The Nervous System
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Nervous System
o Three specific functions that work to maintain homeostasis:
• Sensory input• Integration• Motor output
o Divisions of the Nervous Systemo Nervous Tissue
Nervous System
o Divisions of the Nervous System• Central nervous system (CNS)
Brain Spinal cord
• Peripheral nervous system (PNS) – includes all the cranial and spinal nerves
Afferent (sensory) division Efferent (motor) division
o Nervous Tissue
Fig 8.1
Nervous System
o Nervous Tissue• Neurons (nerve cells) – transmit
impulses• Neuroglia – support and nourish
neurons
Nervous System
• Neuron structure Cell body – contains nucleus and other
organelles Dendrite – receive signals from sensory
receptors or other neurons Axon
Conducts nerve signals away from cell body Nerve – bundle of parallel axons in the PNS Tract – bundle of parallel axons in the CNS May be covered by myelin (lipid coating)
Formed by Schwann cells or neurolemmocytes in PNS
Formed by oligodendrocytes in CNS
Fig 8.2
Nervous System
• Types of neurons Motor neurons
Take nerve impulses from the CNS to muscles, organs, or glands
Multipolar – have many dendrites Sensory neurons
Take nerve impulses from sensory receptors to the CNS
Almost all are unipolar Interneurons
All are in the CNS Typically multipolar Convey nerve impulses between various parts
of the CNS
Nervous System
• Nerve signal conduction Resting potential
Neuron possesses potential energy The cell membrane is polarized
positively charged outside the cell negatively charged inside
Action potential Process of conduction of nerve signals Occurs in the axons Begins with a stimulus Channels in the cell membrane opens and
sodium ions rush into the cell (depolarization) Sodium channels close and the cell repolarizes
Fig 8.3
Nervous System
Conduction of action potentials (APs) In unmyelinated axons
Slow (~1 meter/second) Each section of the axon must be stimulated
In myelinated axons Called saltatory conduction An AP at one node of Ranvier can “jump”
over myelinated portion of axon Much faster (>100 meters/second)
Is an all-or-none event Refractory period
Axon cannot conduct an AP Ensures one-way direction of an impulse
Fig 8.4
Nervous System
• Transmission across a synapse Axon terminal – small swelling at tips of
the branched end of an axon Synapse
Region of close proximity between two neurons Presynaptic membrane – membrane of the first
neuron Postsynaptic membrane – membrane of the
next neuron Synaptic cleft – small gap between the
presynaptic and the postsynaptic neuron Neurotransmitters – molecules that
transmit a nerve impulse across a synapse
Fig 8.5
Nervous System
Graded potentials and synaptic integration
Graded potentials – each small signal from a synapse
Excitatory neurotransmitters produce a graded potential that promotes an AP
Inhibitory neurotransmitters produce a graded potential that inhibits an AP
Integration involves summing up the excitatory and inhibitory signals
Nervous System
Neurotransmitter Molecules At least 50 have been identified Two well-known neurotransmitters:
Acetylcholine (Ach) Norepinephrine (NE)
After a neurotransmitter has initiated a response it is removed from the synaptic cleft Enzymes may inactivate the
neurotransmitter The neurotransmitter may be reabsorbed by
the presynaptic membrane Prevents continuous stimulation (or
inhibition) of postsynaptic membranes
Central Nervous System
o Gray matter – contains cell bodies and unmyelinated fibers
o White matter – contains myelinated axons
o Meninges and Cerebrospinal Fluid• Meninges – protective membranes of the
CNS Dura mater – outer menix composed of tough,
fibrous connective tissue Arachnoid mater – middle menix composed of
spider-web-like connective tissue Pia mater – deepest menix
Fig 8.6
Central Nervous System
• Cerebrospinal fluid (CSF) Produced by ependymal cells Fills the following:
Subarachnoid space – space between the arachnoid mater and the pia mater
Ventricles – hollow, interconnecting cavities of the brain
Central canal – hollow, space of the spinal cord
Central Nervous System
• The spinal cord Extends from the base of the brain to the
lumbar vertebra Protected by vertebral column
Central Nervous System
Structure of the spinal cord Central canal contains CSF Centrally located gray matter
Posterior (dorsal) root – contains sensory fibers
Anterior (ventral) root – contains motor fibers
interneurons White matter
Posterior white matter composed of ascending tracts carrying sensory information to the brain
Anterior white matter composed of descending tracts carrying motor information from the brain
Fig 8.7
Central Nervous System
Functions of the spinal cord Provides communication between the brain and
the peripheral nerves Reflexes
Central Nervous System
• The brain Cerebrum Diencephalon Cerebellum Brain stem Ventricles
Two lateral ventricles (cerebrum) Third ventricle (diencephalon) Fourth ventricle (cerebellum)
Fig 8.8
Central Nervous System
• The Cerebrum Largest portion of the brain Voluntary motor responses Coordinates the activities of the other
parts of the brain Involved in higher thought processes The cerebral hemispheres
Longitudinal fissure divides the left and right Connected by the corpus callosum Gyri (ridges) are separated by sulci (shallow
grooves)
Central Nervous System
Lobes Frontal lobe Parietal lobes Occipital lobe Temporal lobes Insula
Fig 8.9
Central Nervous System
• The cerebral cortex Outer layer of gray matter Accounts for sensation, voluntary
movement, and consciousness Motor and sensory areas
Primary motor area In frontal lobe anterior to central sulcus Voluntary commands to skeletal muscle
