8
The Nervous System
The Nervous System
Two Organ Systems Control All the Other Organ Systems
• Nervous system characteristics
• Rapid response
• Brief duration
• Endocrine system characteristics
• Slower response
• Long duration
The Nervous System
Two Anatomical Divisions
• Central nervous system (CNS)
• Brain
• Spinal cord
• Peripheral nervous system (PNS)
• All the neural tissue outside CNS
• Afferent division (sensory input)
• Efferent division (motor output)
• Somatic nervous system
• Autonomic nervous system
Neural Tissue Organization
Two Classes of Neural Cells
• Neurons
• For information transfer, processing, and storage
• Neuroglia
• Supporting framework for neurons
• Phagocytes
Neural Tissue Organization
Three Classes of Neurons
• Sensory neurons
• Deliver information to CNS
• Motor neurons
• Stimulate or inhibit peripheral tissues
• Interneurons (association neurons)
• Located between sensory and motor neurons
• Analyze inputs, coordinate outputs
Neural Tissue Organization
Neuron Anatomy
• Cell body
• Nucleus
• Mitochondria, RER, other organelles
• Dendrites
• Several branches
• Signal reception (inward)
• Axon
• Signal propagation (outward)
Neural Tissue Organization
The Anatomy of a Representative Neuron
Neural Tissue Organization
Structural Classes of Neurons
• Unipolar
• Dendrite, axon continuous
• Afferent neurons
• Multipolar
• Many dendrites, one axon
• Most common class of neuron
• Bipolar
• One dendrite, one axon
• Very rare
Neuron Function
A Structural Classification of Neurons
Neural Tissue Organization
Types of Neuroglia (glia)
• Astrocytes
• Part of blood-brain barrier
• Oligodendrocytes
• Responsible for myelination
• Microglia
• Phagocytic defense cells
• Ependymal cells
• Lining of brain, spinal cord cavities
• Source of cerebrospinal fluid
Neural Tissue Organization
Neuroglia in the CNS
Neural Tissue Organization
Two Types of Neuroglia in the PNS
• Satellite cells
• Surround cell bodies
• Schwann cells
• Surround all peripheral axons
• Form myelin sheath on myelinated axons
Neural Tissue Organization
Schwann Cells and Peripheral Axons
Neural Tissue Organization
Key Note
Neurons perform all of the communication, information processing,
and control functions of the nervous system. Neuroglia outnumber
neurons and have functions essential to preserving the physical and
biochemical structure of neural tissue and the survival of neurons.
Neural Tissue Organization
Anatomic Organization of CNS Neurons
• Center—Collection of neurons with a shared function
• Nucleus—A center with a discrete anatomical boundary
• Neural cortex—Gray matter covering of brain portions
• White matter—Bundles of axons (tracts) that share origins,
destinations, and functions
Neural Tissue Organization
Anatomic Organization of PNS Neurons
• Ganglia—Groupings of neuron cell bodies
• Nerve—Bundle of axons supported by connective tissue
• Spinal nerves
• To/from spinal cord
• Cranial nerves
• To/from brain
Neural Tissue Organization
The Anatomical Organization of the Nervous System
Neural Tissue Organization
Pathways in the CNS
• Ascending pathways
Carry information from sensory receptors to processing
centers in the brain
• Descending pathways
Carry commands from specialized CNS centers to skeletal
muscles
Neuron Function
The Membrane Potential
• Resting potential
• Excess negative charge inside the neuron
• Created and maintained by Na-K ion pump
• Negative voltage (potential) inside
• -70 mV (0.07 Volts)
Neuron Function
The Cell Membrane at the Resting Potential
Neuron Function
Changes in Membrane Potential
• Result from changes in ion movement
• Ions move in transmembrane channels
• Membrane channels can open or close
• If Na+ channels open  positive charges enter cell  membrane
potential moves positive (depolarization)
• If K+ channels open  positive charges leave cell  membrane
potential moves negative (hyperpolarization)
Neuron Function
Key Note
A membrane potential exists across the cell membrane because (1)
the cytosol and the extracellular fluid differ in their ionic composition,
and (2) the cell membrane is selectively permeable to these ions. The
membrane potential can quickly change, as the ionic permeability of
the cell membrane changes, in response to chemical or physical
stimuli.
