Afferent Division of the Nervous System Sensory pathways (15-1 to 15-4 & 17) Efferent Division of the Nervous System Motor pathways Somatic Nervous system (15-5) Autonomic Nervous system (16-1 to 16-7)

An Overview of Events Occurring Along the Sensory and Motor Pathways. Immediate Involuntary Response Motor Pathway 16-1 to 16-7 (involuntary) Processing centers in the spinal cord or brain stem may direct an immediate reflex response even before sensations reach the cerebral cortex. Sensory Pathway 15-1 to 15-4 Arriving stimulus Depolarization of Receptor Action Potential Generation CNS Processing Propagation A stimulus produces a graded change in the membrane potential of a receptor cell. If the stimulus depolarizes the receptor cell to threshold, action potentials develop in the initial segment. Axons of sensory neurons carry information about the type of stimulus (touch, pressure, temperature) as action potentials to the CNS. Information processing occurs at every relay synapse. Sensory informa- tion may be distributed to multiple nuclei and centers in the spinal cord and brain. Voluntary Response Motor Pathway 15-5 (voluntary) Perception The voluntary response, which is not immediate, can moderate, enhance, or supplement the relatively simple involuntary reflexive response. Only about 1 percent of arriving sensations are relayed to the primary sensory cortex.

Sensory Receptors Sensory Receptors are specialized cells or nerve endings that monitor specific conditions in the body or external environment Connect our internal and external environment with the nervous system When stimulated, a receptor passes information to the CNS In the form of action potentials along the axon of a sensory neuron

15-1 Sensation: Overview Sensation Perception Interpretation of the sensory input occurs at a conscious or subconscious level Sensation the information carried by a sensory pathway to the CNS May be interpreted at a conscious or subconscious level CNS interprets the modality (type of sensation) according to the labeled line over which it arrives For example: Touch, pressure, pain, temp sensations go to primary sensory cortex in parietal lobe of cerebrum Perception Conscious awareness of a sensation Must be received by cerebral cortex

General Senses General sensory receptors are simple in structure and widely distributed throughout the body (skin & internal organs) Temperature Pain Touch Pressure Vibration Proprioception

Special sensory receptors are complex in structure and located in sense organs only Special Senses (Chapter 17) Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing

Figure 15-2 Receptors and Receptive Fields The area is monitored by a single receptor cell is called the Receptive field The larger the receptive field, the more difficult it is to localize a stimulus 8

Central adaptation occurs along sensory pathways within the CNS Adaptation is the reduction in sensitivity in the presence of a constant stimulus Peripheral Adaptation occurs when the level of receptor activity changes Responds strongly at first, then gradually declines Central adaptation occurs along sensory pathways within the CNS Involves inhibition of nuclei along a sensory pathway

Adaptation, a reduction in sensitivity in the presence of a constant stimulus Receptor Labeled line Arriving stimulus CNS processing center Site of peripheral adaptation Site of central adaptation Figure 13.14.4 Receptors for the general senses can be classified by function and by sensitivity Figure 13.14 4 10

Two types of Receptors Phasic receptors –usually off: detect a new stimulus or a change in stimulus Tonic receptors - always active

Phasic receptors detect a new stimulus or a change in stimulus Normally inactive Become active only for a short time Undergo peripheral adaptation: Fast-adapting receptors: Over time, their sensitivity to stimulus decreases: Found in senses of smell, hair movement and cutaneous pressure Tonic receptors Are always active Show little peripheral adaptation Are slow-adapting receptors Proprioceptors are among the most slowly adapting tonic receptors

Increased Normal Normal Stimulus Frequency of action potentials Time b Phasic receptors are normally inactive, but become active for a short time in response to a change in the conditions they are monitoring.

Increased Normal Normal Stimulus Frequency of action potentials Time a Tonic receptors are always active and generate action potentials at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, the rate of action potential generation changes accordingly.

