Chapter 14: Integration of Nervous System Functions

Integration of Nervous System Functions

Lecture Outline

Integration of the nervous system involves:
- Communication of sensory information from areas of the body to the CNS through specific sensory pathways.
- Control of effectors by the CNS through specific motor pathways.

14.1 Sensation

Sensation: Process initiated by stimuli acting on sensory receptors.
Perception: Conscious awareness of sensations.
Senses are the means by which the brain receives information about the environment and body.
- General senses.
- Special senses.

Types of Senses

General senses: Distributed over a large part of the body.
- Receptor generates an action potential called a generator potential that then travels to the brain.
- Somatic (information about the body and environment):
  - Touch, pressure, temperature, proprioception, pain.
  - Proprioception: sense of body position and movement.
- Visceral (information about internal organs):
  - Pain and pressure.
Special senses: Occur in special sense organs.
- Smell, taste, vision, hearing, balance.
- Receptor produces a receptor potential and the receptor then releases a neurotransmitter that binds to receptors on the membrane of a neuron which then travels to the brain.

Classification of the Senses

General Senses
- Somatic: Located in skin, muscles, joints
  - Pain
  - Pressure
  - Temperature
  - Proprioception
- Visceral: Located in internal organs
  - Pain
  - Pressure
Special Senses
- Smell
- Balance
- Taste
- Hearing
- Vision

Classification of Senses - Table 14.1

General Senses
- Somatic
  - Touch
    - Receptor Type: Mechanoreceptors
    - Initiation of Response: Compression of receptors
    - Examples: Meissner corpuscle, Hair follicle receptor, Merkel disk, Pacinian corpuscle, Ruffini end organ, Free nerve endings
  - Pressure
    - Receptor Type: Mechanoreceptors
    - Initiation of Response: Compression of receptors
    - Examples: Merkel disk
  - Proprioception
    - Receptor Type: Mechanoreceptors
    - Initiation of Response: Compression of receptors
    - Examples: Free nerve endings, Pacinian corpuscle, Muscle spindle, Golgi tendon organ
  - Temperature
    - Receptor Type: Thermoreceptors
    - Initiation of Response: Temperature around nerve endings
    - Examples: Free nerve endings, Cold receptors, Warm receptors
  - Pain
    - Receptor Type: Nociceptors
    - Initiation of Response: Irritation of nerve endings (mechanical, chemical, thermal)
    - Examples: Free nerve endings
- Visceral
  - Pain
    - Receptor Type: Nociceptors
    - Initiation of Response: Irritation of nerve endings
    - Examples: Free nerve endings
  - Pressure
    - Receptor Type: Mechanoreceptors
    - Initiation of Response: Compression of receptors
    - Examples: Pacinian corpuscle
Special Senses
- Smell
  - Receptor Type: Chemoreceptors
  - Initiation of Response: Binding of molecules to membrane receptors
  - Examples: Olfactory receptor
- Taste
  - Receptor Type: Chemoreceptors
  - Initiation of Response: Binding of molecules to membrane receptors
  - Examples: Taste receptor
- Vision
  - Receptor Type: Photoreceptors
  - Initiation of Response: Chemical change in receptors initiated by light
  - Examples: Rods and cones
- Hearing
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Bending of microvilli on receptor cells
  - Examples: Hair cells
- Balance
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Bending of microvilli on receptor cells
  - Examples: Hair cells

Sensory Receptors: Types Based on Stimulus

Mechanoreceptors: Compression, bending, stretching of cells.
- Touch, pressure, proprioception, hearing, and balance.
Chemoreceptors: Chemicals become attached to receptors on their membranes.
- Smell and taste.
Thermoreceptors: Respond to changes in temperature.
Photoreceptors: Respond to light: vision.
Nociceptors: Extreme mechanical, chemical, or thermal stimuli. Pain.

Sensory Receptors: Types Based on Location

Cutaneous receptors: Associated with skin.
Visceroreceptors: Associated with organs.
Proprioceptors: Associated with joints, tendons; detect stretch.
Eight types based on structure:
- Free nerve endings, Merkel disk, hair follicle receptor, Pacinian corpuscle, Meissner corpuscle, Ruffini end organ, muscle spindle, Golgi tendon organ.