Neuron Function
Generation of Action Potential
• Depolarization of membrane to threshold
• Rapid opening of voltage-gated Na+ channels
• Na+ entry causes rapid depolarization
• Voltage-gated K+ channels open
• K+ exit causes rapid repolarization
• Refractory period ends as membrane recovers the resting state
Neuron Function
Propagation of an Action Potential
• Continuous propagation
• Involves entire membrane surface
• Proceeds in series of small steps (slower)
• Occurs in unmyelinated axons
• Saltatory propagation
• Involves patches of membrane exposed at nodes
• Proceeds in series of large steps (faster)
• Occurs in myelinated axons
Neuron Function
The Propagation of Action Potentials over Unmyelinated and
Myelinated Axons
Neuron Function
The Propagation of Action Potentials over Unmyelinated and
Myelinated Axons
Neuron Function
Action Potential PropagationNeuron Function
Key Note
“Information” travels within the nervous system primarily in the form of
propagated electrical signals known as action potentials. The most
important information (e.g., vision, balance, movement), is carried by
myelinated axons.
Neural Communication
Synapse Basics
• Intercellular communication
• Axon terminal
• Input to next cell
• Chemical signaling
• Neurotransmitter release
Neural Communication
Structure of a Synapse
• Presynaptic components
• Axon terminal
• Synaptic knob
• Synaptic vesicles
• Synaptic cleft
• Postsynaptic components
• Neurotransmitter receptors
Neural Communication
The Structure of a Typical Synapse
Neural Communication
Synaptic Function and Neurotransmitters
• Cholinergic synapses
• Release neurotransmitter acetylcholine
• Enzyme in synaptic cleft (acetylcholinesterase) breaks it down
• Adrenergic synapses
• Release neurotransmitter norepinephrine
• Dopaminergic synapses
• Release neurotransmitter dopamine
Neural Communication
• Neuronal pools
Groups of interconnected neurons with specific functions
• Divergence
Spread of information from one neuron to several others
• Convergence
Several neurons send information to one other
Neural Communication
Two Common Types of Neuronal Pools
Neural Communication
Key Note
A synaptic terminal releases a neuro-transmitter that binds to the
postsynaptic cell membrane. The result is a brief, local change in the
permeability of the postsynaptic cell. Many drugs affect the nervous
system by stimulating neurotransmitter receptors and thus produce
complex effects on perception, motor control, and emotions.
The Central Nervous System
Meninges—Layers that surround and protect the brain and spinal
cord (CNS)
• Dura mater (“tough mother”)
• Tough, fibrous outer layer
• Epidural space above dura of spinal cord
• Arachnoid (“spidery”)
• Subarchnoid space
• Cerebrospinal fluid
• Pia mater (“delicate mother”)
• Thin inner layer
The Central Nervous System
The Meninges
The Central Nervous System
The Meninges
The Central Nervous System
Spinal Cord Basics
• Relays information to/from brain
• Processes some information on its own
• Divided into 31 segments
• Each segment has a pair of:
• Dorsal root ganglia
• Dorsal roots
• Ventral roots
• Gray matter appears as horns
• White matter organized into columns
The Central Nervous System
Gross Anatomy of the Spinal Cord
The Central Nervous System
Gross Anatomy of the Spinal Cord
The Central Nervous System
Sectional Anatomy of the Spinal Cord
The Central Nervous System
Sectional Anatomy of the Spinal Cord
The Central Nervous System
Key Note
The sensory and motor nuclei (gray matter) of the spinal cord
surround the central canal. Sensory nuclei are dorsal, motor nuclei
are ventral. A thick layer of white matter consisting of ascending and
descending axons covers the gray matter. These axons are
organized into columns of axon bundles with specific functions. This
highly organized structure often enables predicting the impact of
particular injuries.
