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Human Anatomy, Second EditionMcKinley & O'Loughlin

Chapter 17 Lecture Outline:Pathways and Integrative

Functions

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General Characteristics of Nervous System Pathways CNS communicates with body structures

via pathways. sensory or motor information processing and integration occur

continuously along them Pathways travel through the white

matter of the spinal cord.

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Nervous System Pathways Consist of a tract and a nucleus. Tracts are groups or bundles of axons

that travel together in the CNS. Each tract may work with multiple

nuclei groups in the CNS. A nucleus is a collection of neuron cell

bodies located within the CNS.

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Nervous System Pathways Ascending pathways carry sensory

information Descending pathways transmit motor

information

90% of pathways cross over from one side of the body to the other side at some point (decussate). The left side of the brain processes information

from the right side of the body, and vice versa.

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Nervous System Pathways Most exhibit a precise correspondence

between receptors through axons to specific functional areas of the cerebral cortex or between the primary motor cortex and specific body regions that it controls (somatotopy).

All pathways are composed of paired tracts. Both left and right tracts are needed to innervate the body.

Pathways are a series of two or three neurons that work together.

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Nervous System Pathways Tracts named for

white column they are in where cell bodies originate where axons terminate Examples

Anterior spinothalamic tracts Ascending or descending? Sensory or motor?

Corticospinal tracts Ascending or descending? Sensory or motor?

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Sensory Pathways Sensory pathways

conduct information about limb position and the sensations such as touch, temperature, pressure, and pain

Somatosensory pathways process stimuli received from receptors

within the skin, muscles, and joints Viscerosensory pathways

process stimuli received from the viscera

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Sensory Receptors Detect stimuli and then conduct nerve

impulses to the CNS Sensory pathway centers within either

the spinal cord or brainstem process and filter the incoming sensory information. More than 99% of incoming

impulses do not reach the cerebral cortex and our conscious awareness!

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Functional Anatomy of Sensory Pathways Use two or three neurons to

transmit stimulus information from the body periphery to the brain

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Primary (First-Order) Neuron

The dendrites are part of the receptor that detects a specific stimulus.

The cell bodies reside in the posterior root ganglia of spinal nerves or the sensory ganglia of cranial nerves.

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Secondary (Second-Order) Neuron The axon of the primary neuron projects to a

secondary neuron within the CNS. It is an interneuron. The cell body resides within either the

posterior horn of the spinal cord or a medulla oblongata nucleus.

The axon projects to the thalamus, where it synapses with the tertiary neuron.

The second-order neuron is the one that crosses to the other side!

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Tertiary (Third-Order) Neuron Also an interneuron. Its cell body resides within the

thalamus. The thalamus is the central processing

and coding center for almost all sensory information.

It extends to the cerebral cortex.

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Posterior Funiculus-Medial Lemniscal Pathway Projects through the spinal cord, brainstem, and

diencephalon to the cerebral cortex. Tracts within the spinal cord (posterior funiculus or column)) Tracts from the brainstem to the thalamus (medial

lemniscus) Conducts sensory stimuli concerned with

conscious proprioceptive information about limb position and discriminative (fine) touch, precise pressure, and vibration sensations. Fasciculus gracilis - lower body Fasciculus cuneatus - upper body

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Anterolateral (Spinothalamic) Pathway Located in the anterior and lateral white funiculi of the

spinal cord. anterior spinothalamic tract - itch, tickle,

crude touch, pressure lateral spinothalamic tract - pain and

temperature Axons projecting from primary neurons enter the spinal

cord and synapse on secondary neurons within the posterior horns.

Axons of the secondary neurons cross over to the opposite side of the spinal cord through the anterior white commissure before ascending toward the thalamus.

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Spinocerebellar Pathway Conducts subconscious proprioceptive

information to the cerebellum for processing to coordinate body movements.

Information on proprioception stays on the same side

These are different from the other sensory pathways in that they do not use tertiary neurons. only primary and secondary neurons

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Integration of Sensory Information Sensory information joins a large

pool of sensory input - not all acted upon.

Integration in many regions with the help of association neurons (spinal cord, brain stem, cerebellum, cerebrum)

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Motor Pathways The motor response may initiate at

any level. Descending pathways in the brain and

spinal cord control the activities of skeletal muscles.

Formed from the cerebral nuclei, the cerebellum, descending projection tracts, and motor neurons

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Motor Pathways At least two motor neurons in the somatic motor

pathway: Upper motor neuron: within either the

cerebral cortex or a nucleus within the brainstem. Axons of the upper motor neuron synapse either directly on lower motor neurons or on interneurons that synapse directly on lower motor neurons. May be excitatory or inhibitory on lower motor neuron.

Lower motor neuron: the cell body is in the anterior gray horn of the spinal cord or within a brainstem cranial nerve nucleus. Always excitatory on skeletal muscle fibers.

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Motor Pathways The cerebral motor cortex has the most

control over precise skeletal muscle movements.