Structural Classification of General Sensory Receptors - Table 14.2

Free nerve ending
- Structure: Branching, no capsule
- Function: Pain, itch, tickle, temperature, joint movement, and proprioception
Merkel disk
- Structure: Flattened expansions at the end of axons; each expansion is associated with a Merkel cell
- Function: Light touch and superficial pressure
Hair follicle receptor
- Structure: Wrapped around hair follicles or extending along the hair axis; each axon supplies several hairs, and each hair receives branches from several neurons, resulting in considerable overlap
- Function: Light touch; responds to very slight bending of the hair
Pacinian corpuscle
- Structure: Onion-shaped capsule composed of several cell layers with a single central nerve process
- Function: Deep cutaneous pressure, vibration, and proprioception
Meissner corpuscle
- Structure: Several branches of a single axon associated with specialized Schwann cells and surrounded by a connective tissue capsule
- Function: Two-point discrimination
Ruffini end organ
- Structure: Branching axon with numerous small, terminal knobs surrounded by a connective tissue capsule
- Function: Continuous touch or pressure; responds to depression or stretch of the skin
Muscle spindle
- Structure: Three to 10 striated muscle fibers enclosed by a loose connective tissue capsule, striated only at the ends, with sensory nerve endings in the center
- Function: Proprioception associated with detection of muscle stretch; important for control of muscle tone
Golgi tendon organ
- Structure: Surrounds a bundle of tendon fasciculi and is enclosed by a delicate connective tissue capsule; nerve terminations are branched, with small swellings applied to individual tendon fasciculi
- Function: Proprioception associated with the stretch of a tendon; important for control of muscle contraction

General Sensory Receptors

Free nerve endings respond to painful stimuli, temperature, itch, joint movement, and proprioception.
Merkel disks detect light touch and superficial pressure.
Hair follicle receptor detects light touch and slight bending of the hair.
Pacinian corpuscle detects deep cutaneous pressure, vibration, and proprioception.
Meissner corpuscles involved in two-point discrimination.
Ruffini end organ detects continuous touch or pressure and depression or stretch of the skin.

Free Nerve Endings

Simplest, most common sensory receptor.
Responsible for the sensations of pain, temperature, itch, and movement.
Scattered through most of the body, especially epithelia and C.T.
Type responsible for temperature sensation.
- Cold receptor: 10 to 15 times more numerous than warm.
- Warm receptor.
- Pain: responds to extreme cold or heat.

Merkel (Tactile) Disks

Axonal branches end as flattened expansions associated with epithelial cells.
Basal layers of epidermis.
Associated with dome-shaped mounds of thickened epidermis in hairy skin.
Light touch and superficial pressure.

Hair Follicle Receptors

Hair end organs.
Respond to slight bending of hair as occurs in light touch.
End organ receptor fields overlap; sensation not very localized, yet very sensitive.

Pacinian (Lamellated) Corpuscles

Single dendrite to layers of corpuscles arranged like leaves of an onion.
Deep dermis or hypodermis.
Deep cutaneous pressure; vibration.
When associated with joints, involved in proprioception.

Meissner (Tactile) Corpuscles

Distributed throughout dermal papillae.
Two-point discrimination: Ability to detect simultaneous stimulations at two points on the skin.
Used to determine texture of objects.
Numerous and close together on tongue and fingertips.

Ruffini End Organ

Primarily in dermis of fingers.
Respond to continuous touch or pressure.

Muscle Spindles

3 to 10 specialized skeletal muscle cells.
Provide information about the length of muscles.
Involved in the tone of postural muscles and the stretch reflex.

Golgi Tendon Organs

Proprioceptors associated with tendons.
Respond to increased tension on the tendon.

Responses of Sensory Receptors

Graded potential results from the interaction of the sensory receptor with a stimulus.
This graded potential is called a receptor potential.
Primary receptors: Axons conduct action potentials in response to receptor potential. Most receptors are in this category.
Secondary receptors: Cause release of neurotransmitters that bind to receptors on a neuron causing a receptor potential. Smell, taste, hearing, balance.

