neurolemmocytes) –
surround fibers of the PNS
• Satellite cells surround neuron cell bodies
with ganglia
Oligodendrocytes, Schwann Cells, and Satellite
Cells
Figure 11.3d, e
Fig. 2-10, p. 35
Fig. 2-11, p. 36
Neurons (Nerve Cells)
• Structural units of the nervous system
– Composed of a body, axon, and dendrites
– Long-lived, amitotic, and have a high
metabolic rate
• Their plasma membrane function in:
– Electrical signaling
Fig. 2-4, p. 32
Neurons (Nerve Cells)
Figure 11.4b
Fig. 2-2, p. 31
The Cells of the Nervous
System
• The membrane refers to the structure that
separates the inside of the cell from the
outside environment.
• The nucleus refers to the structure that
contains the chromosomes.
• The mitochondria are the structures that
perform metabolic activities and provides
energy that the cells requires.
• Ribosomes are the sites at which the cell
synthesizes new protein molecules
Nerve Cell Body (Perikaryon or
Soma)
• Contains the nucleus and a nucleolus
• Is the major biosynthetic center
• Is the focal point for the outgrowth of
neuronal processes
• Has no centrioles (hence its amitotic
nature)
• Has well-developed Nissl bodies (rough
ER)
• Contains an axon hillock – cone-shaped
area from which axons arise
Processes
• Armlike extensions from the soma
• Called tracts in the CNS and nerves in the
PNS
• There are two types: axons and dendrites
Dendrites of Motor Neurons
• Short, tapering, and diffusely branched
processes
• They are the receptive, or input, regions of
the neuron
Axons: Structure
• Slender processes of uniform diameter
arising from the hillock
• Long axons are called nerve fibers
• Usually there is only one unbranched axon
per neuron
• Rare branches, if present, are called axon
collaterals
• Axonal terminal – branched terminus of an
axon
Axons: Function
• Generate and transmit action potentials
• Secrete neurotransmitters from the axonal
terminals
• Movement along axons occurs in two ways
– Anterograde — toward axonal terminal
– Retrograde — away from axonal terminal
Myelin Sheath
• Whitish, fatty (protein-lipoid), segmented
sheath around most long axons
• It functions to:
– Protect the axon
– Electrically insulate fibers from one another
– Increase the speed of nerve impulse
transmission
Myelin
• Myelin is about 40 % water; the dry mass of
myelin is about 70 - 85 % lipid (cholesterol and
phospholipid)) and about 15 - 30 % proteins.
• Some of the proteins that make up myelin are
myelin basic protein (MBP), myelin
oligodendrocyte glycoprotein (MOG), and
proteolipid protein (PLP).
• The primary lipid of myelin is a glycolipid called
galactocerebroside. The intertwining
hydrocarbon chains of sphingomyelin serve to
strengthen the myelin sheath.
Myelin Sheath and Neurilemma:
Formation
• Formed by Schwann cells in the PNS
• A Schwann cell:
– Envelopes an axon in a trough
– Encloses the axon with its plasma membrane
– Has concentric layers of membrane that make
up the myelin sheath
• Neurilemma – remaining nucleus and
cytoplasm of a Schwann cell
Myelin Sheath and Neurilemma:
Formation
Figure 11.5a–c
Nodes of Ranvier
• Are gaps in the myelin sheath formed by
spaces
between
successive
oligodendrocytes (in CNS) or Schwann
cells (in PNS) along the length of the axon.
• Nodes of Ranvier contain Na+ ion
channels, and are sites where action
potentials are generated by membrane
depolarizations.
• They are the sites where axon collaterals
can emerge
Unmyelinated Axons
• A Schwann cell surrounds nerve fibers but
coiling does not take place
• Schwann cells partially enclose 15 or more
axons
Axons of the CNS
• Both myelinated and unmyelinated fibers
are present
• Myelin sheaths are formed by
oligodendrocytes
• Nodes of Ranvier are widely spaced
• There is no neurilemma
Regions of the Brain and Spinal
Cord
• White matter (diencephalon) – dense
collections of myelinated fibers
• Gray matter – mostly soma and
unmyelinated fibers
White matter
• Situated between the brainstem and
cerebellum, the white matter consists of
structures at the core of the brain such as
the thalamus and hypothalamus
• Certain nuclei within the white matter are
involved in the expression of emotions, the
release of hormones from the pituitary
gland, and in the regulation of food and
water intake
White matter
• The nuclei of the white matter are involved
in the relay of sensory information from the
rest of the body to the cerebral cortex, as
well as in the regulation of autonomic
(unconscious) functions such as body
temperature, heart rate and blood
pressure.
Grey matter
• Grey matter – closely packed neuron cell
bodies form the grey matter of the brain.
• The grey matter includes regions of the
brain involved in muscle control, sensory
perceptions, such as seeing and hearing,
memory, emotions and speech.
Neuron Classification
• Structural:
– Multipolar — three or more processes
– Bipolar — two processes (axon and dendrite)
– Unipolar — single, short process
Neuron Classification
• Functional:
– Sensory (afferent) — transmit impulses
toward the CNS
– Motor (efferent) — carry impulses away from
the CNS
– Interneurons (association neurons) — shuttle
signals through CNS pathways
Comparison of Structural Classes of Neurons
Table 11.1.1
Comparison of Structural Classes of Neurons
Table 11.1.2
Comparison of Structural Classes of Neurons
Table 11.1.3
Blood brain barrier
• The blood-brain barrier is a mechanism
that surrounds the brain and blocks most
chemicals from entering.
• Because neurons in the brain generally
do not regenerate, it is vitally important
for the blood brain barrier to block
incoming viruses, bacteria or other
harmful material from entering.
BBB
Fig. 2-12, p. 37