Most regulation of involuntary activities by the ANS originates in the brain stem and hypothalamus

After integration, there are 2 major descending tracts: Direct pathways responsible for conscious control

of skeletal muscle activity Indirect pathways responsible for unconscious

control of skeletal muscle activity

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Direct (Pyramidal) Pathways Pyramidal tracts

Corticobulbar -> head and neck

Lateral corticospinal -> Limbs

Anterior corticospinal -> Axial muscles

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Corticobulbar Tracts Originate from the facial region of the

motor homunculus within the primary motor cortex.

Axons extend to the brainstem, where they synapse with lower motor neuron cell bodies that are housed within brainstem cranial nerve nuclei.

Axons of these lower motor neurons help form the cranial nerves.

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Corticospinal Tracts The upper motor neurons descend from the cerebral

cortex through the brainstem and form a pair of thick bulges in the medulla called the pyramids.

Continue into the spinal cord to synapse on lower motor neurons in the anterior horn of the spinal cord.

Lateral corticospinal tracts serve the limbs cross in the medulla oblongata (90% of fibers)

Anterior corticospinal tracts serve the axial muscles

cross in the spinal cord at the level of the lower motor neuron

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Indirect (Extrapyramidal) Pathways Several nuclei within the brainstem

initiate motor commands for activities that occur at an unconscious level.

Multisynaptic and exhibit a high degree of complexity and circuitous routes.

Names do not include cortico!!!(Examples: rubrospinal, tectospinal,

vestibulospinal, lateral reticulospinal, and medial reticulospinal)

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Functions of Indirect Pathways Program automatic movements Help coordinate body movements

with visual stimuli Maintain skeletal muscle tone and

posture Major role in equilibrium by

regulating muscle tone in response to head movements

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Final Common Pathway There is a converging pathway

of upper motor neurons to the lower motor neuron.

These neurons may be excitatory and inhibitory from the direct and indirect pathways

The total determines the response

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Role of the Cerebral Nuclei Receive impulses from the entire cerebral

cortex Most of the output goes to the primary

motor cortex. No direct control over lower motor neurons. Provide the patterned background

movements needed for conscious motor activities by adjusting the motor commands issued in other nuclei.

Program habitual or automatic sequences and sets the appropriate muscle tone

Also selectively inhibit other motor neuron circuits

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Role of the Cerebellum Active in learning and performing rapid,

coordinated, highly skilled movements Maintains proper posture and equilibrium Comparator function

Monitors intentions (motor cortex, basal ganglia)

Monitors actual movement (proprioceptors) Compares Provides corrective feedback (nuclei of

brainstem and motor cortex via thalamus)

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Somatic Motor Control Several regions of the brain participate in the

control of motor activities. Motor programs require conscious directions

from the frontal lobes. Movement is initiated when commands are

received by the primary motor cortex from the motor association areas.

The cerebellum is critically important in coordinating movements because it specifies the exact timing of control signals to different muscles.

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Levels of Processing and Motor Control Simple reflexes that stimulate motor neurons

represent the lowest level of motor control. The nuclei controlling these reflexes are

located in the spinal cord and the brainstem. Brainstem nuclei also participate in more

complex reflexes. Initiate motor responses to control motor

neurons directly. Oversee the regulation of reflex centers

elsewhere in the brain.

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Cerebral Cortex Control highly variable and complex voluntary

motor patterns. Occupy the highest level of processing and

motor control. Motor commands may be conducted to

specific motor neurons directly. May be conveyed indirectly by altering the

activity of a reflex control center.

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Cerebrovascular Accident (CVA or Stroke) Most common brain disorder and the 3rd most

common cause of death in the US Two types

Ischemic from thrombus or embolus is most common

Hemorrhagic from ruptured blood vessel especially serious

Consequences depend on location and duration Transient ischemic attack (TIA), a “mini-stroke,”

a temporary, but serious warning Risk factors: age, family history, race and gender Lifestyle changes and treatment can help

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Higher-Order Processing and Integrative Functions Higher-order mental functions:

consciousness, learning, memory, and reasoning

occur within the cortex and involve multiple brain regions connected by complicated networks and arrays of axons

conscious and unconscious processing of information are involved in higher-order mental functions

may be continually adjusted or modified

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Development and Maturation of Higher-Order Processing From infancy on, our motor control and

processing capabilities increase in complexity. First year: increase number of cortical neurons Most CNS neurons continue to myelinate during first

2 yrs. Brain growth 95% complete by age 5 Some CNS axons don’t myelinate until teens (e.g.,

in prefrontal cortex) PNS neurons generally continue to myelinate past

puberty The ability to carry out higher-order mental

functions is a direct result of the level of nervous system maturation.

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Cerebral Lateralization Each hemisphere tends to be specialized

for certain tasks. Higher-order centers in both

hemispheres tend to have different but complementary functions.

