Nervous System 1
Nervous Tissue
Organs and Divisions of the Nervous System
Central nervous system (CNS) brain and spinal cord
Peripheral nervous system (PNS) —all nerves
Autonomic nervous system (ANS)- Sympathetic and Parasympathetic
General Functions of the Nervous System
 Sensory receptors
-Gather information by detecting changes inside and outside the body
 Integrative Functions
 Perception of sensations
 Effectors
 Outside the nervous system
 Somatic-skeletal muscles
 Autonomic nervous system- involuntary
 Heart, smooth Muscle, and Glands
Neurons
 Gather and transmit information by:
 Responding to stimuli
 Producing and sending electrochemical impulses
 Releasing chemical messages
Nervous tissue
 Neurons
 Nerve impulses
 Parts of a typical neuron
 Cell Body
 Dendrites
 Axons
 Nerves
 Neuroglial cells
Neuron Structure
 Cell body
 Granular cytoplasm
network of fine threads extending into axon
 Chromatophilic substance (Nissl bodies)
 Membranous sacs similar to RER
 Attached ribosomes produce proteins
 Dendrites: Usually short, and highly branched
 Bring in information to the cell body
 Axons: Axon hillock
 Conducts impulses away from the cell body
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Cells of the Nervous System
Neurons
Consist of three main parts—dendrites, which conduct impulses to cell body of neuron;
cell body of neuron; and axon, which conducts impulses away from cell body of neuron
Classified according to function:
1. Sensory: conduct impulses to the spinal cord and brain.
2. Motor: conduct impulses away from brain and spinal cord to muscles and
glands.
3. Interneurons: conduct impulses from sensory neurons to motor neurons.
Structural classification of Neurons
 Unipolar (Pseudo unipolar)
 One single process dividing into two
 Found only in dorsal root ganglia
 Bipolar
 One dendrite and one axon arise from opposite poles
 Found in special sense organs e.g. eye, ear, taste buds
 Multipolar: Most common
 Multiple dendrites, and a single axon
 Found in CNS and autonomic ganglia
Neuroglial Cells of the CNS
 Four main types of glial cells of the CNS
 Astrocytes—star-shaped cells that anchor small blood vessels to
neurons
 Most common glial cell
 Involved in:
 Buffering K+ levels
 Recycling neurotransmitters
 Regulating adult neurogenesis
 Releasing transmitters that regulate neuronal activity
Blood-Brain Barrier
 Allows only certain compounds to enter brain
 Formed by capillary specializations in brain
 That appear to be induced by astrocytes
 Capillaries are not as leaky as those in body
 Gaps between adjacent cells are closed by tight junctions
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 Microglia—small cells that move in inflamed brain tissue carrying
on phagocytes
 Oligodendrocytes—form myelin sheaths on axons in the CN
 Ependymal cells line the interior walls of the ventricles of the
brain
Neuroglial cells of PNS
 Schwann cells form myelin sheaths of axons in the PNS
 Neurilemmocytes-Sheath of Schwann cells.
 Satellite cells- found in ganglia
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Nerves
 Nerve-bundle of peripheral axons
 Tract of central axons
 White matter —tissue composed primarily of myelinated axons (nerves or
tracts)
 Gray matter —tissue composed primarily of cell bodies and unmyelinated
fibers
 Nerve coverings —fibrous connective tissue
 Endoneurium —surrounds individual fibers within a nerve
 Perineurium —surrounds a group (fascicle) of nerve fibers
 Epineurium —surrounds the entire nerve
Nerve Impulses
Definition —self-propagating wave of electrical disturbances that travel
along the surface of a neuron membrane
 Mechanism
 A stimulus triggers the opening of Na+ channels in the plasma membrane
of the neuron
 Inward movement of positive sodium ions leaves a slight excess of
negative ions outside at a stimulated point; marks beginning of a nerve
impulse
Neuronal Circuits
 Diverging circuit – one presynaptic neuron synapses with several other neurons
(divergence)
 Converging circuit – many neurons synapse on a single postsynaptic neuron
(convergence)
 Reverberating circuit – circuit that receives feedback via a collateral axon from
a neuron in the circuit
Input Processing
 Serial processing
 Neurons pass a signal to a specific destination along a single pathway
from one to another
 Parallel processing
 Input is delivered along many pathways, a single sensory stimulus results in
multiple perceptions
 Neuromuscular junctions (Synapse)
Presynaptic neuron(membrane)
Synaptic cleft
Postsynaptic membrane( neuron, muscle, gland)
 Names of neurotransmitters:
 Acetylcholine
 Epinephrine
 Nor-epinephrine
 Dopamine – Parkinson’s Disease
 Serotonin
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 GABA
Reflex Arcs
• Nerve impulses are conducted from receptors to effectors over neuron pathways
or reflex arcs; conduction by a reflex arc results in a reflex (that is, contraction by
a muscle or secretion by a gland)
• Simplest reflex arcs are two-neuron arcs—consist of sensory neurons synapsing
in the spinal cord with motor neurons.
