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Chapter 15

Neural Integration

 

Sensory Integration

•      Survival depends upon sensation and perception

•      Sensation is the awareness of changes in the internal and external environment

•      Perception is the conscious interpretation of those stimuli

Organization of the Somatosensory System

•      Input comes from exteroceptors, proprioceptors, and interoceptors

•      The three main levels of neural integration in the somatosensory system are:

•    Receptor level – the sensor receptors

•    Circuit level – ascending pathways

•    Perceptual level – neuronal circuits in the cerebral cortex

Processing at the Receptor Level

•      Receptor potential – a graded potential from a stimulated sensory receptor

•      Generator potential – depolarization of the afferent fiber caused by a receptor that is a separate cell (e.g., hair cell of the ear’s hearing receptor)

•      If the receptor potential is above threshold, an action potential is sent to the CNS

•      Strength of stimulus is determined by the frequency of action potentials

Adaptation of Sensory Receptors

•      Adaptation occurs when sensory receptors are subjected to an unchanging stimulus

•    Receptor membranes become less responsive

•    Receptor potentials decline in frequency or stop

•      Receptors responding to pressure, touch, and smell adapt quickly

•      Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood

Processing at the Circuit Level

•      Chains of three neurons (1st, 2nd, and 3rd order) conduct sensory impulses upward to the brain

•      First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem

•      Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum

•      Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum

Main Ascending Pathways

•      The central processes of fist-order neurons branch diffusely as they enter the spinal cord and medulla

•      Some branches take part in spinal cord reflexes

•      Others synapse with second-order neurons in the cord and medullary nuclei

•      Pain fibers synapse with substantia gelatinosa neurons in the dorsal horn

•      Fibers from touch and pressure receptors form collateral synapses with interneurons in the dorsal horns

Three Ascending Pathways

•      The nonspecific and specific ascending pathways send impulses to the sensory cortex

•    These pathways are responsible for discriminative touch and conscious proprioception

•      The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception

Specific and Posterior Spinocerebellar Tracts

•      Specific ascending pathways within the fasciculus gracilis and fasciculus cuneatus tracts, and their continuation in the medial lemniscal tracts

•      The posterior spinocerebellar tract

Nonspecific Ascending Pathway

•      Nonspecific pathway for pain, temperature, and crude touch within the lateral spinothalamic tract

Processing at the Perceptual Level

•      The thalamus projects fibers to:

•    The somatosensory cortex

•    Sensory association areas

•      First one modality is sent, then those considering more than one

•      The result is an internal, conscious image of the stimulus

Main Aspects of Sensory Perception

•      Perceptual detection – detecting that a stimulus has occurred and requires summation

•      Magnitude – how much of a stimulus is acting

•      Spatial discrimination – identifying the site or pattern of the stimulus

•      Feature abstraction – used to identify a substance that has specific texture or shape

•      Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet  or sour tastes)

•      Pattern recognition – ability to recognize patterns in stimuli (e.g., melody, familiar face)

Motor Integration

•      In the motor system:

•    There are effectors (muscles) instead of sensory receptors

•    The pathways are descending efferent circuits, instead of afferent ascending ones

•    There is motor behavior instead of perception

Levels of Motor Control

•      The three levels of motor control are:

•    Segmental level

•    Projection level

•    Programs/instructions level

Segmental Level

•      The segmental level is the lowest level of motor hierarchy

•      It consists of segmental circuits of the spinal cord

•      Its circuits control locomotion and specific, oft-repeated motor activity

•      These circuits are called central pattern generators (CPGs)

Projection Level

•      The projection level consists of:

•    Cortical motor areas that produce the direct (pyramidal) system

•    Brain stem motor areas that oversee the indirect (mulitneuronal) system

•      Helps control reflex and fixed-pattern activity and houses command neurons that modify the segmental apparatus

Descending (Motor) Pathways

•      Descending tracts deliver efferent impulses from the brain to the spinal cord, and are divided into two groups

•    Direct pathways equivalent to the pyramidal tracts

•    Indirect pathways, essentially all others

•      Motor pathways involve two neurons (upper and lower)

The Direct (Pyramidal) System

•      Direct pathways originate with the pyramidal neurons in the precentral gyri

•      Impulses are sent through the corticospinal tracts and synapse in the anterior horn

•      Stimulation of anterior horn neurons activates skeletal muscles

•      Parts of the direct pathway, called corticobulbar tracts, innervate cranial nerve nuclei

•      The direct pathway regulates fast and fine (skilled) movements

Indirect (Extrapyramidal) System

•      Includes the brain stem, motor nuclei, and all motor pathways not part of the pyramidal system

•      This system includes the rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts

•      These motor pathways are complex and multisynaptic, and regulate:

•    Axial muscles that maintain balance and posture

•    Muscles controlling coarse movements of the proximal portions of limbs

•    Head, neck, and eye movement

Extrapyramidal (Multineuronal) Pathways

•      Reticular nuclei – maintain balance

•      Vestibular nuclei – receive input from the equilibrium apparatus of the ear and from the cerebellum

•      Vestibulospinal tracts – control the segmental apparatus during standing

•      Red nuclei – control flexor muscles

•      Superior colliculi and tectospinal tracts mediate head movements

Programs and Instructions Level

•      The program/instructional level integrates the sensory and motor systems

•      This level is called the precommand area

•      They are located in the cerebellum and basal nuclei 

•    Regulate precise start/stop movements and coordinate movements with posture

•    Block unwanted movements and monitor muscle tone

Brain Waves

•      Normal brain function involves continuous electrical activity

•      An electroencephalogram (EEG) records this activity

•      Patterns of neuronal electrical activity recorded are called brain waves

•      Each person’s brain waves are unique

Types of Brain Waves

•      Alpha waves – low-amplitude, slow, synchronous waves indicating an “idling” brain