Comparison of Primary and Secondary Receptors

Primary Receptor: A mechanoreceptor (Pacinian corpuscle) is subjected to a pressure stimulus, the sensory receptor responds by generating a graded potential, also known as the receptor potential. When the graded potential reaches threshold, action potentials are generated in the axon. The action potentials are propagated towards the CNS.
Secondary Receptor: A chemoreceptor (taste cell) is subjected to a chemical stimulus in the form of a salty substance. The sensory receptor responds by generating a graded potential called a receptor potential. The sensory receptor releases a neurotransmitter into the synaptic cleft, which in turn stimulates the sensory neuron. When the stimulation reaches threshold, the neuron generates action potentials, which are propagated toward the CNS.

Responses of Sensory Receptors

Adaptation: Decreased sensitivity to a continued stimulus.
Proprioceptors: Provide information about the precise position and the rate of movement of various body parts, the weight of an object being held in the hand, and the range of movement of a joint.
Tonic receptors: Accommodate very slowly. Examples: Merkel disks and Ruffini end organs.
Phasic receptors: Accommodate rapidly and are more sensitive to changes. Examples: Pacinian and Meissner corpuscles.

Sensory Pathways

Transmit action potentials from the periphery to the brain.
Ascending pathways (tracts).
Each pathway is involved with a specific modality (type of information transmitted).
The first half of word indicates origin, the second half indicates termination.

Anterolateral System: Spinothalamic Tract

Major pathway in conscious perception of external stimuli.
Conveys pain, temperature, light touch, pressure, tickle, and itch.
Three-neuron system.
- Primary: From the periphery to the posterior horn of the spinal cord. Synapse with interneurons.
- Secondary: Cross to the opposite side, enter the spinothalamic tract, ascend to the thalamus.
- Tertiary: Thalamus to somatic sensory cortex.

Anterolateral System

Spinoreticular tract and spinomesencephalic tract carry pain and touch to other parts of the brain where the information is not consciously perceived.
Axons ascend with the spinothalamic tract through the spinal cord but then divert to the midbrain and other brainstem nuclei.
A portion of the spinomesencephalic tract is part of the spinotectal tract that ends in the superior colliculi of the midbrain; involved in reflexes that turn the head toward the point of cutaneous stimulation.

Dorsal-Column/Medial-Lemniscal System

Carries sensations of two-point discrimination, proprioception, pressure, vibration to cerebrum, and cerebellum.
Fasciculus gracilis: Sensations from inferior to midthoracic level.
Fasciculus cuneatus: Impulses from above midthorax.
Primary neurons have cell bodies in the dorsal root ganglion. Axons enter the spinal cord and ascend to the medulla oblongata without decussating where they synapse with secondary neurons.
Secondary neurons: Axons decussate and ascend to the thalamus.
Tertiary neurons: Project to somatic sensory cortex.

Trigeminothalamic Tract

Cranial nerve V.
Fibers join the spinothalamic tract in the brainstem.
Secondary neurons decussate in the brainstem; tertiary neurons run from the thalamus to the primary somatosensory cortex.
Carries similar information to that of the spinothalamic and dorsal-column/medial-lemniscal system, but from the face, nasal cavity, and oral cavity.

Spinocerebellar Tracts

Carries proprioceptive information to the cerebellum, most of which is unconscious.
Actual movements monitored and compared to cerebral information representing intended movement.
Posterior and anterior spinocerebellar tracts.
Some fibers cross but then cross again in cerebellum so information goes to the same side of the cerebellum as the sensation came from.

Descending Pathways Modifying Sensation

Descending pathways pass from the brain to the spinal cord.
Descending tracts send branches to ascending tracts and release neuromodulators such as endorphins and enkephalins.
Modification of sensation: The cortex may reduce the conscious perception of sensations.

Pain Pathways

Pain: A sensation characterized by unpleasant perceptual and emotional experiences that trigger autonomic, psychological, and somatic motor responses.
Two components to pain pathways:
- Rapidly conducted action potentials on large, myelinated axons resulting in sharp, localized, cutting pain.
- Slowly propagated action potentials on smaller, less myelinated axons resulting in diffuse burning or aching pain.

Pain Pathways

Dorsal-column/medial-lemniscal system neurons involved in gate-control theory of pain control.
Pain action potentials in the spinothalamic tract can be suppressed by action potentials in the dorsal-column/medial-lemniscal system.
Analgesics: Pain-relieving medications that act similar to gate control.

Referred Pain

Referred pain: Sensation in one region of the body that is not the source of the stimulus.
Organ pain is usually referred to the skin.
Both the organ and that region of the skin input to the same spinal segment and converge on the same ascending neurons.

