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Lecture 9
Nervous Tissue and the Spinal Cord
Neurons – nerve cells
 Universal properties of neurons
o __________________ – all neurons have the ability to respond to environmental changes
o Conductivity – Neurons produce traveling electrical signals that quickly reach other cells
at _________________ locations
o Secretion – when the electrical signal reaches the end of a nerve fiber, the neuron usually
secretes a chemical called a _________________________ that stimulates the next cell
 Functional classes of neurons
o Sensory (afferent) neurons – neurons specialized to detect _______________ such as
light, temperature, or pressure and to transmit information about them towards the CNS
o Interneurons (association neurons) – neurons located entirely within the brain or spinal
cord
 These neurons carry out the integrative function of the nervous system – they
process, store and ___________________ information, and “make decisions”
about how the body responds to stimuli
o Motor (efferent) neurons – neurons that send signals to muscle or gland cells, sending
signals ____________________ the central nervous system
 Structure of a neuron
o Soma (cell body) – control center of the neuron
 Has a ______________ and cytoplasm containing organelles like other cells
o Dendrites - processes of a neuron that receives information from other cells or from
environmental stimuli
 They conduct signals _____________ the soma
 Dendrites are usually shorter, more branched, and more numerous than the axon
o Axon – process of a neuron that conducts action potentials __________________ the
soma
 Each neuron has only one axon
 The _____________ end of an axon usually has terminal arborization
 Terminal arborization is an extensive complex of fine branches
 Each branch ends in a synaptic knob
 Synaptic knob (terminal button)
o Swollen tip of the distal end of an axon; the site of synaptic
_________________ and neurotransmitter release
 Neuronal variety
o Multipolar neurons – Have one axon and __________ or more dendrites
 This type is common in the brain and spinal cord
o Bipolar neurons – have one axon and one dendrite
 Includes ___________________ cells and sensory cells of the inner ear
o Unipolar neurons – have only one process leading away from the soma
 Also called pseudounipolar neurons
 Includes neurons that carry signals to the ____________________
o Anaxonic neurons – have multiple dendrites but no axon
 This type is found in the brain and ________________
Supportive Cells
 Types of Neuroglia
o Oligodendrocytes – Forms a myelin sheath around nerve fibers in the _______________
nervous system
o Astrocytes – form the blood-brain barrier that controls which substances are able to get
from the bloodstream into the ______________ tissue
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Ependymal Cells – ______________ cerebrospinal fluid and help to circulate
cerebrospinal fluid
Microglia – develop from _____________ blood cells and phagocytize dead nervous
tissue, microorganisms, and other foreign matter
Schwann Cells – envelop peripheral nervous system fibers with myelin and assist in the
regeneration of damaged fibers
Myelin
o Insulating layer around a nerve fiber
o Formed by oligodendrocytes in the CNS and Schwann cells in the PNS
o Axons are covered in segments
 Gaps between segments are called _____________ of Ranvier
 Myelin-covered areas between nodes of Ranvier are called internodes
Myelin and signal conduction
o In an unmyelinated nerve fiber, the signal spreads by diffusion of sodium and potassium
ions through the plasma membrane at every point along the fiber
 The ion movement creates a sudden voltage change called an action potiential at
each point
 Each action potential triggers another one just ____________ of it
 The nerve signal consists of a wave of action potentials traveling down the axon
 This signal travels at about .5 to 2 m/sec
o In a myelinated nerve fiber, the ion movements through the membrane occur only at the
_________________ of Ranvier (gaps between segments of myelin)
 In the internodes (myelin covered portions), signals travel by a much faster
process of ion diffusion along the length of the nerve fiber immediately under
the plasma membrane
 Since most of the fiber is covered with myelin, the signal can travel as fast as
___________ m/sec
Synapses and Neural Circuits
 Synapses – the meetings between ______________ and any other cells
o Chemical synapses – junctions in which the presynaptic neuron releases a
neurotransmitter to stimulate the ____________________ cell
 At a chemical synapse, a terminal branch of a presynaptic fiber ends in a
swelling called the synaptic ________________
 Between the synaptic knob and the next cell there is a 20-40 nm gap called the
synaptic ____________________
 A nerve signal arrives at the end of the presynaptic neuron and triggers the
release of neurotransmitters that either excite or inhibit the postsynaptic cell
o Electrical synapses – junctions in which adjacent cells are joined by gap
______________
 Ions diffuse directly from one cell to the next for quick transmission
The Spinal Cord
 Functions
o Conduction – the ___________________ contains fibers that conduct information up and
down the body
 It enables sensory information to reach the brain
 It enables motor commands to reach the receptors
 Input received at one level of the spinal cord can affect output at
________________ level
o Locomotion – the simple repetitive muscle contractions that put one foot in front of
another are controlled by central pattern generators in the spinal cord
 The spinal cord does not control the ______________ or direction of
locomotion (those are under control of the motor neurons in the brain)
o Reflexes – the spinal cord is responsible for involuntary stereotyped responses to stimuli