The Central Nervous System
Brain Regions
• Cerebrum
• Diencephalon
• Midbrain
• Pons
• Medulla oblongata
• Cerebellum
The Central Nervous System
The Brain
The Central Nervous System
The Brain
The Central Nervous System
The Brain
The Central Nervous System
Brain—The four hollow chambers in the center of the brain filled with
cerebrospinal fluid (CSF)
• CSF produced by choroid plexus
• CSF circulates
• From ventricles and central canal
• To subarachoid space
• Accessible by lumbar puncture
• To blood stream
The Central Nervous System
The Ventricles of the Brain
The Central Nervous System
The Formation and Circulation of Cerebrospinal Fluid
The Central Nervous System
The Formation and Circulation of Cerebrospinal Fluid
The Central Nervous System
Functions of the Cerebrum
• Conscious thought
• Intellectual activity
• Memory
• Origin of complex patterns of movement
The Central Nervous System
Anatomy of Cerebral Cortex
• Highly folded surface
• Elevated ridges (gyri)
• Shallow depressions (sulci)
• Cerebral Hemispheres
• Longitudinal fissure
• Central sulcus
• Boundary between frontal and parietal lobes
• Other lobes (temporal, occipital)
The Central Nervous System
Functions of the Cerebral Cortex
• Hemispheres serve opposite body sides
• Primary motor cortex (precentral gyrus)
• Directs voluntary movement
• Primary sensory cortex (postcentral gyrus)
• Receives somatic sensation (touch, pain, pressure,
temperature)
• Association areas
• Interpret sensation
• Coordinate movement
The Central Nervous System
The Surface of the Cerebral Hemispheres
The Central Nervous System
Hemispheric Lateralization
• Categorical hemisphere (usually left)
• General interpretative and speech centers
• Language-based skills
• Representational Hemisphere (usually right)
• Spatial relationships
• Logical analysis
The Central Nervous System
Hemispheric Lateralization
The Central Nervous System
Brain Waves
(Electroencephalogram)
The Central Nervous System
The Basal Nuclei
• Lie deep within central white matter of the brain
• Responsible for muscle tone
• Coordinate learned movements
• Coordinate rhythmic movements (e.g., walking)
The Central Nervous System
The Basal Nuclei
The Central Nervous System
The Basal Nuclei
The Central Nervous System
Functions of the Limbic System
• Establish emotions and related drives
• Link cerebral cortex intellectual functions to brain stem
autonomic functions
• Control reflexes associated with eating
• Store and retrieve long-term memories
The Central Nervous System
The Limbic System
The Central Nervous System
The Diencephalon
• Switching and relay center
• Integration of conscious and unconscious motor and sensory
pathways
• Components include:
• Epithalamus
• Choroid plexus
• Pineal body
• Thalamus
• Hypothalamus
The Central Nervous System
The Diencephalon and Brain Stem
The Central Nervous System
The Diencephalon and Brain Stem
The Central Nervous System
Functions of the Thalamus
• Relay and filter all ascending (sensory) information
• Relay a small proportion to cerebral cortex (conscious
perception)
• Relay most to basal nuclei and brain stem centers
• Coordinate voluntary and involuntary motor behavior
The Central Nervous System
Functions of the Hypothalamus
• Produce emotions and behavioral drives
• Coordinate nervous and endocrine systems
• Secrete hormones
• Coordinate voluntary and autonomic functions
• Regulate body temperature
The Central Nervous System
Anatomy and Function of the Brain Stem
• Midbrain
• Process visual, auditory information
• Generate involuntary movements
• Pons
• Links to cerebellum
• Involved in control of movement
• Medulla oblongata
• Relay sensory information
• Regulate autonomic function
The Central Nervous System
Anatomy and Function of the Cerebellum
• Oversees postural muscles
• Stores patterns of movement
• Fine tunes most movements
• Links to brain stem, cerebrum, spinal cord
• Communicates over cerebellar peduncles
The Central Nervous System
Functions of the Medulla Oblongata
• Links brain and spinal cord
• Relays ascending information to cerebral cortex
• Controls crucial organ systems by reflex
• Cardiovascular centers
• Respiratory rhythmicity centers
The Central Nervous System
Key Note
The brain, a large mass of neural tissue, contains internal
passageways and chambers filled with CSF. The six major regions of
the brain have specific functions. As you ascend from the medulla
oblongata to the cerebrum, those functions become more complex
and variable. Conscious thought and intelligence are provided by the
cerebral cortex.