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Cerebral Lateralization Left hemisphere is the categorical hemisphere

and it functions in categorization and symbolization.

contains Wernicke’s area and the motor speech area

specialized for language abilities important in performing sequential and analytical

reasoning tasks (science and mathematics) appears to direct or partition information into

smaller fragments for analysis Speech-dominant hemisphere.

controls speech in almost all right-handed people as well as in many left-handed ones

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Cerebral Lateralization Right hemisphere is called the representational

hemisphere. concerned with visuospatial relationships and

analyses the seat of imagination and insight, musical and

artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes

Both cerebral hemispheres remain in constant communication through commissures, especially the corpus callosum, which contains hundreds of millions of axons that project between the hemispheres.

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Cerebral Lateralization Use both hemispheres for most activities,

share memories, look almost identical Cerebral dominance refers to language

dominance, about 90% are left dominant Usually, right-handed, but about 10% are reversed

or shared. Lefties are more often male. If shared dominance, then increased dexterity and

strength in non-dominant hand and ambidexterity or cerebral confusion and learning disabilities, e.g., dyslexia

Each side exerts control over the other. Communication is good and fast through fiber tracts. Integrate functions well.

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Hemispherectomies and Cerebral Lateralization Epilepsy usually originates in one

hemisphere Drastic removal of the hemisphere is

used in some very severe cases High success rate in stopping epilepsy

but brain function not returned to normal Remaining hemisphere can take over

some functions of removed hemisphere especially in younger patients

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Language Higher-order processes involved in

language include reading, writing, speaking, and understanding words

Wernicke’s area key to ability to recognize written and spoken language

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Language Wernicke’s area is in the categorical

hemisphere The corresponding area in the

representational hemisphere recognizes the emotional content of speech. A lesion here results in an inability to understand emotional nuances in speech.

A lesion in the representational area opposite Broca’s area results in speech without emotional content

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Include awareness, knowledge, memory, perception and thinking

Association areas (70% of cerebrum) are responsible for cognition and processing and integration of info between sensory input and motor output areas

The frontal association area, prefrontal cortex, integrates info from sensory, motor, and association areas to enable thinking, planning and executing appropriate behavior (remember Phineas Gage)

Cognition

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An inherited learning disability, characterized by problems with single-word decoding

May have problems with reading, writing and spelling accurately

Occasionally, the ability to recognize and interpret the meaning of pictures and objects is also impaired.

May improve in many over time. Some researchers think it is a form of disconnect

syndrome where information transfer through the corpus callosum is impaired

Dyslexia

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Different lengths of time and storage capacity

Sensory memory occurs when we form important associations on sensory input - unlimited capacity

Working memory is necessary when we are trying to perform several different mental activities simultaneously

Memory

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Short-term memory follows sensory memory, associations based on sensory input Limited capacity (7 bits of info) Brief duration (seconds to hours)

Long-term memory May last limitless time Needs to be retrieved occasionally or be “lost”

Ability to store and retrieve information decreases with age

Memory

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Memory Brain must organize complex info into STM

before storing it in LTM. Conversion of STM to LTM is called

encoding, or memory consolidation Requires proper functioning of amygdala

and hippocampus Long-term memories are stored in the

association areas Different anatomic structures so the loss of

the ability to form STM does not affect the maintenance or accessibility of LTM

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Includes awareness of sensation, voluntary control of motor activities, and activities for higher mental processing

Reticular formation is in the brain stem and a bit in the diencephalon and spinal cord

Consciousness

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The reticular formation is a functional brain system with motor and sensory components The motor component communicates with the

spinal cord and regulates muscle tone esp. at rest and helps with autonomic motor functions

The reticular activating system (RAS) is the sensory component. Alerts the sensory cortex to incoming information through neurons. Responsible for maintaining a state of awareness (or consciousness), i.e., the physical basis for consciousness.

Consciousness

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Consciousness Levels of consciousness are continuous from sleep

to alertness RAS processes visual, auditory, and touch stimuli

and uses this information to keep cortex alert Filters and selects what info is sent on

to the conscious mind: essential, unusual, dangerous

RAS arouses us from sleep. Lack of stimuli to RAS allows us to sleep.

Damage can lead to a coma General anaesthetics suppress reticular activating

system

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Syncope or fainting Stupor - hard to arouse Coma - can’t arouse or respond to

environment Persistent vegetative state - may follow

a coma, may look somewhat normal and have spontaneous movements but unable to speak and do not respond to commands

Pathologic States of Unconsciousness

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Noticeable effects of aging on the brain and nervous system start about 30 yrs.

Decrease in the number of neurons and, therefore, number of synapses and decrease in sensory info

Decreased blood flow so less nutrients and less waste removal

Above lead to impaired cognitive capacity

Aging and the Nervous System

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Leading cause of dementia, loss of cognitive abilities.

11% of people over 65 yrs. 4th leading cause of death in elderly

Cause: ? Genetics and environment may play a role Changes in brain:

Cerebral atrophy, neurofibrillary tangles, amyloid precursor protein seen at autopsy

Histological changes esp. in hippocampus Decreased neurotransmitter, acetylcholine

Symptoms: Memory loss, confusion, disorientation, episodes of paranoia,

hallucination, or violent changes in mood No cure yet but some treatments help

Alzheimer Disease: The “Long Goodbye”