A&P: Chapter 15

Levels of Processing and Motor Control

All sensory and motor pathways involve a series of synapses, one after the other General pattern • Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over short term

Six Types of Tactile Receptors in the Skin: Tactile Corpuscles

Also called Meissner corpuscles Perceive sensations of fine touch, pressure, and low-frequency vibration Adapt to stimulation within 1 second after contact Fairly large structures Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia

Six Types of Tactile Receptors in the Skin: Tactile Discs

Also called Merkel discs Fine touch and pressure receptors Extremely sensitive to tonic receptors Have very small receptive fields

Nociceptors(also called pain receptors)

Are common in the superficial portions of the skin, joint capsules, within the periostea of bones, and around the walls of blood vessels May be sensitive to temperature extremes, mechanical damage, and dissolved chemicals, such as chemicals released by injured cells

Specical Sensory Receptors

Are located in sense organs such as the eye or ear Are protected by surrounding tissues

Are Free Nerve Endings with large Receptive Fields

Branching tips of dendrites Not protected by accessory structures Can be stimulated by many different stimuli Two types of axons: Type A and Type C fibers

Type C Fibers

Carry sensations of slow pain, or burning and aching pain Cause a generalized activation of the reticular formation and thalamus You become aware of the pain but only have a general idea of the area affected

Temperature Sensations

Conducted along the same pathways that carry pain sensations Sent to: • the reticular formation • the thalamus • the primary sensory cortex (to a lesser extent)

Conscious & Subconscious Motor Commands

Controlskeletal muscles by traveling over three integrated motor pathways 1) Corticospinal pathway 2) Medial pathway 3) Lateral pathway

Sensory Pathways

Deliver somatic and visceral sensory information to their final destinations inside the CNS using Nerves: PNS Nuclei: CNS Tracts: CNS

Sensory homunculus

Functional map of the primary sensory cortex Distortions occur because area of sensory cortex devoted to particular body region is not proportional to region's size, but to number of sensory receptors it contains

Three Major Groups of Proprioceptors: Golgi Tendon Organs

Located at the junction between skeletal muscle and its tendon Stimulated by tension in tendon Monitor external tension developed during muscle contraction

2 & 3 The Medial and Lateral Pathways

Medial pathway help control gross movements of trunk and proximal limb muscles Lateral pathway help control distal limb muscles that perform more precise movements

Proprioceptors

Monitor Position of joints Tension in tendons and ligaments State of muscular contraction

Baroreceptors

Monitor change in pressure Consist of free nerve endings that branch within elastic tissues in wall of distensible organ (such as a blood vessel) Respond immediately to a change in pressure, but adapt rapidly

Six Types of Tactile Receptors in the Skin: Root Hair Plexus Nerve Endings

Monitor distortions and movements across the body surface wherever hairs are located Adapt rapidly, so are best at detecting initial contact and subsequent movements

Three Major Groups of Proprioceptors: Muscle

Monitor skeletal muscle length Trigger stretch reflexes

Somatic Nervous System(SNS)

Motor neurons and pathways that control skeletal muscles

Somatic Motor Pathways: Review

Neurons of the primary motor cortex innervate motor neurons in the brain and spinal cord responsible for stimulating skeletal muscles Higher centers in the brain can suppress or facilitate reflex responses Reflexes can complement or increase the complexity of voluntary movements

Special Senses

Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing

Monitors

Proprioceptive (position) sensations Visual information from the eyes Vestibular (balance) sensations from inner ear as movements are under way

The Detection of Stimuli

Receptor sensitivity Each receptor has a characteristic sensitivity Receptive field Area is monitored by a single receptor cell The larger the receptive field, the more difficult it is to localize a stimulus

Somatic Motor Pathways

SNS, or the somatic motor system, controls contractions of skeletal muscles (discussed next) ANS, or the visceral motor system, controls visceral effectors, such as smooth muscle, cardiac muscle, and glands (Ch. 16)

Mechanoreceptors

Sensitive to stimuli that distort their plasma membranes Contain mechanically gated ion channels whose gates open or close in response to Stretching Compression Twisting Other distortions of the membrane

Six Types of Tactile Receptors in the Skin: Free Nerve Endings

Sensitive to touch and pressure Situated between epidermal cells Free nerve endings providing touch sensations are tonic receptors with small receptive fields

Sensory Recptors

Specialized cells that monitor specific conditions in the body or external environment When stimulated, a receptor passes information to the CNS in the form of action potentials along the axon of a sensory neuron

Sensory Receptors

Stimulation of a receptor produces action potentials along the axon of a sensory neuron The frequency and pattern of action potentials contain information about the strength, duration, and variation of the stimulus Your perception of the nature of that stimulus depends on the path it takes inside the CNS

Sensory Information: Afferent and Efferent

- Afferent Division of the Nervous System Receptors Sensory neurons Sensory pathways - Efferent Division of the Nervous System Nuclei Motor tracts Motor neurons

Six Types of Tactile Receptors in the Skin: Lamellated Corpuscles

- Also called Pacinian corpuscles - Sensitive to deep pressure - Fast-adapting receptors - Most sensitive to pulsing or high-frequency vibrating stimuli

The Basal Nuclei

- Also called cerebral nuclei - Are masses of gray matter - Are embedded in white matter of cerebrum - Direct subconscious activities