Chronic Pain

Not a response to immediate direct tissue injury.
Some chronic pain has a known cause such as tissue damage from arthritis.
Some have no clear cause such as migraine and back pain.
Associated with depression, frustration, helplessness, hopelessness.

Sensory Areas of the Cerebral Cortex

Primary sensory areas: Regions of cortex to which sensory pathways project.
- Primary somatic sensory cortex (general sensory area): Posterior to the central sulcus. Postcentral gyrus. Receives general sensory input (pain, pressure, temperature, etc).
  - Arranged as an upside-down half homunculus, where the size of regions corresponds to the number of sensory receptors in that area of the body.
  - Localizes the site of sensation on the surface of the body in projection.
- Taste area: Insula.
- Olfactory cortex: Smell; inferior surface of the temporal lobe.
- Primary auditory cortex: Hearing; superior part of the temporal lobe.
- Visual cortex: Vision; occipital lobe.

Sensory Processing

Association areas: Adjacent to primary sensory areas.
- Somatosensory association area is posterior to the primary somatosensory area.
- Visual association area is anterior to the primary visual area.
Receive and integrate information from primary sensory areas.
Evaluation and recognition of sensory input.

14.2 Control of Skeletal Muscles

Motor system: Maintains posture and balance; moves limbs, trunk, head, eyes; facial expression, speech.
Reflexes: Involuntary movements that occur without conscious thought.
Voluntary movements: Consciously activated to achieve a specific goal.
Two neurons: Upper and lower.
- Upper motor neurons: Directly or through interneurons connect to lower.
- Lower motor neurons: Axons leave the CNS, extend through PNS to skeletal muscles. Cell bodies in anterior horns of spinal cord and in cranial nerve nuclei of brainstem.

Control of Skeletal Muscles

Three steps to voluntary movements:
1. Cerebral cortex communicates with the basal nuclei and cerebellum to plan, coordinate, and execute movements.
2. Upper motor neurons in the premotor areas of the cerebral cortex are stimulated and send action potentials down the descending tracts to the lower motor neurons.
3. Lower motor neurons are stimulated and they then stimulate skeletal muscles to contract.

Motor Areas of the Cerebral Cortex

Primary motor cortex (primary motor area): 30% of upper motor neurons. Another 30% in premotor area, rest in somatosensory cortex.
Premotor area: Anterior to primary motor cortex. Motor functions organized before initiation.
Prefrontal area: Motivation, foresight to plan and initiate movements, emotional behavior, mood.

Motor Pathways

Direct pathways (pyramidal system): Maintenance of muscle tone, controlling speed and precision of skilled movements. Upper motor neurons synapse directly with lower motor neurons in the brainstem or spinal cord.
Indirect pathways (extrapyramidal system): Less precise movements. Upper motor neurons do not directly synapse with lower motor neurons.

Direct Pathways

Control muscle tone and conscious fine, skilled movements in the face and distal limbs.
Direct synapse of upper motor neurons of the cerebral cortex with lower motor neurons in the brainstem or spinal cord.
Tracts:
- Corticospinal: Direct control of movements below the head.
- Corticobulbar: Direct control of movements in the head and neck.

Corticospinal Tracts

Axons of upper motor neurons descend through internal capsules and cerebral peduncles to pyramids of medulla oblongata.
75 to 85% decussate and descend in the lateral corticospinal tracts. Supply all levels of body.
Remaining fibers descend uncrossed in anterior corticospinal tracts but decussate near the level of synapse with lower neurons. Supply neck; upper limbs.
Most fibers synapse with interneurons in the lateral portions of the spinal cord gray matter which synapse with lower motor neurons of the ventral gray horns.

Corticobulbar Tracts

Innervate the head.
Upper neurons enter the cranial nerve nuclei after forming the reticular formation.
Lower motor neurons control eye and tongue movement, mastication, facial expression, palatine, pharyngeal, and laryngeal movements.

Indirect Pathways

Control conscious and unconscious muscle movements in trunk and proximal limbs.
Synapse in some intermediate nucleus rather than directly with lower motor neurons.
Tracts:
- Rubrospinal: Upper neurons synapse in the red nucleus. Similar to comparator function of the cerebellum. Regulates fine motor control of muscles in the distal part of the upper limb.
- Vestibulospinal: Influence neurons innervating extensor muscles in the trunk and proximal portion of the lower limbs; help maintain upright posture.
- Reticulospinal: Maintenance of posture.
- Tectospinal: Controls reflex movement of the head to bright lights, noises, and rapid movements.