Primary somatosensory area Posterior to central sulcus in parietal lobes Receives sensory information from the skin
and skeletal muscles
Fig 8.10
Central Nervous System
Primary taste area Located in insula Taste sensations
Primary visual area Located in the occipital lobe Receives information from our eyes
Primary auditory area Located in the temporal lobe Receives information from our ears
Central Nervous System
Association areas Where integration occurs Where memories are stored
Processing centers Prefrontal area Uses information from other association areas Reasoning and planning actions Motor speech area (Broca’s area) Wernicke’s area (general interpretive area)
Central Nervous System
Central white matter Tracts communicate information between the
different sensory, motor, and association areas Corpus callosum join the two cerebral hemispheres
Basal nuclei Masses of gray matter Integrate motor commands
Limbic system Inferior to the cerebral cortex Contains neural pathways that connect portions of
the cerebral cortex and the temporal lobes with the thalamus and the hypothalamus
Causes emotion Involved in memory and learning Hippocampus involved in processing short-term
memory to become long-term memory
Central Nervous System
• The diencephalon Hypothalamus
Forms the floor of the third ventricle Integrating center involved in homeostasis Regulates hunger, sleep, thirst, body
temperature, and water balance Link between nervous and endocrine systems
Thalamus Located in sides and roof of the third ventricle Functions as a sensory relay center Involved in arousal of the cerebrum Participates in memory and emotions
Pineal gland regulates body’s daily rhythms
Central Nervous System
• The cerebellum Separated from brain stem by the fourth
ventricle Two hemispheres composed primarily of
white matter Receives sensory input from the eyes,
ears, joints, and muscles Receives motor output from the cerebral
cortex Maintains posture and balance and
ensures smooth, coordinated voluntary movement
Assists in learning of new motor skills
Central Nervous System
• The brain stem Midbrain
Relay station between cerebrum and the spinal cord or cerebellum
Reflex centers for visual, auditory, and tactile responses
Pons Contains bundles of axons traveling between the
cerebellum and the rest of the CNS Helps regulate breathing rate
Medulla oblongata Vital reflex centers Reflex centers for vomiting, coughing, sneezing,
hiccupping, and swallowing Reticular formation
Involved in maintaining muscle tone Assists in regulating respiration, heart rate, and
blood pressure Helps rouse a sleeping person
Peripheral Nervous System
o Lies outside of CNSo Composed of nerves and gangliao Subdivisions
• Afferent (sensory) Somatic sensory
Serves the skin, skeletal muscles, joints, and tendons Special senses
Visceral sensory supplies the internal organs• Efferent (motor)
Somatic motor system carries commands from CNS to the skeletal muscles
Autonomic motor system regulates cardiac and smooth muscle and glands
Peripheral Nervous System
o Cranial nerves• 12 pairs• Sensory nerves contain only sensory fibers• Motor nerves contain only motor fibers• Mixed nerves contain both sensory and
motor fibers• Mostly involved with the head, neck, and
facial regions of the body• The vagus nerve (cranial nerve X) has
sensory and motor branches to the face and most of the internal organs
Fig 8.11a
Peripheral Nervous System
o Spinal nerves• 31 pairs• Designated according to their location
in relation to the vertebrae• Mixed nerves
Sensory fibers enter at the posterior root Motor fibers exit at the anterior root
• The cell body of a sensory neuron is in a posterior-root ganglion
Fig 8.11b
Peripheral Nervous System
o Somatic Motor Nervous System and Reflexes
• Voluntary actions that originate in the motor cortex
• Reflexes are automatic involuntary responses
Cranial reflexes involve the brain Spinal reflex involves only the spinal cord
Peripheral Nervous System
o Autonomic Motor Nervous System and Visceral Reflexes
• Two divisions of ANS Function automatically and involuntarily Innervate all internal organs Utilize two motor neurons and one
ganglion for each impulse
• Visceral reflexes are important to the maintenance of homeostasis
Peripheral Nervous System
• Sympathetic division of ANS (“Fight or Flight”) Preganglionic fibers arise from the thoracic-lumbar
portion of the spinal cord Preganglionic fibers are short and postganglionic fibers
are long Accelerates heartbeat and dilates the bronchi Inhibits the digestive tract Neurotransmitter released by the postganglionic neuron
is primarily norepineprhine (NE)
TA 8.3
Peripheral Nervous System
• Parasympathetic division of ANS (“Rest and Digest”) Includes a few cranial nerves and preganglionic fibers
that arise from the sacral portion of the spinal cord (craniosacral portion of ANS)
Preganglionic fiber is long and postganglionic fiber is short
Promotes digestion of food, slows heart rate, and decreases the strength of cardiac contraction
Acetylcholine (Ach) is the neurotransmitter of the parasympathetic division
TA 8.4
Table 8.4 not available in this printed version
Fig 8.14
Effects of Aging
o Brain mass decreaseso Learning, memory, and reasoning
declineo Neurotransmitter production
decreaseso Although structural changes occur,
mental impairment is not an automatic consequence of getting older
Homeostasis
o Detects, interprets, and responds to changes in the internal and external environment
o With the endocrine system, it coordinates and regulates the functioning of other systems
o The hypothalamus and medulla oblongata control vital functions
o Sympathetic division of the ANS works to keep us from danger
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