 Surface Anatomy
o
____ pairs of spinal nerves over five regions
 8 Cervical (C1-C8)
 12 Thoracic (T1-T12)
 5 Lumbar (L1-L5)
 5 Sacral (S1-S5)
 1 Coccygeal
o Enlargements
 The diameter of the spinal cord is relatively constant except for the cervical
__________________ and lumbar enlargement
o Conus medullaris
 Location in which the cord tapers to a _____________ below the lumbar
enlargement
o Cauda equina
 Bundle of nerves resembling a horse’s tail that innervates the pelvic organs and
lower _______________
 Meninges of the Spinal Cord
o Dura mater – outermost meninx that forms a dural sheath around the spinal cord
 Epidural space is found between the sheath and the vertebral bone
 It is a space filled with blood vessels, loose connective tissue, and
adipose tissue
 It is a site where anesthetics are sometime introduced to block
___________ signals
o Arachnoid- middle meninx that adheres to the inside of the dura mater composed of a
loose mesh of collagenous and elastic fibers
 Subarachnoid space is the gap between the arachnoid and the pia mater
 Filled with cerebrospinal _______________
 Lumbar cistern is a (subrachnoid) space occupied by the cauda equina below the
medullary cone
o Pia mater – innermost layer of the meninges that closely follows the contours of the
spinal cord
 Cross-Sectional Anatomy
o Gray matter – has a dull color because it contains very little myelin
 Contains the somas, dendrites, and proximal parts of the axons of neurons
o White matter – has a pearly white color because it contain ________________
 Composed of axons that carry signals from one part of the CNS to another
 Spinal Tracts
o Ascending tracts – those that carry signals ___ the spinal cord
o Descending tracts – those that carry motor signals _______ the brainstem and spinal cord
The Spinal Nerves
 General Anatomy of Nerves and Ganglia
o Nerve – cordlike organ composed of axons bound together by ________________ tissue
 Mixed nerve – consists of both sensory and motor fibers and transmits signals in
______________ directions (but any one fiber transmits in only one direction)
 Sensory nerve – consists of sensory axons, including those of the olfactory and
optic nerves
 ______________ nerve – consists of motor fibers only
 Many motor nerves are actually mixed nerves because they also carry
sensory signals from muscles back to the CNS
o Ganglion – a _______________ of cell bodies outside the CNS (resembling a knot).
 Spinal Nerves
o Proximal Branches
 Dorsal root- ______________ signals
 Ventral root- efferent signals
o Distal Branches
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Dorsal ramus – innervates the muscles and joints in the region of the spine the
nerve is in. It also innervates and the skin of the ____________
Ventral ramus – innervates the ventral and lateral skin and muscles of the trunk
and gives rise to the nerves of the _______________
 In the thoracic region, it forms the intercostal _________________
 In other regions, it forms nerve plexuses
Nerve Plexuses
o Except in the thoracic region, the ventral rami form web-like nerve plexuses which carry
signals from bones, joints, muscles, and the __________
o Cervical Plexus (C1-C5) –
 Great Auricular nerve (sensory nerve of skin of and around the ear)
 Transverse _____________ nerve (sensory nerve of skin of ventral and lateral
neck)
 Ansa Cervicalis (motor nerve of omohyoid, sternohyoid, and sternothyroid)
 _________________ nerve (motor nerve of the diaphragm)
o Brachial Plexus (C5-T1)
 Axillary nerve
 Radial nerve
 Musculocutaneous nerve
 Median nerve
 _______________ nerve
o Lumbar Plexus (L1-L4)
 Ilioinguinal nerve
 Lateral femoral cutaneous nerve
 ____________________ nerve
 Obturator nerve
o Sacral Plexus (L4-S4)
 Superior gluteal nerve
 Inferior gluteal nerve
 __________________ nerve
 Tibial nerve
 Common fibular (peroneal) nerve
 Cutaneous Innervation and Dermatomes
o Each spinal nerve receives sensory input from a specific area of skin called a
____________________
o Dermatomes overlap at their edges by as much as 50%, so severing one sensory nerve
root does not entirely deaden sensation from a dermatome
Somatic Reflexes
 Reflexes have 4 properties
o They require stimulation – they are responses to sensory input
o They are quick – they involve few if any interneurons and ______________ synaptic
delay
o They are involuntary – they occur without intent, often without our awareness, and they
are difficult to suppress
o They are _________________ – they occur in essentially the same way every time, in a
predictable manner
 Visceral vs. Somatic
o Visceral reflexes are responses of _________________ muscle, cardiac muscle, or glands
o Somatic reflexes are responses of skeletal muscle, such as the quick withdrawal of your
hand from a hot stove
 Somatic reflexes use simple neural pathways called reflex arcs that send signals
from the sensory nerve ending to the spinal cord or brainstem and back to a
skeletal muscle
 Monosynaptic reflex arc – simplest type of reflex arc, consisting only of a sensory neuron and a
motor neuron (with just _____________ synapse between neurons)
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Polysynaptic reflex arc – reflex arc containing one or more association neurons
Ipsilateral reflex – CNS input and output are on the _______________ side of the body
Contralateral reflex – sensory input enters the spinal cord on one side of the body and the motor
output leaves from the opposite side
Intersegmental reflex – Sensory signal enters the spinal cord at one level, and the motor output
leaves the cord from a ___________________ or lower level.
o Example – stepping on something sharp influences trunk muscles that flex the waist, so
that as the foot is lifted, the center of gravity is shifted, so that you don’t fall over