• Three-neuron arcs consist of sensory neurons synapsing in the spinal cord with
interneurons that synapse with motor neurons
Axon Regeneration
 Occurs much more readily in PNS than CNS
 Oligodendrocytes produce proteins that inhibit regrowth
 And form glial scar tissue that blocks regrowth
 When axon in PNS is severed:
 Distal part of axon degenerates
 Schwann cells survive; form regeneration tube
 Tube releases chemicals that attract growing axon
 Tube guides regrowing axon to synaptic site
Multiple Sclerosis (MS) is a crippling disease
Central Nervous System
Central Nervous System (CNS)-two parts
Brain
Spinal cord
THE SPINAL CORD
The Spinal Cord
 Functions of the spinal cord
 Spinal nerves attach to it
 Provides two-way conduction pathway
 Major center for reflexes
 Location of the spinal cord
 Runs through the vertebral canal
 Extends from the foramen magnum to the level of the vertebra L1 or L2
 Conus medullaris
 The inferior end of the spinal cord
 Filum terminale
 Long filament of connective tissue
 Attaches to the coccyx inferiorly
 Cervical and lumbar enlargements
 Where nerves for upper and lower limbs arise
 Cauda equina
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 Collection of spinal nerve roots
 Spinal cord segments
 Indicate the region of the spinal cord from which spinal nerves emerge
 Designated by the spinal nerve that issues from it
 T1 is the region where the first thoracic nerve emerges
 Two deep grooves run the length of the cord
 Posterior median sulcus
 Anterior median fissure
White Matter of the Spinal Cord
 White matter
 Outer region of the spinal cord
 Composed of myelinated and unmyelinated axons
 Allow communication between spinal cord and brain
 Fibers classified by type
 Ascending fibers
 Descending fibers
 Commisural fibers
Gray Matter of the Spinal Cord and Spinal Roots
 Shaped like the letter “H”
 Gray commissure – contains the central canal
 Dorsal horns
 Consist of interneurons
 Ventral and lateral horns
 Contain cell bodies of motor neurons
Protection of the Spinal Cord
 Protected by vertebrae, meninges, and CSF
 Meanings
 Dura mater – a single layer surrounding spinal cord
 Arachnoid mater – lies deep to the dura mater
 Pia mater – innermost layer
 Delicate layer of connective tissue
 Extends to the coccyx
 Denticulate ligaments – lateral extensions of pia mater
Cerebrospinal Fluid
 Fills the hollow cavities of the brain and spinal cord
 Provides a liquid cushion for the spinal cord and brain
 Other functions
 Nourishes brain and spinal cord
 Removes wastes
 Carries chemical signals between parts of the CNS
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Peripheral Nervous System
 Nervous structures outside the brain and spinal cord
 Nerves allow the CNS to receive information and take action
 Functional components of the PNS
 Sensory inputs and motor outputs
 Categorized as somatic or visceral
 Sensory inputs are also classified as general or special
 Autonomic nervous system (ANS)
 General visceral motor part of the PNS
 ANS has two divisions
 Parasympathetic
 Sympathetic
Basic Structural Components of the PNS
 Sensory receptors – pick up stimuli from inside or outside the body
 Motor endings – axon terminals of motor neurons
 Innervate effectors (muscle fibers and glands)
 Nerves and ganglia
 Nerves – bundles of peripheral axons
 Ganglia – clusters of peripheral neuronal cell bodies
Peripheral Sensory Receptors
 Structures that pick up sensory stimuli
 Initiate signals in sensory axons
 Two main categories of sensory receptors
 Free nerve endings of sensory neurons
 Monitor general sensory information
 Complete receptor cells – specialized epithelial cells or small neurons
 Monitor most types of special sensory information
 Sensory receptors also classified according to
 Location
 Type of stimulus detected
 Structure
 Exteroceptors – sensitive to stimuli arising from outside the body
 Located at or near body surfaces
 Include receptors for touch, pressure, pain, and temperature
 Interceptors – (visceroceptors) receive stimuli from internal viscera
 Monitor a variety of stimuli
 Proprioceptors – monitor degree of stretch
 Located in musculoskeletal organs
Classification by Stimulus Detected
 Mechanoreceptors – respond to mechanical forces
 Thermoreceptors – respond to temperature changes
 Chemoreceptors
 Respond to chemicals in solution
