Neurons – nerve cells
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Universal properties of neurons o Excitability – all neurons have the ability to respond to environmental changes o
Conductivity – Neurons produce traveling electrical signals that quickly reach other cells at distant locations o Secretion – when the electrical signal reaches the end of a nerve fiber, the neuron usually secretes a chemical called a neurotransmitter that stimulates the next cell
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Functional classes of neurons o Sensory (afferent) neurons – neurons specialized to detect stimuli 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
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These neurons carry out the integrative function of the nervous system – they process, store and retrieve 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 away from the central nervous system
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Structure of a neuron o
Soma (cell body) – control center of the neuron
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Has a nucleus and cytoplasm containing organelles like other cells o Dendrites - processes of a neuron that receives information from other cells or from environmental stimuli
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It conducts signals toward the soma
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Dendrites are usually shorter, more branched, and more numerous than the axon o Axon – process of a neuron that conducts action potentials away from the soma
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Each neuron has only one axon
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The distal end of an axon usually has termina arborization
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Terminal arborization is an extensive complex of fine branches
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Each branch ends in a synaptic knob
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Synaptic knob (terminal button) o Swollen tip of the distal end of an axon; the site of synaptic vesicles and neurotransmitter release
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Neuronal variety o
Multipolar neurons – Have one axon and two or more dendrites
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This type is common in the brain and spinal cord o Bipolar neurons – have one axon and one dendrite
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Includes olfactory cells and sensory cells of the inner ear o
Unipolar neurons – have only one process leading away from the soma
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Also called pseudounipolar neurons
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Includes neurons that carry signals to the spinal cord o
Anaxonic neurons – have multiple dendrites but no axon
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This type is found in the brain and retina
Supportive Cells
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Types of Neuroglia o Oligodendrocytes – Forms a myelin sheath around nerve fibers in the central nervous system o Astrocytes – form the blood-brain barrier that controls which substances are able to get from the bloodstream into the brain tissue o Ependymal Cells – produce cerebrospinal fluid and help to circulate cerebrospinal fluid

o Microglia – develop from white blood cells and phagocytize dead nervous tissue, microorganisms, and other foreign matter o Schwann Cells – envelop peripheral nervous system fibers with myelin and assist in the regeneration of damaged fibers
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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
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Gaps between segments are called nodes of Ranvier
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Myelin-covered areas between nodes of Ranvier are called internodes
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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
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The ion movement creates a sudden voltage change called an action potiential at each point
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Each action potential triggers another one just ahead of it
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The nerve signal consists of a wave of action potentials traveling down the axon
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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 nodes of Ranvier (gaps between segments of myelin)
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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
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Since most of the fiber is covered with myelin, the signal can travel as fast as
120 m/sec
Synapses and Neural Circuits
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Synapses – the meetings between neurons and any other cells o
Chemical synapses – junctions in which the presynaptic neuron releases a neurotransmitter to stimulate the postsynaptic cell
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At a chemical synapse, a terminal branch of a presynaptic fiber ends in a swelling called the synaptic knob
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Between the synaptic knob and the next cell there is a 20-40 nm gap called the synaptic cleft
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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 junctions
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Ions diffuse directly from one cell to the next for quick transmission
The Spinal Cord
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Functions o Conduction – the spinal cord contains fibers that conduct information up and down the body
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It enables sensory information to reach the brain
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It enables motor commands to reach the receptors
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Input received at one level of the spinal cord can affect output at another 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
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The spinal cord does not control the speed 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
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Surface Anatomy o 31 pairs of spinal nerves over five regions
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8 Cervical (C1-C8)
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12 Thoracic (T1-T12)
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5 Lumbar (L1-L5)

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5 Sacral (S1-S5)
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1 Coccygeal o Enlargements
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The diameter of the spinal cord is relatively constant except for the cervical enlargement and lumbar enlargement o
Conus medullaris
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Location in which the cord tapers to a point below the lumbar enlargement o Cauda equina
 Bundle of nerves resembling a horse’s tail that innervates the pelvic organs and lower limbs
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Meninges of the Spinal Cord o Dura mater – outermost meninx that forms a dural sheath around the spinal cord
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Epidural space is found between the sheath and the vertebral bone
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It is a space filled with blood vessels, loose connective tissue, and adipose tissue
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It is a site where anesthetics are sometime introduced to block pain signals o Arachnoid- middle meninx that adheres to the inside of the dura mater composed of a loose mesh of collagenous and elastic fibers
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Subarachnoid space is the gap between the arachnoid and the pia mater
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Filled with cerebrospinal fluid
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Lumbar cistern is a (subarachnoid) 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
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Cross-Sectional Anatomy o Gray matter – has a dull color because it contains very little myelin
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Contains the somas, dendrites, and proximal parts of the axons of neurons o
White matter – has a pearly white color because it contain myelin
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Composed of axons that carry signals from one part of the CNS to another
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Spinal Tracts o Ascending tracts o Descending tracts
The Spinal Nerves
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General Anatomy of Nerves and Ganglia o
Nerve – cordlike organ composed of axons bound together by connective tissue
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Mixed nerve – consists of both sensory and motor fibers and transmits signals in two directions (but any one fiber transmits in only one direction)
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Sensory nerve – consists of sensory axons, including those of the olfactory and optic nerves
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Motor nerve – consists of motor fibers only
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Many motor nerves are actually mixed nerves because they also carry sensory signals from muscles back to the CNS o
Ganglion – a cluster of cell bodies outside the CNS (resembling a knot).
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Spinal Nerves o Proximal Branches
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Dorsal root- afferent signals
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Ventral root- efferent signals o Distal Branches
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Dorsal ramus – innervates the muscles and joints in that region fo the spine and the skin of the back
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Ventral ramus – innervates the ventral and lateral skin and muscles of the trunk and gives rise to the nerves of the limbs
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In the thoracic region, it forms the intercostals nerve
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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 skin o Cervical Plexus (C1-C5) –
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Great Auricular nerve (sensory nerve of skin of and around the ear)
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Transverse Cervical nerve (sensory nerve of skin of ventral and lateral neck)
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Ansa Cervicalis (motor nerve of omohyoid, sternohyoid, and sternothyroid)
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Phrenic nerve (motor nerve of the diaphragm) o Brachial Plexus (C5-T1)
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Axillary nerve
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Radial nerve
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Musculocutaneous nerve
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Median nerve
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Ulnar nerve o
Lumbar Plexus (L1-L4)
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Ilioinguinal nerve
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Lateral femoral cutaneous nerve
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Femoral nerve
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Obturator nerve o Sacral Plexus (L4-S4)
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Superior gluteal nerve
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Inferior gluteal nerve
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Sciatic nerve
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Tibial nerve
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Common fibular (peroneal) nerve
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Cutaneous Innervation and Dermatomes o Each spinal nerve receives sensory input from a specific area of skin called a dermatome 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
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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 minimal synaptic delay o
They are involuntary – they occur without intent, often without our awareness, and they are difficult to suppress o They are stereotyped – they occur in essentially the same way every time, in a predictable manner
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Visceral vs. Somatic o Visceral reflexes are responses of smooth 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
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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
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Monosynaptic reflex arc – simplest type of reflex arc, consisting only of a sensory neuron and a motor neuron (with just one synapse between neurons)
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Polysynaptic reflex arc – reflex arc containing one or more association neurons
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Ipsilateral reflex – CNS input and output are on the same side of the body
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Contralateral reflex – sensory input enters the spinal cord on one side of the body and the motor output leaves from the opposite side
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Intersegmental reflex – Sensory signal enters the spinal cord at one level, and the motor output leaves the cord from a higher 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