Chapter 7 The Nervous System

Chapter 7 The Nervous System

Be Able To

• List the general functions of the nervous system.

• Explain the structural and functional classifications of the nervous system.

• Define central and peripheral nervous system.

• State the function of neurons and neuroglia.

• Describe the anatomy of a neuron.

Organization of the Nervous System

• The nervous system has 3 overlapping functions:

1) Monitors changes inside/outside the body.

2) Processes and interprets sensory input and performs integration 3) Effects a response called motor

output.

• The structural classification has 2 subdivisions: central nervous

system (CNS) consists of brain and spinal cord and the peripheral

nervous system (PNS) consists of spinal and cranial nerves.

• The functional classification is based on PNS structures in 2 subdivisions:

sensory or afferent division and the motor or efferent division.

The average human brain has 100 billion neurons; average octopus

brain has 30 billion neurons.

Organization of the Nervous System

• Afferent division consists of nerve fibers that convey impulses to the CNS from sensory receptors, somatic or visceral sensory fibers.

• Efferent division carries impulses from the CNS to effector structures.

• The efferent division has 2 subdivsions:

1) Somatic nervous system allows voluntary control.

2) Autonomic nervous system regulates events involuntarily.

There are more neurons in the brain than stars in the Milky Way.

Nervous Tissue:

• Supporting cells of the CNS are called neuroglia or glia which

support, insulate and protect neurons

• The CNS neuroglia or glia include:

1) Astrocytes brace and anchor

neurons to blood capillaries, protect, and control the chemical environment of the brain.

2) Microglia dispose of debris.

3) Ependymal cells circulate cerebrospinal fluid in the CNS.

4) Oligodendrocytes form a insulating layer around nerve fibers.

• Supporting cells of the PNS include Schwann cells that form myelin sheath around nerve fibers and satellite cells that protect and cushion.

The diameter of a neuron ranges from 4-100 microns.

Nervous Tissue:

• Neurons transmit messages from one part of the body to another.

• All have a cell body and 1 or more slender processes.

• Nissl substance and

neurofibrils are important in maintaining shape.

• Dendrites convey incoming

messages toward the cell body, whereas axons conduct

messages away.

• Each neuron has 1 axon but could have hundreds of

dendrites.

• Each axon branches at its tips called axonal terminals.

A fetus gains 250,000 neurons a minute.

http://faculty.washington.edu/chudler/cells.html

Nervous Tissue:

• The synaptic cleft is a junction between neurons.

• Most nerve fibers are covered by myelin which protects and insulates fibers.

• Axons outside the CNS are myelinated by Schwann cellls forming a myelin sheath.

• Gaps between Schwann cells are called nodes of Ranvier.

• In the CNS oligodendrcytes form myelin sheaths, however they are unable to regenerate.

• Clusters of neurons in the CNS are called nuclei, whereas they are called ganglia in the PNS and bundles of

nerve fibers in the CNS are called nerve tracts, whereas in the PNS they are

called nerves.

The longest axon is 15 feet long.

What animal is this found in?

Chapter 7 The Nervous System

Be Able To

• Classify neurons according to structure and function.

• List the types of general sensory receptors and describe their functions.

• Describe the events that lead to the generation of a nerve impulse and its conduction from one neuron to another.

• Define reflex arc and list its elements.

Nervous Tissue: Classifucaton

• Functional classification groups neurons by the direction of a nerve impulse.

• Sensory or afferent neurons carry impulses from sensory receptors, e.g.

cutaneous or proprioceptors.

• Motor or efferent receptors carry impulses from the CNS.

• Association neurons or interneurons connect sensory and motor neurons.

• Structural classification is based on number of processes.

• Multipolar neurons have several processes.

• Bipolar neurons have 1 axon and dendrite.

• Unipolar neurons have a single

process emerging from the cell body.

Lined up, how far would the neurons in our bodies stretch?

Nervous Tissue: Physiology of Nerve Impulse

• Resting neurons are polarized, fewer positive ions inside the neuron.

• Upon stimulation by a stimulus, the

permeability of the membrane changes briefly.

• The inward rush of Na+ changes the polarity or depolarization.

• If the stimulus is strong enough an action potential or nerve impulse is generated.

• The neuron is almost immediately repolarized by the loss of K+ ions returning to rest.

• ATP is required to power the NA+ K+

pump.

• The transmission of an impulse is an electrochemical event.

Nerve impulses can reach up to 250 miles/hour along a neuron.

How Neurons Communicate

Nervous Tissue: Reflex Arcs

• Reflexes are rapid, predictable, and involuntary responses to stimuli.

• Reflex arcs occur in one direction.

• Autonomic reflexes regulate digestion, elimination, blood pressure, and sweating.

• Somatic reflexes are reflexes that stimulate skeletal muscles.

