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AP Lecture 16

• Chapter 12 Neural Tissue
• Part 1

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Neural Tissue

• I. An Overview of the Nervous System, p. 380
• The nervous system includes all the neural tissue
  in the body.
• Neural tissue contains 2 kinds of cells
• 1. neurons the cells that send and receive
  signals
• 2. neuroglia (glial cells) the cells that
  support and protect the neurons

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The organs of the nervous system include

• the brain
• the spinal cord
• sensory receptors of sense organs (eye, ears,
  etc.)
• the nerves that connect the nervous system with
  other systems

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The nervous system has 2 major anatomical
divisions

• 1. the central nervous system (CNS)
• 2. the peripheral nervous system (PNS)

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The Anatomical Divisions of the Nervous System,
p. 380

• The central nervous system (CNS) consists of the
  spinal cord and brain, which contain neural
  tissue, connective tissues and blood vessels.

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The CNS is responsible for processing and
coordinating

• sensory data from inside and outside the body.
• motor commands that control activities of
  peripheral organs such as the skeletal muscles.
• higher functions of the brain such as
  intelligence, memory, learning and emotion.

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The peripheral nervous system (PNS)

• includes all neural tissue outside the CNS.

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The PNS is responsible for

• delivering sensory information to the CNS
• carrying motor command to peripheral tissues and
  systems

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Nerves

• Sensory information and motor commands in the PNS
  are carried by bundles of axons (with their
  associated connective tissues and blood vessels)
  called peripheral nerves (nerves)
• 1. cranial nerves are connected to the brain
• 2. spinal nerves are attached to the spinal cord

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The Functional Divisions of the Nervous System,
p. 380

• The PNS is separated into 2 divisions
• 1. the afferent division
• 2. the efferent division

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The Functional Divisions of the Nervous System,
p. 380

• 1. The afferent division, which carries sensory
  information from sensory receptors of the PNS to
  the CNS.
• Receptors include neurons or specialized cells
  that detect changes or respond to stimuli, and
  complex sensory organs such as the eyes and ears.

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The Functional Divisions of the Nervous System,
p. 380

• 2. The efferent division, which carries motor
  commands from the CNS to muscles and glands of
  the PNS.
• The cells or organs that respond to efferent
  signals by doing something are called effectors.

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The efferent division is divided into 2 parts

• 1. the somatic nervous system (SNS), which
  controls skeletal muscle contractions
• a. voluntary muscle contractions
• b. involuntary muscle contractions (reflexes)
• 2. the autonomic nervous system (ANS), which
  controls subconscious actions such as
  contractions of smooth muscle and cardiac muscle,
  and glandular secretions.

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The ANS is separated into 2 divisions

• 1. the sympathetic division, which has a
  stimulating effect
• 2. the parasympathetic division, which has a
  relaxing effect

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Neurons

• Neurons are the basic functional units of the
  nervous system.

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II. Neurons, p. 380

• The Structure of Neurons, p. 381
• Figure 12-1
• The multipolar neuron is a common type of neuron
  in the central nervous system. It consists of
• 1. a cell body (soma)
• 2. several short, branched dendrites, and
• 3. a long, single axon

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The cell body includes

• a relatively large nucleus and nucleolus
• the cytoplasm, called the perikaryon, which
  contains mitochondria that provide energy, and
  dense areas of RER and ribosomes that produce
  neurotransmitters. These dense areas, called
  Nissl bodies, make neural tissues appear gray
  (the gray matter).
• the cytoskeleton with neurofilaments and
  neurotubules (in place of microfilaments and
  microtubules) Bundles of neurofilaments called
  neurofibrils support the dendrites and axon.
• most nerve cells do not contain centrioles and
  cannot divide

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Dendrites

• Dendrites are highly branched, with many fine
  dendritic spines that receive information from
  other neurons. Dendritic spines make up 80-90 of
  the neurons surface area.