•      Beta waves – rhythmic, more irregular waves occurring during the awake and mentally alert state

•      Theta waves – more irregular than alpha waves; common in children but abnormal in adults

•      Delta waves – high-amplitude waves seen in deep sleep and when reticular activating system is damped

Brain Waves: State of the Brain

•      Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body

•      EEGs can be used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions

•      A flat EEG (no electrical activity) is clinical evidence of death

Epilepsy

•      A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure

•      Epilepsy is not associated with, nor does it cause, intellectual impairments

•      Epilepsy occurs in 1% of the population

Epileptic Seizures

•      Absence seizures, or petit mal – mild seizures seen in young children where the expression goes blank

•      Temporal lobe epilepsy – the victim loses contact with reality and may experience hallucinations, flashbacks, and emotional outburst

•      Grand mal seizures – victim loses consciousness, bones are often broken due to intense convulsions, loss of bowel and bladder control, and severe biting of the tongue

Control of Epilepsy

•      Epilepsy can usually be controlled with anticonvulsive drugs

•      Valproic acid, a nonsedating drug, enhances GABA and is a drug of choice

•      Vagus nerve stimulators can be implanted under the skin of the chest and can keep electrical activity of the brain from becoming chaotic

Consciousness

•      Encompasses perception of sensation, voluntary initiation and control of movement, and capabilities associated with higher mental processing

•      Involves simultaneous activity of large areas of the cerebral cortex

•      Is superimposed on other types of neural activity

•      Is holistic and totally interconnected

•      Clinical consciousness is defined on a continuum that grades levels of behavior – alertness, drowsiness, stupor, coma

Types of Sleep

•      There are two major types of sleep:

•    Non-rapid eye movement (NREM)

•    Rapid eye movement (REM)

•      One passes through four stages of NREM during the first 30-45 minutes of sleep

•      REM sleep occurs after the fourth NREM stage has been achieved

Types and Stages of Sleep: NREM

•      NREM stages include:

•    Stage 1 – eyes are closed and relaxation begins; the EEG shows alpha waves; one can be easily aroused

•    Stage 2 – EEG pattern is irregular with sleep spindles (high-voltage wave bursts); arousal is more difficult

•    Stage 3 – sleep deepens; theta and delta waves appear; vital signs decline; dreaming is common

•    Stage 4 – EEG pattern is dominated by delta waves; skeletal muscles are relaxed; arousal is difficult

Types and Stages of Sleep: REM

•      REM sleep is characterized by:

•    EEG pattern reverts through the NREM stages to the stage 1 pattern

•    Vital signs increase

•    Skeletal muscles (except ocular muscles) are inhibited

•    Most dreaming takes place

Sleep Patterns

•      Alternating cycles of sleep and wakefulness reflect a natural circadian rhythm

•      Although RAS activity declines in sleep, sleep is more than turning off RAS

•      The brain is actively guided into sleep

•      The suprachiasmatic and preoptic nuclei of the hypothalamus regulate the sleep cycle

•      A typical sleep pattern alternates between REM and NREM sleep

Importance of Sleep

•      Slow-wave sleep is presumed to be the restorative stage

•      Those deprived of REM sleep become moody and depressed

•      REM sleep may be a reverse learning process where superfluous information is purged from the brain

•      Daily sleep requirements decline with age

Sleep Disorders

•      Narcolepsy – lapsing abruptly into sleep from the awake state

•      Insomnia – chronic inability to obtain the amount or quality of sleep needed

•      Sleep apnea – temporary cessation of breathing during sleep

Memory

•      Memory is the storage and retrieval of information

•      The three principles of memory are:

•    Storage – occurs in stages and is continually changing

•    Processing – accomplished by the hippocampus and surrounding structures

•    Memory traces – chemical or structural changes that encode memory

Stages of Memory

•      The two stages of memory are short-term memory and long-term memory

•      Short-term memory (STM, or working memory) – a fleeting memory of the events that continually happen

•      STM lasts seconds to hours and is limited to 7 or 8 pieces of information

•      Long-term memory (LTM) has limitless capacity

Transfer from STM to LTM

•      Factors that effect transfer of memory from STM to LTM include:

•    Emotional state – we learn best when we are alert, motivated, and aroused

•    Rehearsal – repeating or rehearsing material enhances memory

•    Association – associating new information with old memories in LTM enhances memory

•    Automatic memory – subconscious information stored in LTM

Categories of Memory

•      The two categories of memory are fact memory and skill memory

•      Fact (declarative) memory:

•    Entails learning explicit information

•    Is related to our conscious thoughts and our language ability

•    Is stored with the context in which it was learned

Skill Memory

•      Skill memory is less conscious than fact memory and involves motor activity

•      It is acquired through practice

•      Skill memories do not retain the context in which they were learned

Structures Involved in Fact Memory

•      Fact memory involves the following brain areas:

•    Hippocampus and the amygdala, both limbic system structures

•    Specific areas of the thalamus and hypothalamus of the diencephalon

•    Ventromedial prefrontal cortex and the basal forebrain

Major Structures Involved with Skill Memory

•      Skills memory involves:

•    Corpus striatum – mediates the automatic connections between a stimulus and a motor response

•    Portion of the brain receiving the stimulus (visual in this figure)

•    Premotor and motor cortex

Mechanisms of Memory

•      The engram, a hypothetical unit of memory, has never be elucidated

•      Changes that take place during memory include:

•    Neuronal RNA content is altered

•    Dendritic spines change shape

•    Unique extracellular proteins are deposited at synapses involved in LTM

•    Presynaptic terminals increase in number and size, and release more neurotransmitter