• All reflex arcs involve:

– A sensory receptor, an effector organ, afferent and efferent neurons to connect the two.

Chapter 7 The Nervous System

Be Able To

• Identify and indicate the functions of the major

regions of the cerebral hemispheres, diencephalon, brain stem, and cerebellum on a human brain

model or diagram.

• Name the three meningeal layers and state their functions.

• Discuss the formation and function of cerebrospinal fluid

and the blood-brain barrier.

• Compare the signs of a CVA with those of Alzheimer's disease;

of a contusion with those of a concussion.

• Define EEG and explain how it evaluates neural

functioning.

Central Nervous System:

• During embryonic

development the CNS begins as the neural tube.

• Brain formation begins during the 4

week of development.

• 4 chambers or ventricles form in the brain as well.

• The brain may be divided into 4 major regions:

1) Cerebral hemispheres 2) Diencephalon

3) Brain stem 4) cerebellum

Fetus at 7 weeks

Central Nervous System:

• The paired cerebral

hemispheres are the most superior & largest regions.

• Gyri are large elevated ridges of tissue separated by shallow grooves called sulci.

• Fissures are deeper grooves that separate large regions.

• The longitudinal fissure separates the left and right hemispheres.

• Lobes of the brain are regions that are named by the cranial bones above them.

An elephant’s brain weighs 13 lbs.

Central Nervous System:

• Parietal lobe contains the somatic sensory area.

• The sensory pathways are crossed.

• Occipital lobe is a visual area.

• Temporal lobe is the auditory area and olfactory area.

• Frontal lobe is the primary motor area that allows for conscious

skeletal muscle movement.

• The pyramidal or corticospinal tract forms from neurons in the frontal lobe.

• Broca’s area found at the base of the precentral gyrus is responsible for speech.

78% of the human brain is made of water.

Central Nervous System:

• Intellectual reasoning originates in the frontal lobe.

• Complex memories are stored in the temporal and frontal lobes.

• Language comprehension is housed in the frontal lobe.

• The cerebral cortex is the

outermost region of the cerebrum where consciousness takes place.

• The remaining deep cerebral

tissue is composed of fiber tracts.

• The corpus callosum connects the left and right hemispheres.

The cerebral cortex is 77% of the human brain

Central Nervous System:

• The diencephalon is the central core of the forebrain composed of: the thalamus, hypothalamus, and

epithalamus.

• The thalamus makes up 80% of the diencephalon.

• All sense impulses converge on the thalamus which plays a key role in mediating

sensation, motor cavities,

learning, and memory.

Central Nervous System:

• The hypothalamus caps the top of the brain stem.

• The paired mammillary bodies associated with the hypothalamus are relay stations for olfaction.

• The hypothalamus is the main

visceral control center of the body and is vital to homeostasis.

• The hypothalamus influences blood pressure, digestion,

respiratory rate and depth, perception of pleasure and

biological rhythms, body temp.

regulation, hunger, thirst, sleep, endocrine system.

hypothalamus

Central Nervous System:

• The brain stem is

composed of the midbrain, pons, and medulla

oblongata.

• Brain stem produces rigid automatic behaviors

necessary for survival.

• The brain stem provides a a pathway for fiber tracts.

• It is also associated with 10-12 pairs of cranial

nerves.

Brain stem

Central Nervous System:

• The midbrain is located

between the diencephalon and the pons.

• Two ventral bulging cerebral peduncles contain the large pyramidal tracts.

• A hollow cerebral aqueduct connects the 3

and 4

ventricles.

• The corpora quadrigemina is a group of four protrusions that

are the visual reflex centers that coordinate our involuntary head and eye movements.

• The midbrain is the most

superior part of the CNS that contains motor neurons.

Posterior view of the midbrain

Central Nervous System:

• The pons is a bulging region between the

midbrain and the medulla oblongata.

• The pons is composed of nerve tracts that connect the higher brain centers and the spinal cord.

• The pneumotaxic center is an area of the pons

involved in the normal rhythm of breathing .

Pons and medulla oblongata anterior view

Pons

Central Nervous System:

• The medulla oblongata or

medulla is the most inferior region of the brain stem.

• It blends with the spinal cord at the foramen magnum.

• The central canal of the spinal column passes through the medulla.

• The medulla has a crucial role in controlling heart rate, blood

pressure, rate and depth of breathing, and other activities such as vomiting, hiccupping, swallowing, coughing, and sneezing.

Anterior view of the medulla oblongata

Central Nervous System:

• The reticular formation is an intricate system of clustered neurons of gray matter.

• Neurons in the reticular formation are involved in motor control of visceral organs.

• The reticular activating system or RAS maintain cerebral cortical alertness by filtering out repetitive stimuli (awake/sleep cycles).