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Axons

• The long axon carries the electrical signal
  (action potential) to its target. The structure
  of an axon is critical to its function.
• axoplasm the cytoplasm of the axon, which
  contains neurotubules, neurofibrils, enzymes and
  various organelles
• axolemma a specialized cell membrane, covers the
  axoplasm
• the initial segment of the axon attaches to the
  cell body at a thick section called the axon
  hillock
• collaterals are branches of a single axon
• telodendria are the fine extensions at the
  synaptic terminal of the axon

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Fig. 12-1, p. 381
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Fig. 12-1a, p. 381
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Fig. 12-1b, p. 381
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The synapse Figure 12-2

• The synapse is the critical area where one neuron
  communicates with another cell or neuron.
• The neuron that sends the message is the
  presynaptic cell, and the cell that receives the
  message is the postsynaptic cell.

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The synapse Figure 12-2

• Within the synapse, the 2 cells do not actually
  touch. A small gap called the synaptic cleft
  separates the presynaptic membrane and the
  postsynaptic membrane. The message carried by the
  action potential is carried between the membranes
  by chemicals.

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The synapse Figure 12-2

• The expanded area of the axon, called the
  synaptic knob, contains synaptic vesicles filled
  with chemical messengers called
  neurotransmitters, which affect receptors on the
  postsynaptic membrane.

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The synapse Figure 12-2

• The neurotransmitter chemical is then broken down
  and reassembled at the synaptic knob. The
  transport of raw materials between the cell body
  and the synaptic knob by neurotubules within the
  axon is called axoplasmic transport (powered by
  mitochondria and kinesins).

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The synapse Figure 12-2

• The postsynaptic cell can be a neuron, or another
  type of cell.
• A synapse between a neuron and a muscle is a
  neuromuscular junction.
• A neuroglandular junction is a synapse between a
  neuron and a gland.

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Fig. 12-2, p. 382
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Fig. 12-2 left, p. 327
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Fig. 12-2 right, p. 327
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The Classification of Neurons, p. 383

• Neurons can be classified by structure or by
  function.

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There are 4 classifications of neurons based on
structure

• Figure 12-3
• 1. Anaxonic neurons
• 2. Bipolar neurons
• 3. Unipolar neurons
• 4. Multipolar neurons

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Fig. 12-3, p. 383
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1. Anaxonic neurons

• small
• all cell processes look alike
• found in brain and sense organs

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2. Bipolar neurons

• small
• one dendrite and one axon
• found in special sensory organs (sight, smell,
  hearing)

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Fig. 12-3, part 1, p. 383
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3. Unipolar neurons

• very long axons
• dendrites and axon are fused, with cell body to
  one side
• found in sensory neurons of the peripheral
  nervous system

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4. Multipolar neurons

• very long axons
• 2 or more dendrites and 1 axon
• common in the CNS
• includes all motor neurons of skeletal muscles

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Fig. 12-3, part 2, p. 383
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There are 3 classifications of neurons based on
function

• 1. Sensory neurons or afferent neurons, (the
  afferent division of the PNS)
• 2. Motor neurons or efferent neurons (the
  efferent division of the PNS)
• 3. Interneurons or association neurons

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1. Sensory neurons or afferent neurons, (the
afferent division of the PNS)

• Cell bodies of sensory neurons are grouped in
  sensory ganglia.
• Sensory neurons collect information about our
  internal environment (visceral sensory neurons)
  and our relationship to the external environment
  (somatic sensory neurons).
• Sensory neurons are unipolar. Their processes,
  called afferent fibers, extend (deliver messages)
  from sensory receptors to the CNS.

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1. Sensory neurons or afferent neurons, (the
afferent division of the PNS)

• Sensory receptors are categorized as
• a. interoceptors monitor digestive,
  respiratory, cardiovascular, urinary and
  reproductive systems
• provide internal senses of taste, deep pressure
  and pain
• b. exteroceptors
• external senses of touch, temperature, and
  pressure
• distance senses of sight, smell and hearing
• c. proprioceptors
• monitor position and movement of skeletal muscles
  and joints

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2. Motor neurons or efferent neurons (the
efferent division of the PNS)

• carry instructions from the CNS to peripheral
  effectors of tissues and organs via axons called
  efferent fibers.

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the 2 major efferent systems are

• the somatic nervous system (SNS), including all
  the somatic motor neurons that innervate skeletal
  muscles.
• the autonomic nervous system (ANS), including the
  visceral motor neurons that innervate all other
  peripheral effectors (smooth muscle, cardiac
  muscle, glands and adipose tissue).