Descending Spinal Pathways - Table 14.4

Direct Pathways
- Corticospinal Tract
  - Lateral: Movements below the head, especially of the hands, and of the neck, trunk, upper and lower limbs, especially the fingers. Crossing at the Inferior end of the medulla oblongata.
  - Anterior: Movements of the neck and trunk crossing at the Level of the lower motor neuron.
- Corticobulbar Tract: Movements of the head and face, crossing is Varies for the different cranial nerves.
Indirect Pathways
- Rubrospinal: Movement coordination for the Positioning of digits and the palm of the hand when reaching out to grasp, crossing at the Midbrain.
- Vestibulospinal: Maintenance of upright posture and balance for the Extension of the upper limbs when falling, uncrossed.
- Reticulospinal: Posture adjustment and walking Maintenance of posture when standing on one foot, crossing is Some uncrossed; some cross at termination.
- Tectospinal: Movements of the head and neck in response to visual and auditory reflexes for the Movement of the head and neck away from a sudden flash of light, crossing at the Midbrain.

Modifying and Refining Motor Activities

Basal nuclei and cerebellum.
Basal nuclei:
- Important in planning, organizing, coordinating movements, and posture.
- Feedback loops among basal nuclei, thalamus, and cerebral cortex.
- Stimulatory: Facilitate muscle activity like rising from a chair.
- Inhibitory: Inhibit activity in antagonistic muscles.

Modifying and Refining Motor Activities

Cerebellum:
- Helps maintain muscle tone in postural muscles, helps control balance during movement, and coordinate eye movements.
- Three functional parts:
  - Vestibulocerebellum, spinocerebellum, and cerebrocerebellum.
- Spinocerebellum helps coordinate fine movements by means of its comparator function.

Cerebellar Comparator Function

The motor cortex sends action potentials to lower motor neurons in the spinal cord to initiate voluntary movement.
Action potentials from the motor cortex inform the cerebellum of the intended movement.
Lower motor neurons in the spinal cord send action potentials to skeletal muscles, causing them to contract. Proprioceptive signals from the skeletal muscles and joints to the cerebellum convey information concerning the status of the muscles and the structure being moved during contraction.
The cerebellum compares the information from the motor cortex with the proprioceptive information from the skeletal muscle joints.
If a difference is detected, the cerebellum sends action potentials to the motor cortex, via the thalamus, and to the spinal cord to correct the discrepancy.
The result is smooth, coordinated movements.

Role of the Cerebrocerebellum

Communicates with the motor, premotor, and prefrontal cortices in planning and practicing rapid, complex motor actions that require coordination and training. Allows faster, highly skilled movements than the comparator functions will allow.
Also involved with cognitive functions such as rhythm, some word associations, and conceptualization of time intervals.

Cerebellar Disfunction

Results in:
- Decreased muscle tone
- Balance impairment
- Tendency to overshoot when reaching for or touching an object
- An intention tremor (shaking hands)

14.3 Brainstem Functions

Most ascending and descending pathways pass through the brainstem.
Nuclei of cranial nerves III - X and XII located here.
Many reflexes important to survival are located here:
- Heart rate, blood pressure, respiration, sleep, swallowing, vomiting, coughing, and sneezing.
Receives sensory input from collateral branches of sensory spinal cord pathways and from most cranial nerves.

Reticular Activating System (RAS)

The reticular formation is a group of nuclei scattered throughout the brainstem involved in regulating cyclical motor functions.
The part called the reticular activating system (RAS) controls the sleep/wake cycle.
RAS receives input from cranial nerves II (optic), V (trigeminal), and VIII (vestibulocochlear), ascending tactile sensory pathways, and descending neurons from the cerebral cortex. Wakefulness is maintained by information coming in from the trigeminothalamic tract.
General anesthetics depress the RAS.