• Permanent damage to this area

could result in a coma.

Central Nervous System:

• The cauliflower-like cerebellum

accounts for 11% of total brain mass.

• The 2 cerebral hemispheres protrude under the occipital lobes.

• The cerebellum processes inputs from the cerebral motor cortex, brain stem nuclei, and sensory receptors for

skeletal and smooth muscle.

• It controls are balance and equilibrium.

• The cerebellum sends messages to

skeletal and smooth muscle to initiate

appropriate corrective measures for

efficient and effective muscle control.

Protection of the Central Nervous System:

Meninges

• The meninges are 3 connective tissue membranes that cover and protect the CNS, blood vessels, form partitions in the skull, and contain CSF.

• The dura mater is the outermost layer composed of the periosteal layer and the deeper meningeal layer.

• The inner meningeal extends inward to form several folds that act as partitions.

• The arachnoid mater is the middle membrane that attaches to the

innermost layer.

• The delicate pia mater is the innermost layer that clings tightly to the brain and spinal cord and is rich in blood vessels.

Protection of the Central Nervous System:

Cerebrospinal Fluid

• The CSF is a liquid cushion found around the brain and spinal cord.

• The CSF gives buoyancy to the brain, reducing its overall weight.

• CSF is similar in composition to blood plasma, but contains less

protein, more vitamin C, and differs in ion concentration.

• The choroid plexuses hang from the roof of each ventricle and form CSF.

• CSF volume is replaced every 3-4 hours.

• CSF flows freely through the ventricles.

Circulation of CSF

Protection of the Central Nervous System:

THE BLOOD BRAIN BARRIER

• The blood-brain barrier is a

protective mechanism that helps maintain a stable environment for the brain.

• If chemical concentrations fluctuated neurons would fire uncontrollably.

• The blood-brain barrier is selective.

• Capillaries of the choroid plexuses are porous but ht

ependymal cells that surround them have tight junctions.

• Since newborns have an

incomplete blood-brain barrier

they are more susceptible to

toxins.

Protection of the Central Nervous System:

HOMEOSTATIC IMBALANCES OF THE BRAIN

• A concussion is a slight brain injury, usually with no permanent damage.

• A contusion is an area of tissue destruction.

• Hemorrhages of the subdural or subarachnoid space can increase intracranial pressure.

• Cerebral edema or swelling of the brain is the uptake of water by brain tissue following head trauma.

• Cerbrovascular accidents or strokes occur when blood circulation to a brain area is blocked and vital brain tissue dies.

• Transient ischemic attacks are

temporary strokes that last from 5 to 50 minutes with various symptoms.

hydrocephalus

Chapter 7 The Nervous System

Be Able To

• List two important functions of the spinal cord and its structure.

• Describe the general structure of a nerve.

• Identify the cranial nerves by number and by name, and list the major functions of each.

• Describe the origin and fiber composition of

dorsal and ventral nerve roots, spinal nerves,

and dorsal and ventral rami.

Central Nervous System:

• The spinal cord extends from the foramen magnum to the 1^st or 2^nd lumbar vertebra.

• It is a two-way conduction pathway to and from the brain.

• Spinal reflexes are initiated and completed at the spinal cord level.

• The single layered dura mater is called the spinal dural sheath.

• Between the vertebrae and the dural sheath is the epidural space filled with fat and

veins.

• CSF fills the space between the arachnoid and pia mater meninges.

• The meninges extend below the end of the spinal cord and for this reason spinal taps or lumbar punctures are done there.

• 31 pairs of spinal nerves arise from the spinal cord by paired roots.

The cauda equina is a collection of nerve roots at the end of the vertebral canal.

Central Nervous System:

gray matter of the spinal cord and spinal roots

• The gray matter consists of a mixture of cell bodies, unmyelinated processes, and neuroglia.

• The anterior or ventral horns house somatic motor neurons that exit via the ventral roots to muscles.

• Lateral horn neurons exiting via the ventral roots are autonomic motor neurons that serve visceral organs.

• The posterior or dorsal horns contain

association neurons or interneurons that enter the cord via the dorsal root.

• The nerve cell bodies of these neurons are found in the dorsal root ganglion.

• The dorsal and ventral roots fuse laterally to form the spinal nerves.

• Polio results from the destruction of anterior horn motor neurons by the poliovirus.

• Damage to the ventral root results in paralysis of muscles served.

Central Nervous System:

white matter of the spinal cord

• The white matter is composed of

myelinated and unmyelinated nerve fibers that allow communication between different parts (ascending, descending, and

transversely) of the spinal cord and brain.

• White matter is divided into anterior, lateral, and posterior funiculi.

• Each funiculus contains several nerve tracts.