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autonomic ganglia

• signals from CNS motor neurons to visceral
  effectors pass through synapses at autonomic
  ganglia, dividing efferent axons into 2 groups
• preganglionic fibers
• postganglionic fibers

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3. Interneurons or association neurons

• located in the brain, spinal cord and some
  autonomic ganglia, between sensory neurons and
  motor neurons
• responsible for distribution of sensory
  information and coordination of motor activity
• involved in higher functions such as memory,
  planning and learning

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III. Neuroglia, p. 384

• Neuroglia make up half the volume of the nervous
  system.
• There are many types of neuroglia, with more
  variety in the CNS than in the PNS.

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III. Neuroglia, p. 384

• Central Nervous System, p. 384
• Figure 12-4
• The central nervous system has 4 types of
  neuroglia
• 1. ependymal cells
• 2. astrocytes
• 3. oligodendrocytes
• 4. microglia

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Ependymal Cells

• The central canal of the spinal cord and
  ventricles of the brain, filled with circulating
  cerebrospinal fluid (CSF), are lined with
  ependymal cells which form an epithelium called
  the ependyma.
• Some ependymal cells secrete cerebrospinal fluid,
  and some have cilia or microvilli that help
  circulate CSF. Others monitor the CSF or contain
  stem cells for repair. Processes of ependymal
  cells are highly branched and contact neuroglia
  directly.

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Astrocytes

• Astrocytes are large and have many functions,
  including
• maintaining the blood-brain barrier that isolates
  the CNS
• creating a 3-dimensional framework for the CNS
• repairing damaged neural tissue
• guiding neuron development
• controlling the interstitial environment

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Oligodendrocytes

• Oligodendrocytes have smaller cell bodies and
  fewer processes than astrocytes. Processes may
  contact other neuron cell bodies, or wrap around
  axons to form insulating myelin sheaths. An axon
  covered with myelin (myelinated) increases the
  speed of action potentials.
• Myelinated segments of an axon are called
  internodes. The gaps between internodes, where
  axons may branch, are called nodes (nodes of
  Ranvier).
• Because myelin is white, regions of the CNS that
  have many myelinated nerves are called white
  matter, while unmyelinated areas are called gray
  matter

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Microglia

• Microglia are small, with many fine-branched
  processes.
• They migrate through neural tissue, cleaning up
  cellular debris, waste products and pathogens.

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Fig. 12-4a, p. 385
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Fig. 12-4b, p. 385
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Neuroglia in the PNS

• The cell bodies of neurons in the PNS are
  clustered in masses called ganglia, which are
  surrounded and protected by support cells called
  neuroglia.

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Neuroglia of the Peripheral Nervous System, p.
387

• There are 2 types of neuroglia in the PNS
• satellite cells
• and Schwann cells.

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Neuroglia of the Peripheral Nervous System, p.
387

• 1. Satellite cells (amphicytes) surround ganglia
  and regulate the environment around the neuron.

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Neuroglia of the Peripheral Nervous System, p.
387

• Figure 12-5
• 2. Schwann cells (neurilemmacytes) form a myelin
  sheath called the neurilemma around peripheral
  axons. One Schwann cell encloses only one segment
  of an axon, so it takes many Schwann cells to
  sheath an entire axon.

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Fig. 12-5, p. 388
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Fig. 12-5 top, p. 388
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Fig. 12-5 bottom, p. 388
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Neurons

• Key
• Neurons perform all communication, information
  processing, and control functions of the nervous
  system.
• Neuroglia preserve the physical and biochemical
  structure of neural tissue, and are essential to
  the survival and function of neurons.

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Neural Responses to Injuries, p. 387

• Figure 12-6
• In the PNS, peripheral nerves can regenerate
  after injury. Scwann cells assist in a process
  called Wallerian degeneration. As the axon distal
  to the injury site degenerates, Schwann cells
  form a line along the path of the original axon,
  and wrap the new axon as it grows.
• In the CNS, nerve regeneration is limited because
  astrocytes block growth by releasing chemicals
  and producing scar tissue.

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Fig. 12-6, Step 1, part 1, p. 389
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Fig. 12-6, Step 1, part 2, p. 389
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Fig. 12-6, Step 2, p. 389
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Fig. 12-6, Step 3, p. 389
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Fig. 12-6, Step 4, p. 389