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 (1^st, 2^nd, and 3^rd 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