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 Photoreceptors – respond to light
 Located in the eye
 Nociceptors
 Respond to harmful stimuli that result in pain
Classification by Structure
 General sensory receptors
 Widely distributed
 Nerve endings of sensory neurons monitor
 Touch
 Pressure
 Vibration
 Stretch
 Pain
 Temperature
 Proprioception
 General sensory receptors are
 Divided into two groups
 Free nerve endings
 Encapsulated nerve endings
Free Nerve Endings
 Abundant in epithelia and underlying connective tissue
 Respond to pain and temperature
 Two specialized types of free nerve endings
 Merkel discs – lie in the epidermis
 Slowly adapting receptors for light touch
 Hair follicle receptors – wrap around hair follicles
 Rapidly adapting receptors
Encapsulated Nerve Endings
 Consist of one or more end fibers of sensory neurons
 Enclosed in connective tissue
 Mechanoreceptors
 Include four main types
 Meissner’s corpuscles
 Spiraling nerve ending surrounded by Schwann cells
 Occur in the dermal papillae
 Rapidly adapting receptors for discriminative touch
 Occur in sensitive, hairless areas of the skin
 Pacinian corpuscles
 Single nerve ending surrounded by layers of flattened Schwann cells
 Occur in the hypodermis
 Sensitive to deep pressure – rapidly adapting receptors
 Ruffini’s corpuscles
 Located in the dermis and respond to pressure
 Monitor continuous pressure on the skin – adapt slowly
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 Proprioceptors
 Monitor stretch in locomotory organs
 Three types of proprioceptors
Three Types of Proprioceptors
 Muscle spindles – measure the changing length of a muscle
 Imbedded in the perimysium between muscle fascicles
 Golgi tendon organs – located near the muscle-tendon junction
 Monitor tension within tendons
 Joint kinesthetic receptors
 Sensory nerve endings within the joint capsules
SPINAL NERVES
Spinal Nerves
 31 pairs – contain thousands of nerve fibers
 Connect to the spinal cord
 Named for point of issue from the spinal cord
 8 pairs of cervical nerves (C1–C8)
 12 pairs of thoracic nerves (T1–T12)
 5 pairs of lumbar nerves (L1–L5)
 5 pairs of sacral nerves (S1–S5)
 1 pair of coccygeal nerves (Co1)
Spinal Nerves
 Connect to the spinal cord by the dorsal root and ventral root
 Dorsal root – contains sensory fibers
 Cell bodies – located in the dorsal root ganglion
 Ventral root – contains motor fibers arising from anterior gray
column
 Branch into dorsal ramus and ventral ramus
 Dorsal and ventral rami contain sensory and motor fibers
 Rami communicants connect to the base of the ventral ramus
 Lead to the sympathetic chain ganglia
Introduction to Nerve Plexuses
 Nerve plexus – a network of nerves
 Ventral rami (except T2 – T12)
 Branch and join with one another
 Form nerve plexuses
 In cervical, brachial, lumbar, and sacral regions
 Primarily serve the limbs
 Fibers from ventral rami crisscross
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The Cervical Plexus
 Formed by ventral rami of first four cervical nerves (cn 1 – 4)
Phrenic nerve – the most important nerve of the cervical plexus
The Brachial Plexus and Innervation of the Upper Limb
The Lumbar Plexus and Innervation of the Lower Limb
The Sacral Plexus
Innervation of the Skin: Dermatomes
 Dermatome – an area of skin
 Innervated by cutaneous branches of a single spinal nerve
 Upper limb
 Skin is supplied by nerves of the brachial plexus
 Lower limb
 Lumbar nerves – anterior surface
 Sacral nerves – posterior surface
Disorders of the PNS
 Shingles (herpes zoster)
 Viral infection
 Stems from childhood chicken pox
 Often brought on by stress
 Mostly experienced by those over 50
 Migraine headache
 Relates to sensory innervation of cerebral arteries
 Arteries dilate and compresses and irritate sensory nerve
endings
 Myasthenia gravis
 Progressive weakening of the skeletal muscles
 An autoimmune disorder
 Antibodies destroy acetylcholine receptors
SENSORY and MOTOR TRACTS
Ascending tracts carry sensory info to the brain, whereas descending tracts carry
motor info from the brain.
Many tracts cross the midline at a decussation.
Tract origin is contralateral to its destination.
Thus a right brain stroke impairs the left side of body’s function.
Some tracts do not cross the midline.
The tract origin is ipsilateral to its destination
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