2. 11-2
Functions of the
Nervous System
1. Sensory input: Monitor internal and external
stimuli
2. Integration. Brain and spinal cord process
sensory input and initiate responses
3. Motor output: Controls of muscles and glands
4. Homeostasis. Regulate and coordinate
physiology
5. Mental activity. Consciousness, thinking,
memory, emotion
3. 11-3
The Nervous System
• Components
–Brain, spinal cord, nerves, and
sensory receptors
• Subdivisions
–Central nervous system (CNS):
brain and spinal cord
–Peripheral nervous system (PNS)
•Nerves: Sensory and Motor
4. Peripheral Nervous System
11-4
• Divided into
– Sensory (Afferent) Division
– Motor (Efferent Division
• Sensory Division
– Use sensory neurons to transmit nerve impulses toward
the brain and spinal cord
– Receptors in various body location react to stimuli
(touch, pressure, heat, stretch, light, etc) and trigger a
nerve impulse in the sensory neuron
• Motor Division
– Use motor neurons to transmit nerve impulses away
from the brain and spinal cord
– Stimulate effectors (muscle and glands)
5. 11-5
Divisions of PNS
• Sensory (afferent):
transmits nerve impulses
from receptors to CNS.
• Motor (efferent):
transmits nerve impulses
from CNS to effectors
(muscles, glands)
7. Sensory Division of the PNS
• Transmit nerve impulses over sensory
neurons to the CNS from receptors
• Receptors are classified as:
–Somatic receptors - those found in skin, joints,
skeletal muscles, and special sense organs
• Respond to touch, pressure, heat, stretch, pain, light
–Visceral receptors - located in walls of viscera
• Respond to stretch, pain, temperature, chemical
stimuli (CO2)
11-7
8. Motor Division of PNS
• Transmits impulses away from the CNS to effectors
• Somatic nervous system: from CNS to skeletal muscles.
– Voluntary.
– Single neuron system. Use anterior horn motor neurons to
stimulate the skeletal muscle
• Autonomic nervous system (ANS): from CNS to smooth
muscle, cardiac muscle and certain glands.
– Subconscious or involuntary control.
– Two neuron system: first from CNS to ganglion; second from
ganglion to effector.
– Divisions of the ANS
• Sympathetic. Prepares body for physical activity.
• Parasympathetic. Regulates resting or vegetative functions
such as digesting food or emptying of the urinary bladder.
11-8
10. Cells of Nervous System
• Neurons or nerve cells
receive stimuli and
transmit action
potentials to other
neurons or effectors
11-10
– Organization
• Cell body or soma
• Dendrites: input
• Axons: output
• Neuroglia or glial cells
– Support and protect
neurons
11. 11-11
The Neuron
• The human body contains billions of neurons
– Basic functional unit of the nervous system
• Specialized cells conduct electrical impulses along the plasma
membrane
– Nerve impulse: a series of action potentials
• Characteristics
– Longevity – can live and function for a lifetime
– Do not divide – fetal neurons lose their ability to
undergo mitosis; neural stem cells are an exception
– High metabolic rate – require abundant oxygen and
glucose
12. Parts of the
Neuron
11-12
• Cell Body. Aka Soma or Perikaryon
– Contains nucleus and nucleolus
– Major biosynthetic center
– No centrioles (hence its amitotic
nature)
– Nissl bodies = chromatophilic
substance = rough E.R: primary site of
protein synthesis.
– Tapers to form axon hillock
– Cytoskeleton of neurofilaments and
nerutubules
• Dendrites: short, often highly branched.
– Receptive regions of the neuron
• Axons. Long cytoplasmic process capable
of propagating a nerve impulse
– Can branch to form collaterals.
– Axon hillock: Initial segment:
beginning of axon
– Axoplasm - cytoplasm of the axon
– Axolemma - membrane of the axon
– Presynaptic terminals (terminal
13. Structure of a Neuron
11-13
• Dendrites: short, often highly branched.
– Receptive regions of the neuron
• Axons. Long cytoplasmic process
capable of propagating a nerve impulse
– Axoplasm - cytoplasm of the axon
– Axolemma - membrane of the axon
– Axon hillock: Initial segment:
beginning of axon
– Can branch to form collaterals.
– Presynaptic terminals (terminal
boutons)
– Telodendria - the fine terminal
branches of the main axon; end at
– Synaptic terminals; aka synaptic
knob
• Contain vesicles filled with neuro-transmitter
(NT)
14. Axoplasmic Transport
11-14
• Anterograde:
– Axoplasm moved from cell body toward terminals.
– Supply materials for growth, repair, renewal.
– Can move cytoskeletal proteins, organelles away from
cell body toward axon terminals.
• Retrograde
– Away from axonal terminal toward the cell body
– Damaged organelles, recycled plasma membrane, and
substances taken in by endocytosis can be transported
up axon to cell body.
– Rabies and herpes virus can enter axons in damaged
skin and be transported to CNS. Would include toxins
such as heavy metals (the chemical, not the noise)
15. 11-15
Types of Neurons
• Functional classification
– Sensory or afferent: action
potentials toward CNS
– Motor or efferent: action
potentials away from CNS
– Interneurons or
association neurons:
within CNS from one neuron
to another
• Structural classification
– Multipolar: most neurons
in CNS; motor neurons
– Bipolar: sensory in retina of
the eye and nose
– Unipolar: single process
that divides into two
branches. Part that extends
to the periphery has
dendrite-like sensory
20. Neuroglia of CNS: Astrocytes
• Largest and most numerous
• Functions include:
1. Form the blood-brain barrier
– Processes form feet that cover the
surfaces of neurons and blood vessels
and the pia mater. Regulate what
substances reach the CNS from the
blood
1. Repair damaged neural tissue
2. Guide neuron development
3. Control the interstitial environ-ment.
– Regulate concentration of Na, K, and
11-20
CO2
– Control the volume of blood flow
through cerebral capillaries
– Absorb and recycle some NTs
21. Neuroglia of CNS: Ependymal Cells
• Line brain ventricles
and spinal cord central
canal. Specialized
versions of ependymal
form choroid plexuses.