The Peripheral Nervous System
PNS Basics
• Links the CNS with the body
• Carries all sensory information and motor commands
• Axons bundled in nerves
• Cell bodies grouped into ganglia
• Includes cranial and spinal nerves
The Peripheral Nervous System
The Cranial Nerves
• 12 Pairs
• Connect to brain not the cord
• Olfactory (CN I)
• Sense of smell
• Optic (CN II)
• Sense of vision
• Oculomotor (CN III)
• Eye movement
The Peripheral Nervous System
The Cranial Nerves (continued)
• Trochlear (CN IV)
• Eye movement
• Trigeminal (CN V)
• Eye, jaws sensation/movement
• Abducens (CN VI)
• Eye movement
• Facial (CN VII)
• Face, scalp, tongue sensation/movement
• Vestibulocochlear (CN VIII)
• Hearing, balance
The Peripheral Nervous System
The Cranial Nerves (continued)
• Glossopharyngeal (CN IX)
• Taste, swallowing
• Vagus (CN X)
• Autonomic control of viscera
• Accessory (CN XI)
• Swallowing, pectoral girdle movement
• Hypoglossal (CN XII)
• Tongue movement
The Peripheral Nervous System
The Cranial Nerves
The Peripheral Nervous System
The Cranial Nerves
The Peripheral Nervous System
Key Note
The 12 pairs of cranial nerves are responsible for the special senses
of smell, sight, and hearing/balance, and control movement of the
eye, jaw, face, tongue, and muscles of the neck, back, and shoulders.
They also provide sensation from the face, neck, and upper chest
and autonomic innervation to thoracic and abdominopelvic organs.
The Peripheral Nervous System
The Spinal Nerves
• 31 Pairs
• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• Dermatome—Region of the body surface monitored by a pair of
spinal nerves
The Peripheral Nervous System
Nerve Plexus—A complex, interwoven network of nerves
• Four Large Plexuses
• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus
The Peripheral Nervous System
Peripheral Nerves and Nerve Plexuses
The Peripheral Nervous System
Dermatomes
The Peripheral Nervous System
Reflex—An automatic involuntary motor response to a specific
stimulus
• The 5 steps in a reflex arc
• Arrival of stimulus and activation of receptor
• Activation of sensory neuron
• CNS processing of information
• Activation of motor neuron
Response by effector (muscle or gland)
The Peripheral Nervous System
Examples of Reflexes
• Monosynaptic reflex—Simplest reflex arc; sensory neuron
synapses directly on motor neuron
• Stretch reflex—Monosynaptic reflex to regulate muscle length
and tension (example: patellar reflex)
• Muscle spindle—Sensory receptor in a muscle that stimulates
the stretch reflex
The Peripheral Nervous System
Polysynaptic reflex—A reflex arc with at least one interneuron
between the sensory afferent and motor efferent
• Has a longer delay than a monosynaptic reflex (more synapses)
• Can produce more complex response
• Example: flexor reflex, a withdrawal reflex
• Brain can modify spinal reflexes
The Peripheral Nervous System
The Flexor Reflex, a Type of Withdrawal Reflex
The Peripheral Nervous System
Key Note
Reflexes are rapid, automatic responses to stimuli that “buy time” for
the planning and execution of more complex responses that are often
consciously directed.
The Peripheral Nervous System
Sensory Pathway
• Afferent axon signals from a sensory receptor
• Posterior column pathway
• Carries fine touch, pressure, proprioception
• Ascending neurons synapse in medulla oblongata
• Axons cross over and synapse in thalamus
• Thalamus sends axons to primary sensory cortex
• Organized as sensory homunculus
The Peripheral Nervous System
The Posterior Column Pathway
The Peripheral Nervous System
Motor Pathways
• Corticospinal pathway (tract)
• Provides conscious muscle control
• Organized as motor homunculus
• Medial & lateral pathways (tract)
• Provide subconscious muscle control
The Peripheral Nervous System
The Corticospinal Pathway
The Peripheral Nervous System
The Autonomic Nervous System
Autonomic Nervous System
Branch of nervous system that coordinates cardiovascular,
digestive, excretory, and reproductive functions
The Autonomic Nervous System
Anatomy of ANS
• Preganglionic neuron
• Cell body resides in CNS
• Axon synapses in PNS in autonomic ganglion
• Postganglionic axons (postganglionic fibers) synapse on
peripheral effectors
• Cardiac muscle
• Smooth muscle
• Glands
• Adipose tissues
The Autonomic Nervous System
Divisions of the ANS
• Sympathetic division
• Preganglionic neurons in the thoracic and lumbar segments of
the spinal cord
• “Fight or flight” system
• Parasympathetic division
• Preganglionic neurons in the brain and sacral segments
• “Rest and digest” system
The Autonomic Nervous System
Key Note
The two divisions of the ANS operate largely without our awareness.