Motor Output and Reflexes Projecting Through the Brainstem

The brainstem is the integration site for many descending motor pathways and reflexes.
Motor output is functionally classified as somatic motor and parasympathetic.
Somatic motor:
- CN III, CN IV, and CN VI are involved in eye movements and reflexes; nerves are controlled by superior colliculi.
- CN V: Mastication; CN VII: Facial expression; CN IX and X: Swallowing, and speech.
Parasympathetic:
- CN III: Pupil constriction.
- CN V: Sneeze, salivation reflexes.
- CN IX: Gag reflex.
- CN X: Cough reflex.

14.4 Higher Brain Functions

Speech
- Area normally in the left cerebral cortex.
- Wernicke area: Sensory speech- understanding what is heard and thinking of what one will say.
- Broca area: Motor speech- sending messages to the appropriate muscles to actually make the sounds.
- Wernicke and Broca areas connected by arcuate fasciculus.
To read aloud a word that is seen:
- Primary visual cortex → Wernicke area → Broca area → primary motor cortex.
To repeat a word that has been heard:
- Primary auditory cortex → Wernicke area → Broca area → primary motor cortex.

Demonstration of Cortical Activities During Speech

Visual cortex: Action potentials from the eyes reach the visual cortex, where the word is seen. The word is then recognized in the visual association area.
Wernicke area: The signal representing the word is understood in parts of the Wernicke area.
Broca area: Action potentials representing the word are conducted through association fibers that connect the Wernicke and Broca areas. In the Broca area, the word is formulated as it will be spoken.
Primary motor cortex: Action potentials are then propagated to the premotor area, where the movements are programmed, and finally to the primary motor cortex, where the proper movements are triggered.

Communication Between the Right and Left Hemispheres

Right: Controls muscular activity in and receives sensory information from the left side of the body.
Left: Controls muscular activity in and receives sensory information from the right side of the body.
Sensory information of both hemispheres shared through commissures: corpus callosum.
Language, and possibly other functions like artistic activities, not shared equally.
- Left: Mathematics and speech.
- Right: Three-dimensional or spatial perception, recognition of faces, musical ability.

Brain Waves and Sleep

Electroencephalogram (EEG): Record of brain’s electrical activity. Summation of all of the action potentials occurring at a particular moment sensed by electrodes placed on the scalp.
Brain wave patterns:
- Alpha: Resting state with eyes closed; REM sleep occurs.
- Beta: During intense mental activity.
- Theta: Occur in children but also in adults experiencing frustration or brain disorders.
- Delta: Occur in deep sleep, infancy, and severe brain disorders.
Sleep: Cyclic pattern of NREM and REM sleep.

Memory

Working memory: Task-associated memory, occurring when the brain briefly stores info to perform a task (for example, a phone number).
Short-term memory: Information retained for minutes to days; involves long-term potentiation.
Long-term memory: Retention for days to years or a lifetime.
- Involves consolidation, strengthening synaptic connections.
Declarative (explicit) memory: Retention of facts; hippocampus and amygdala.
Procedural (implicit; reflexive) memory: Development of skills such as riding a bicycle; cerebellum and premotor area.

Memory Processing

Working memory
Short-term memory: Long-term potentiation
Long-term memory: Consolidation

Cellular Mechanisms of Long-Term Potentiation

The amount of glutamate released by the presynaptic neuron increases.
Glutamate binds its receptors and causes ligand-gated ion channels to open. Calcium enters the cell.
${2Ca^{++}}$ -calmodulin activates calmodulin-dependent protein kinase II (CaM kinase II), which activates the glutamate receptors.
CaM kinase II also causes translocation of more glutamate receptors to the cell surface to increase the number of receptors on the postsynaptic membrane.

Long Term Memory

Requires the prior formation of short-term memory.
Consolidation – the gradual process of transferring short-term memory to long-term memory by forming new and stronger synaptic connections.
- Synthesizing new proteins that increase the size and number of synaptic contacts to form dendritic spikes on which new synapses are made.
- Increased long-term potentiation
End result is changes in the cytoskeleton of the postsynaptic neuron.
Memory becomes more or less permanent.

Storage and Retrieval of Memories

Memory engram (memory trace): Series of interconnected neurons and their pattern of activity. Involved in long-term retention of information, a thought, or an idea. Repetition and association of the new information with existing memories assist in the transfer of info from short-term to long-term memory.
The retrieval of a complete memory required accessing parts of the memory from different “pigeonholes” in the brain.