• Choroid plexus
– Secrete cerebrospinal
fluid. Cilia help move
fluid thru the cavities of
the brain.
11-21
22. Neuroglia of CNS: Microglia
and Oligodendrocytes
• Microglia: specialized macrophages. Respond to
inflammation, phagocytize necrotic tissue,
microorganisms, and foreign substances that invade
the CNS.
• Oligodendrocytes: form myelin sheaths if surround
axon. Single oligodendrocytes can form myelin sheaths
around portions of several axons.
11-22
23. 11-23
Neuroglia of PNS
• Schwann cells or neurolemmocytes:
– Wrap around portion of only one axon to form myelin sheath.
– Wrap around many times.
– As cells grow around axon, cytoplasm is squeezed out and
multiple layers of cell membrane wrap the axon. Cell
membrane primarily phospholipid.
– Outer surface of Schwann cell called the neurilemma
• Satellite cells: surround neuron cell bodies in
ganglia, provide support and nutrients
24. 11-24
Myelinated and
Unmyelinated
Axons
• Myelinated axons
– Myelin protects and insulates
axons from one another, speeds
transmission, functions in repair of
axons.
– Not continuous
– Nodes of Ranvier
– Completion of development of
myelin sheaths at 1 yr.
– Degeneration of myelin sheaths
occurs in multiple sclerosis and
some cases of diabetes mellitus.
• Unmyelinated axons: rest
in depressions of Schwann
cells or oligoden- drocytes.
Not wrapped around the
axon; gray matter.
25. Regions of the Brain & SC
White vs. Gray Matter
11-25
• White matter
– Dense collections of myelinated axons.
– Only axons are myelinated!!
• Gray matter
– Unmyelinated axons, cell bodies, dendrites, neuroglia.
– Integrative functions
• In brain
– Gray matter forms the outer layer (cortex) as well as
inner nuclei;
– White matter is deeper.
• In spinal cord
– White matter is outer layer,
– Gray matter is deeper.
26. 11-26
Nerve Fiber Types
• Type A: large-diameter, myelinated. Conduct
at 15-120 m/s. Motor neurons supplying
skeletal and most sensory neurons
• Type B: medium-diameter, lightly
myelinated. Conduct at 3-15 m/s. Part of ANS
• Type C: small-diameter, unmyelinated.
Conduct at 2 m/s or less. Part of ANS
27. 11-27
The Synapse
• Junction between two neurons
• Site where nerve impulse in one cell cause excitation in the
next cell
• Types of cells in synapse
– Presynaptic neuron - conducts impulse toward the synapse
– Postsynaptic neuron - conducts impulse away from the synapse
• Two major types of synapses
– Electrical synapses - rare in the CNS
– Chemical synapses - most common type in CNS
• Synapses between a neuron and its effector:
– Neuromuscular junction - between a neuron and a muscle
– Neuroglandular synapse - between a neuron and a gland
28. 11-28
Electrical Synapses
• Gap junctions that allow
local current to flow between
adjacent cells. Connexons:
protein tubes in cell
membrane that allow flow of
ions from one cell to the
next.
• Found in cardiac muscle and
many types of smooth
muscle. Action potential of
one cell causes action
potential in next cell, almost
as if the tissue were one cell.
• Important where contractile
activity among a group of
cells important.
30. 11-30
Chemical Synapse
• Presynaptic bulb has secretory
vesicles that contain neurotrans-mitter
chemical (NT)
• NT must pass across the
synaptic cleft, space that
separates pre- and
postsynaptic membranes
• Postsynaptic membrane
contains receptors specific
for each type of NT
• Binding of NT to its receptor
causes ion channels to open
or close
• Postsynaptic membrane is thus
either stimulated or inhibited
31. 11-31
Nerves
• Nerves - bundles of axons
– If only sensory axons, called sensory nerves
– If only motor axons, called motor nerves
– If both sensory and motor axons, called mixed nerves
• Connective Tissue Coverings
– Endoneurium – layer of delicate connective tissue
surrounding the axon
• Nerve fascicles – groups of axons bound into bundles
– Perineurium – connective tissue wrapping
surrounding a nerve fascicle
– Epineurium – whole nerve is surrounded by tough
fibrous sheath
32. A. Note the similarity of a nerve to a
11-32
Structure of a Nerve
Figure 12.16a
muscle
1. Just as a muscle is a collection of
muscle fibers, a nerve is a
collection of nerve fibers (axons).
2. Each is broken up in smaller units
known as fascicles
3. Each is covered by connective
tissue:
• Epimysium vs. Epineurium
• Perimysium vs. Perineurium
• Endomysium vs. Endoneurium
33. Neuronal Pathways and Circuits
• Organization of neurons in CNS varies in complexity
– Convergent pathways: many neurons converge and synapse
with smaller number of neurons. E.g., synthesis of data in brain.
– Divergent pathways: small number of presynaptic neurons
synapse with large number of postsynaptic neurons. E.g., important
information can be transmitted to many parts of the brain.
11-33
