Sensation (perception): conscious awareness of stimuli received by sensory receptors

Steps to Sensation:– Sensory receptors detect the stimulus– Send action potential by nerves to CNS

Steps to sensation :– Within the CNS, nerve tracts convey action potentials to

the cerebral cortex and to other areas of the CNS

– In CNS, AP are translated so the person can be aware of the stimulus

Three ways to classify Sensory receptors:

Type of stimuli detected

By location

Structural Complexity

Mechanoreceptors: respond to touch, pressure, vibration, stretch, and itch, hearing and balance

Chemoreceptors: respond to chemicals (e.g. smell, taste)

Thermoreceptors: respond to changes in temperature

Photoreceptors: respond to light: vision

Nociceptors: respond to pain-causing stimuli

Exteroceptors: Respond to stimuli arising outside the body

Found near the body surface

Contains touch, pressure, pain, and temperature receptors in skin

And receptors of special sense organs (vision, hearing, taste, smell)

Visceroreceptors: Respond to stimuli arising within the body

Found in viscera organs and blood vessels

Sensitive to chemical changes, stretch, and temperature changes

Proprioceptors: Found in skeletal muscles, tendons, joints, ligaments, and

connective tissue coverings of bones and muscles

Respond to degree of stretch of the organs they occupy

Receptors are structurally classified as: Simple receptors Complex receptors

Simple Receptors: Are distributed throughout the body and associated with general senses

Divided into two groups: -- Somatic senses (provides sensory information about the body and environment): touch, pressure, temperature, proprioception, pain

– Visceral senses (information about internal organs): pain and pressure

Complex Receptors: Receptors are present in specific organs and associated with special senses

– Special senses are smell, taste, sight, hearing, balance

• Free nerve endings• Merkel (tactile) discs• Hair follicle receptors• Pacinian corpuscles

(lamellated corpuscles)• Meissner’s corpuscles

(tactile corpuscles) • Ruffini’s end organs• Muscle spindles• Golgi tendon organs

• Simplest, most common sensory receptor

• In all body tissues (esp. epithelium)– Detect pain, temp., and pressure

• Free nerve with disc shaped endings

• Found in deep epidermis

• Detect light touch and superficial pressure

• Hair follicle Receptors– Respond to slight bending

of hair as occurs in light touch

• Pacinian Corpuscles– In hypodermis of skin,

periostea, ligaments, joint capsules, fingers, soles of feet, external genitalia and nipples

– Deep pressure and stretching

– Respond only when pressure first applied

• Numerous in dermal papillae of hairless skin (lips, nipples, fingertips)

• Ability to detect simultaneous stimulations at two points on the skin (Two-point discrimination)

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

• 3-10 specialized skeletal muscle cells

• Provide information about length of muscles

• Involved in stretch reflex

• Proprioceptors associated with tendons

• Respond to increased tension on tendon

• Sensory receptor generates Graded potential or Receptor potential when interacts with stimulus

– Primary receptors: axons conduct action potentials in response to graded potential

eg. Simple sensory receptors

• Secondary receptors: Have no axons or have short axon like projections

• Causes release of neurotransmitters that bind to receptors on a neuron causing a receptor potential eg. Smell, taste, hearing, balance

• SC and brain stem contains no. of sensory pathway

• They transmit action potentials from periphery to various parts of brain

• Each pathway involved with specific modality (type of information transmitted)

• Names of ascending pathway or tracts in CNS indicate their origin & termination

• First half of word indicates origin, second half indicates termination

• Located in the anterior and lateral white columns of spinal cord

• Convey sensory information such as pain, temp, light touch, pressure, tickle, and itch

• Consist of three Neuron system:– Primary neuron: Relay sensory input

from periphery to posterior horn of SC– And synapse with interneurons– Interneurons synapse with sec. neuron

– Secondary neuron : cross to opposite side of SC & enter spinothalamic tract, ascend to thalamus

– Tertiary neuron: thalamus to somatic sensory cortex

• Carries sensations of two-point discrimination, proprioception, pressure, vibration to cerebrum, and cerebellum

• Primary neurons: Located in dorsalroot ganglion

• Axons enter spinal cord and ascend to the medulla oblongata where they synapse with secondary neurons

• Secondary neurons: axons travel contralaterally and ascend to thalamus

• Tertiary neurons: thalamus to somatic sensory cortex

• Fibers of Trigeminothalamic tract join the spinothalamic tract in the brainstem

• Made up of afferent (sensory) fibers from Cranial nerve V

• Carries similar information to that of the spinothalamic and dorsal-column/medial-lemniscal system, but from the face, nasal cavity and oral cavity

• Carries proprioceptive information to cerebellum

• Information concerning actual movements is monitored

• Two spinocerebellar tracts extend through spinal cord:

• Posterior spinocerebellar tract : – Carries information from thoracic and

upper lumbar regions to cerebellum

• Anterior spinocerebellar tract:– Carries information from lower trunk

and lower limbs to cerebellum

• Sensory Areas:– Primary somatic sensory cortex

(general sensory area): posterior to the central sulcus, Post central gyrus area

– General sensory input for pain, pressure, temperature

– Taste area: located at inferior end of post central gyrus

– Olfactory cortex: at inferior surface of frontal lobe

– Primary auditory cortex: superior part of temporal lobe

– Visual cortex: occipital lobe

• Association areas: Involved in process of recognition

Somatic sensory Association Area:– posterior to primary somatic sensory

cortex

Visual Association area:– anterior to visual cortex: – Where present visual

information is compared to past visual experience

• Motor system of brain and SC: maintains posture and balance; moves limbs, trunk, head, eyes; facial expression, speech

• Voluntary movements: consciously activated to achieve a specific goal, eg. walking

• Voluntary movements depend on two neurons: – Upper motor neurons: directly or through interneurons

connect to lower motor neurons

– Lower motor neurons: Have axons that leave the CNS, and supply to skeletal muscles

1. Initiation of voluntary movement begins in the premotor areas of the cerebral cortex and results in the stimulation of upper motor neurons

2. The axons of the upper motor neurons form the descending nerve tracts. They stimulate lower motor neurons which stimulate skeletal muscles to contract

3. The cerebral cortex interacts with the basal nuclei and cerebellum in the planning, coordination and execution of movements

• Precentral gyrus (primary motor cortex, primary motor area): 30% of upper motor neurons. Another 30% in premotor area, rest in somatic sensory cortex

• Premotor area: ant. to primary motor cortex. Motor functions organized here & initiation takes place in motor cortex area

• Prefrontal area: motivation, foresight to plan and initiate movements takes place

• Also involved in emotional behavior, mood

• Axons carry AP from cerebrum or cerebellum to brain stem or SC

• Descending motor fibers are divided into two groups:

• Direct pathways (pyramidal system): Involved in maintenance of muscle tone, controlling speed and precision of skilled movements

• Indirect pathways (extrapyramidal system): Involved in less precise movements, such as posture

• Control muscle tone and conscious fine, skilled movements in the face and distal limbs

• Direct synapse of upper motor neurons of cerebral cortex with lower motor neurons in brainstem or spinal cord

• Tracts – Corticospinal: direct control of

movements below the head

– Corticobulbar: direct control of movements in head and neck

• Axons of upper motor neurons descend through internal capsules and cerebral peduncles to pyramids of medulla oblongata

• 75-85% decussate and descend in the lateral Corticospinal tracts. Supply all levels of body

• Remaining fibers descend uncrossed in anterior Corticospinal tracts but decussate near level of synapse with lower neurons. Supply neck; upper limbs

• 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

• 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

red nucleus– Regulates fine motor control of muscles in

distal part of upper limb

– Vestibulospinal: Originate in vestibular nuclei and synapse in SC

– influence neurons innervating extensor muscles in trunk and proximal portion of lower limbs; help maintain upright posture

– Reticulospinal: Present in reticular formation of pons and medulla and synapse in SC

– maintenance of posture

• Important in planning, organizing, coordinating movements and posture

• Complex neural circuits between basal nuclei, thalamus, and cerebral cortex

– Stimulatory circuit : facilitate muscle activity like rising from a chair

– Inhibitory circuit : inhibit activity in antagonistic muscles

• Helps maintain muscle tone in postural muscles, helps control balance during movement, and coordinate eye movements

• Cerebellum compares the intended movement with the actual movement

• e.g. Touching the nose

• All ascending and descending pathways pass through the brainstem

• Nuclei of cranial nerves II-XII located here

• Many reflexes important to survival located here: heart rate, blood pressure, respiration, sleep, swallowing, vomiting, coughing, and sneezing

• Reticular activating system (RAS)- controls sleep/wake cycle

• RAS receives input from cranial nerves II (optic), V (trigeminal), VIII (Vestibulocochlear), ascending tactile sensory pathways and descending neurons from the cerebral cortex

• Speech Area normally is in left cerebral cortex• Wernicke's area: sensory speech area- Understanding written and

spoken language

• Broca's area: motor speech area - sending messages to the appropriate muscles to actually make the sounds

• Aphasia: absent or defective speech or language comprehension. Caused by lesion in the language area of the cortex

• Right: controls muscular activity in and receives sensory information from left side of body

• Left: controls muscular activity in and receives sensory information from right side of 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

• 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– 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

• 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

• Sensory: very short-term retention of sensory input received by brain• Short-term: information retained for few seconds to minutes• Long-term: Two types• Explicit or declarative memory:

– Retention of facts, such as names, dates– Accessed by hippocampus, part of temporal lobe (actual memory)– and amygdaloid nucleus – emotional, such as like or dislike

• Implicit (procedural; reflexive) memory: development of skills such as riding a bicycle, playing a piano

• Influences emotions, visceral responses to emotions, motivation, mood, sensations of pain and pleasure

• Limbic system is associated with Basic survival instincts: acquisition of food and water; reproduction

• Major source of sensory input in Limbic System is Olfactory nerve

• Pheromones: molecules released by one organism that have an effect on another organism; e.g., females release pheromones that affect menstrual cycle of other women

• Cingulate gyrus: satisfaction center

• Gradual decline in sensory and motor function• Reflexes slow• Size and weight of brain decrease• Decreased short-term memory in most people• Long-term memory unaffected or improved• Changes in sleep patterns