Limbic System

Includes the olfactory cortex (major source of sensory input), deep cortical regions, and various nuclei.
Influences emotions, visceral responses to emotions, motivation, mood, sensations of pain and pleasure.
Basic survival instincts: Acquisition of food and water; reproduction.
Pheromones: Molecules released by one organism that have an effect on another organism.
- Females release pheromones that affect the menstrual cycle of other women.
Cingulate gyrus: Satisfaction center.
Hippocampus connection to the limbic system is important to survival.

Effects of Aging on the Nervous System

Gradual decline in sensory and motor function.
Receptor and neuron numbers decrease.
Balance and coordination decrease.
Reflexes slow.
Size and weight of brain decrease.
Decreased short-term memory in most people.
Long-term memory is unaffected or improved.
Changes in sleep patterns; less REM sleep.

Stroke

Stroke - cerebrovascular accident (CVA) involving death of brain tissue due to disruption in vascular supply.
- Hemorrhagic stroke – bleeding from brain arteries
- Ischemic stroke – arteries are blocked

Integration of the nervous system involves several crucial functions:

Communication of sensory information from diverse areas of the body to the Central Nervous System (CNS) occurs through specific sensory pathways, which are responsible for relaying data from sensory receptors located throughout the body to the brain, where it is processed and interpreted.
Control of effectors by the CNS is achieved through specific motor pathways that dictate the actions of muscles and glands, allowing the body to respond to stimuli and maintain homeostasis.

14.1 Sensation

Sensation: The process initiated by stimuli that act on specialized sensory receptors, generating neural signals that travel to the brain.
Perception: The conscious awareness and interpretation of these sensations, shaped by past experiences and context.
The senses serve as the primary means by which the brain acquires information about both the external environment and the internal condition of the body; they can be categorized into general and special senses.

Types of Senses

General senses: These are distributed extensively throughout the body, including the skin, muscles, and internal organs. The receptor generates an action potential known as a generator potential, which then travels to the brain.
- Somatic senses (information about the body and environment): Include touch, pressure, temperature, proprioception (the sense of body position and movement), and pain.
- Visceral senses (information from internal organs): Primarily include sensations of pain and pressure associated with visceral organs.
Special senses: These are associated with specialized sense organs located in specific areas of the body. The modalities include smell, taste, vision, hearing, and balance. Each receptor produces a receptor potential and releases neurotransmitters that bind to neurons, allowing the information to ascend to the brain for processing.

Classification of the Senses

General Senses
- Somatic senses:
- Touch
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Compression of receptors
  - Examples: Meissner corpuscle, Hair follicle receptor, Merkel disk, Pacinian corpuscle, Ruffini end organ, Free nerve endings
- Pressure
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Compression of receptors
  - Examples: Merkel disk
- Proprioception
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Compression of receptors
  - Examples: Free nerve endings, Pacinian corpuscle, Muscle spindle, Golgi tendon organ
- Temperature
  - Receptor Type: Thermoreceptors
  - Initiation of Response: Temperature changes around nerve endings
  - Examples: Free nerve endings, Cold receptors, Warm receptors
- Pain
  - Receptor Type: Nociceptors
  - Initiation of Response: Irritation of nerve endings (mechanical, chemical, thermal)
  - Examples: Free nerve endings
- Visceral senses:
- Pain
  - Receptor Type: Nociceptors
  - Initiation of Response: Irritation of nerve endings
  - Examples: Free nerve endings
- Pressure
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Compression of receptors
  - Examples: Pacinian corpuscle
Special Senses:
- Smell (Olfaction)
  - Receptor Type: Chemoreceptors
  - Initiation of Response: Binding of odorant molecules to membrane receptors
  - Examples: Olfactory receptor neurons
- Taste (Gustation)
  - Receptor Type: Chemoreceptors
  - Initiation of Response: Binding of taste molecules to lingual receptors
  - Examples: Taste receptor cells
- Vision
  - Receptor Type: Photoreceptors
  - Initiation of Response: Light-induced chemical changes in photoreceptor cells
  - Examples: Rods and cones in the retina
- Hearing
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Sound waves causing vibration of hair cells
  - Examples: Inner ear hair cells
- Balance
  - Receptor Type: Mechanoreceptors
  - Initiation of Response: Movement of fluid in vestibular apparatus affecting hair cells
  - Examples: Hair cells in the semicircular canals and otoliths.