Six Types of Tactile Receptors in the Skin: Ruffini Corpuscles

- Also sensitive to pressure and distortion of the skin - Located in the reticular (deep) dermis - Tonic receptors that show little if any adaptation

Thermoreceptors: Also called temperature receptors

- Are free nerve endings located in: The dermis Skeletal muscles The liver The hypothalamus

The Interpretation of Sensory Information: Arriving stimulus Sensations

- Arriving stimulus Takes many forms: • physical force (such as pressure) • dissolved chemical • sound • Light - Sensations Taste, hearing, equilibrium, and vision provided by specialized receptor cells Communicate with sensory neurons across chemical synapses

Somatic Sensory Pathways: Posterior Column Pathway

- Carries sensations of highly localized ("fine") touch, pressure, vibration, and proprioception - Spinal tracts involved: left and right fasciculus gracilis left and right fasciculus cuneatus - Processing in the Thalamus - Ability to Determine Stimulus

Tactile Receptors: Fin and Cruce Touch/Pressure

- Fine touch and pressure receptors Are extremely sensitive Have a relatively narrow receptive field Provide detailed information about a source of stimulation, including: • its exact location, shape, size, texture, movement - Crude touch and pressure receptors Have relatively large receptive fields Provide poor localization Give little information about the stimulus

Motor homunculus

- Primary motor cortex corresponds point by point with specific regions of the body - Cortical areas have been mapped out in diagrammatic form - Homunculus provides indication of degree of fine motor control available: • hands, face, and tongue, which are capable of varied and complex movements, appear very large, while trunk is relatively small • these proportions are similar to the sensory homunculus

Somatic Sensory Pathways: The Spinothalamic Pathway

- Provides conscious sensations of poorly localized ("crude") touch, pressure, pain, and temperature - Feeling Pain (Lateral Spinothalamic Tract) An individual can feel pain in an uninjured part of the body when pain actually originates at another location Strong visceral pain Sensations arriving at segment of spinal cord can stimulate interneurons that are part of spinothalamic pathway Activity in interneurons leads to stimulation of primary sensory cortex, so an individual feels pain in specific part of body surface: • also called referred pain

Chemoreceptors

- Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid - Receptors exhibit peripheral adaptation over period of seconds; central adaptation may also occur - Located in the Carotid bodies: • near the origin of the internal carotid arteries on each side of the neck Aortic bodies: • between the major branches of the aortic arch - Receptors monitor pH, carbon dioxide, and oxygen levels in arterial blood

Sensory Receptors: Sensation, Perception, General Senses

- Sensation The arriving information from these senses - Perception Conscious awareness of a sensation - General Senses Describe our sensitivity to Temperature Pain Touch Pressure Vibration Proprioception

3 Major Somatic Sensory Pathways

1) The posterior column pathway 2) The spinothalamic pathway 3) The spinocerebellar pathway

Tonic Receptors

Are always active Show little peripheral adaptation Are slow-adapting receptors Remind you of an injury long after the initial damage has occurred

Phasic Receptors

Are normally inactive Become active for a short time whenever a change occurs Provide information about the intensity and rate of change of a stimulus Are fast-adapting receptors

Somatic Sensory Pathways

Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs

Somatic Sensory Pathways: The Spinocerebellar Pathway

Cerebellum receives proprioceptive information about position of skeletal muscles, tendons, and joints

Baroreceptors

Detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts

Three Major Groups of Proprioceptors: Receptors in joint capsules

Free nerve endings detect pressure, tension, movement at the joint

Classifying Sensory Receptors

General sensory receptors are divided into four types by the nature of the stimulus that excites them 1) Nociceptors (pain) 2) Thermoreceptors (temperature) 3) Mechanoreceptors (physical distortion) 4) Chemoreceptors (chemical concentration)

Proprioceptors

Monitor the positions of joints and muscles The most structurally and functionally complex of general sensory receptors

Nociceptors

Myelinated Type A fibers Carry sensations of fast pain, or prickling pain, such as that caused by an injection or a deep cut Sensations reach the CNS quickly and often trigger somatic reflexes Relayed to the primary sensory cortex and receive conscious attention

Adaptation

Reduction in sensitivity of a constant stimulus Your nervous system quickly adapts to stimuli that are painless and constant A) Central B) Peripheral

Three Classes of Mechanoreceptors

Tactile receptors Baroreceptors Proprioceptors

Interoceptors

monitor visceral organs and functions

Exteroceptors

provide information about the external environment

Tactile Receptors

provide the sensations of touch, pressure, and vibration: • touch sensations provide information about shape or texture • pressure sensations indicate degree of mechanical distortion • vibration sensations indicate pulsing or oscillating pressure

Proprioceptors

report the positions of skeletal muscles and joints Provide a purely somatic sensation No proprioceptors in the visceral organs of the thoracic and abdominopelvic cavities You cannot tell where your spleen, appendix, or pancreas is at the moment

Cranial and Spinal Reflexes

• Control the most basic motor activities Integrative centers in the brain • Perform more elaborate processing • As we move from medulla oblongata to cerebral cortex, motor patterns become increasingly complex and variable Primary motor cortex • Most complex and variable motor activities are directed by primary motor cortex of cerebral hemispheres