• Tabes dorsalis is a slow deterioration of the posterior white matter tracts that result in loss of muscle coordination caused by the syphilis bacterium.

• Amyotrophic lateral sclerosis ALS or Lou Gehrig’s disease is a progressive

deterioration of anterior horn motor neurons and fibers resulting eventually in inability to breathe.

Peripheral Nervous System:

structure of nerves

• A nerve is a cordlike organ that consists of parallel bundles of peripheral axons

enclosed by connective tissue.

• Each neuron is surrounded by endoneurium.

• Groups of fibers are bundled into a

fascicle surrounded by coarse connective tissue.

• The epinerium is a tough fibrous sheath that encloses fascicles to form a nerve.

• Nerves are classified by the direction in which they transmit impulses (afferent, efferent, and mixed nerves).

• Peripheral nerves are further classified as cranial or spinal nerves.

• Ganglia are collections of neuron cell bodies associate with nerves of the PNS.

Axonal regeneration is possible in some situations.

Peripheral Nervous System:

• 12 pairs of cranial nerves are associated with the brain.

• The 1^st two pairs attach to the

forebrain and the remaining originate from the brain stem.

• The paired vagus nerves are the only ones to extend below the neck.

• The names of the cranial nerves

reveal the structures or functions they serve.

• They are numbered in roman

numerals from the most rostral to most caudal.

• Most are mixed nerves except for the olfactory, optic, and

vestibulocochlear which are afferent.

Peripheral Nervous System:

• 31 pairs of spinal nerves arise from the spinal cord and supply all parts of the body except the head and neck.

• All are mixed nerves.

• They are named by the point of issue from the spinal cord:

8 pairs of cervical nerves(C₁-C₈), 12 pairs of thoracic nerves (T₁-T₁₂), 5 pairs of lumabr

nerves (L₁-L₅), 5 pairs of sacral nerves (S₁-S₅, and 1 pair of tiny coccygeal nerves (C₀)

• Each spinal nerve connects to the spinal cord via the dorsal or ventral roots.

• After emerging from its foramen it divides into dorsal and ventral rami.

• Each ramus is a mixed nerve.

• The smaller dorsal rami serve the skin and muscles of the posterior trunk.

• The ventral rami T₁-T₁₂ supply the muscles of the ribs and skin of the anterior and lateral trunk.

• The other ventral rami form a complex of nerves called plexuses.

Autonomic Nervous System

• The ANS innervates smooth and cardiac muscles and glands.

• Makes adjustments as necessary to ensure optimal support for body

activities.

• The ANS has a chain of 2 motor

neurons: the first in the brain or spinal cord and the second outside the CNS.

• The first neuron’s axon is the

preganglionic axon and the second motor neuron’s axon is the

postganglionic axon.

• The 2 divisions of the ANS are the sympathetic and parasympathetic.

• The 2 divisions serve the same organs but cause opposite effects to keep the body running smoothly.

Autonomic Nervous System:

parasympathetic anatomy and physiology

• The parasympathetic division is active in nonstressful situations.

• Several cranial nerves contain parasympathetic outflow.

• The preganglionic axons synapse with the 2^nd neurons in a terminal ganglion and from their the postganglionic axons extends a short

distance to the organs they serve.

• Cranial nerves III, VII, and IX supply the entire parasympathetic innervation of the head.

• The Vagus nerves X account for 90% of all preganglionic fibers in the body where they

“ride” the esophagus down to enter the abdominal cavity.

• In S₁-S₄ of the spinal cord the preganglionic axons leave to form the pelvic sphlanchic nerves.

• The parasympathetic division functions to regulate body activities at low normal levels.

Autonomic Nervous System:

sympathetic anatomy and physiology

• The sympathetic division or

thoracolumbar division is also known as the “fight-or-flight” system.

• It supplies both internal organs and visceral structures in the somatic body.

• All preganglionic fibers arise from cell bodies of neurons in spinal cord gray matter segments T

–L

.

• They leave through the ventral root and pass through ramus communicans to enter a chain of ganglion called the

sympathetic chain or trunk along the vertebral column.

• The function of this division is to provide optimal conditions for appropriate

responses to threatening situations.

Developmental Aspects of the Nervous System

• The nervous system is very active during early embyronic development, therefore, it is very susceptible to pathogens and

chemicals.

• Cerebral palsy is a neuromuscular disability of voluntary muscles.

• Hydrocephalus, anencephaly, and spina bifida are other congenital malformations.

• The hypothalamus is one of the last regions to develop.

• Brain size reaches its peak in young adulthood.

• As we age the sympathetic division becomes less efficient.

• Deterioration of the circulatory system causes many nervous system disorders.

• Shrinking of the brain is common in the aging process, but may be increased in speed by lifestyles.

Spina bifida can be so severe that spinal cord and

its coverings protrude from the back.