The sympathetic division increases alertness, metabolic rate, and
muscular abilities; the parasympathetic division reduces metabolic
rate and promotes visceral activities such as digestion.
The Autonomic Nervous System
The Somatic and Autonomic Nervous Systems
The Autonomic Nervous System
The Somatic and Autonomic Nervous Systems
The Autonomic Nervous System
Sympathetic Division Organization
• Preganglionic neurons in segments T1 to L2
• Ganglia near the vertebral column
• Sympathetic ganglia
• Paired sympathetic chain ganglia
• Unpaired collateral ganglia
• Preganglionic fibers to adrenal medullae
• Epinephrine (adrenalin) into blood stream
The Autonomic Nervous System
The Sympathetic Division
The Autonomic Nervous System
Effects of Sympathetic Activation
• Generalized response in crises
•
•
•
•
•
Increased alertness
Feeling of euphoria and energy
Increased cardiovascular activity
Increased respiratory activity
Increased muscle tone
The Autonomic Nervous System
Parasympathetic Division Organization
• Preganglionic neurons in brain stem and sacral spinal segment
• Ganglionic neurons (peripheral ganglia) in or near target organ
• Sacral fibers form pelvic nerves
The Autonomic Nervous System
The Parasympathetic Division
The Autonomic Nervous System
Effects of Parasympathetic Activation
• Relaxation
• Food processing
• Energy absorption
• Brief effects at specific sites
The Autonomic Nervous System
Relationship between the Two Divisions
• Sympathetic division reaches visceral and somatic structures
throughout the body
• Parasympathetic division reaches only visceral structures via
cranial nerves or in the abdominopelvic cavity
• Many organs receive dual innervation
• In general, the two divisions produce opposite effects on the
their target organs
Aging and the Nervous System
Age-Related Changes
• Reduction in brain size and weight
• Loss of neurons
• Decreased brain blood flow
• Changes in synaptic organization of the brain
• Intracellular and extracellular changes in CNS neurons
The Integumentary System
• Provides sensations of touch, pressure, pain, vibration, and
temperature; hair provides some protection and insulation for skull
and brain; protects peripheral nerves
• Controls contraction of arrector pili muscles and secretion of sweat
glands
The Skeletal System
• Provides calcium for neural function; protects brain and spinal cord
• Controls skeletal muscle contractions that produce bone thickening
and maintenance and determine bone position
The Muscular System
• Facial muscles express emotional state; intrinsic laryngeal muscles
permit communication; muscle spindles provide proprioceptive
sensations
• Controls skeletal muscle contractions; coordinates respiratory and
cardiovascular activities
The Endocrine System
• Many hormones affect CNS neural metabolism; reproductive
hormones and thyroid hormone influence CNS development
• Controls pituitary gland and many other endocrine organs; secretes
ADH and oxytocin
The Cardiovascular System
• Endothelial cells of capillaries maintain blood-brain barrier when
stimulated by astrocytes; blood vessels (with ependymal cells)
produce CSF
• Modifies heart rate and blood pressure; astrocytes stimulate
maintenance of blood-brain barrier
The Lymphatic System
• Defends against infection and assists in tissue repairs
• Release of neurotransmitters and hormones affects sensitivity of
immune response
The Respiratory System
• Provides oxygen and eliminates carbon dioxide
• Controls pace and depth of respiration
The Digestive System
• Provides nutrients for energy production and neurotransmitter
synthesis
• Regulates digestive tract movement and secretion
The Urinary System
• Eliminates metabolic wastes; regulates body fluid pH and
electrolyte concentrations
• Adjusts renal blood pressure; controls urination
The Reproductive System
• Sex hormones affect CNS development and sexual behaviors
• Controls sexual behaviors and sexual function