Chapter 15

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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
• 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 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 • Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to • 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
http://rmoskowitz.tripod.com/2002neuralintegration.htm 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 http://rmoskowitz.tripod.com/2002neuralintegration.htm 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
• 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
• 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
Consciousness
• Encompasses perception of sensation, voluntary initiation and control of movement, and capabilities associated with
• 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:
http://rmoskowitz.tripod.com/2002neuralintegration.htm • 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 http://rmoskowitz.tripod.com/2002neuralintegration.htm • 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

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