A Functional Classification of General Sensory Receptors Nociceptors Thermoreceptors Chemoreceptors Mechanoreceptors Pain receptors Temperature receptors Respond to water-soluble and lipid- soluble substances dissolved in body fluids Sensitive to stimuli that distort their plasma membranes Figure 13.14.2 Receptors for the general senses can be classified by function and by sensitivity Myelinated Type A fibers (carry sensations of fast pain) Unmyelinated Type C fibers (carry sensations of slow pain) Proprioceptors (monitor the positions of joints and muscles) Baroreceptors (detect pressure changes) Tactile receptors (provide the sensations of touch, pressure, and vibration) 15

Nociceptors (Pain Receptors) Nociceptors are free nerve endings (branching tips of dendrites) with large receptive fields Can be stimulated by many different stimuli Located in every organ of the body except the brain Superficial portions of the skin, joint capsules, periostea of bones, around the walls of blood vessels Notifies brain about injuries or changes that may harm the body, used as a sign during diagnosis of disease or injury. Tonic receptors: Adaptation is slight if at all

Nociceptors (Pain Receptors) Fast pain Acute, sharp, prickly pain-Such as from injection or deep cut Carried by myelinated Type A fibers Quickly reach CNS and trigger fast reflexive responses Relayed to primary sensory cortex for conscious attention Stimulus can be located to an area within a few cm Slow pain Chronic, burning or aching pain-such as back pain Carried by unmyeleinated Type C fibers Individual aware of pain but only general location

Phantom pain: sensations of pain in a limb that has been amputated Brain interprets sensations from the remaining part of the limb as sensations from the amputated part or Neurons in the brain that received input from the missing limb are still firing

Organ pain usually referred to skin Referred pain: sensation of pain in one region of body that is not the source of the stimulus. Organ pain usually referred to skin Both the organ and that region of the skin input to the same spinal segment, activate the same interneurons and converge on the same ascending neurons For example; pain of heart attack frequently felt in medial left arm

Heart Liver and gallbladder Stomach Small intestine Ureters Appendix Figure 15-6 Referred Pain Heart Liver and gallbladder Stomach Small intestine Ureters Appendix Colon 20

Thermoreceptors (temperature receptors) are free nerve endings Located in the skin and hypothalamus Maintain body temperature Phasic (fast-adapting) receptors

Detect changes in the concentration of specific chemicals Respond only to water-soluble and lipid-soluble substances dissolved in body fluids (interstitial fluid, plasma and CSF) Monitor pH, CO2 and O2 levels in the blood Information sent to autonomic control centers in brainstem: we do not perceive this sensation Receptors exhibit peripheral adaptation over period of seconds; Central adaptation may also occur Chemoreceptors

The body’s chemoreceptors, which play important roles in the reflexive control of respiration and cardiovascular function Chemoreceptors in Respiratory Centers in the Medulla Oblongata Trigger reflexive adjustments in depth and rate of respiration Respond to the concentrations of hydrogen ions (pH) and carbon dioxide (PCO2) in cerebrospinal fluid Chemoreceptors of Carotid Bodies Via cranial nerve IX Sensitive to changes in the pH, PCO2 , and PO2 in arterial blood Trigger reflexive adjustments in respiratory and cardiovascular activity Chemoreceptors of Aortic Bodies Via cranial nerve X Figure 14.8.2 Baroreceptors and chemoreceptors initiate important autonomic reflexes involving visceral sensory pathways Sensitive to changes in the pH, PCO2, and PO2 in arterial blood 23

Mechanoreceptors Stimulated by mechanical stimulus (touch, pressure, stretching, vibration) that distort the plasma membrane Mechanically gated ion channels whose gates open or close in response to stimuli

Three Classes of Mechanoreceptors Tactile receptors Provide the sensations of touch, pressure, and vibration Six Types of Tactile Receptors in the Skin Baroreceptors Detect pressure changes in the walls of blood vessels and in portions of digestive, reproductive and urinary tract Proprioceptors Monitor the positions of joints, tendons and muscles

Mechanoreceptors: A. Tactile Receptors Meissner’s (Tactile) Corpuscles Capsule Dendrites Tactile corpuscle Hair Dermis Afferent fiber Free Nerve Endings Pacinian (Lamellated) Corpuscles Free nerve endings Layers of Collagen fibers and fibroblasts Sensory nerve Dendrite Dermis Root Hair Plexus Hair shaft Ruffini Corpuscles Root hair plexus Sensory nerves Capsule Module 9.1 General sensory receptors in the skin vary widely in form and function Dendrites Afferent fiber Merkel (tactile) discs Merkel cells Merkel disc

Mechanoreceptors: B. Baroreceptors Detect changes in stretch or distension or pressure in walls of blood vessels and in portions of digestive, reproductive and urinary tract. Consist of free nerve endings that branch within elastic tissues in the wall of the vessel/organ Phasic receptors: Respond immediately to a change in pressure, but adapt rapidly Involved, for example, in blood pressure regulation

Mechanoreceptors: B. Baroreceptors Baroreceptors of Carotid Sinus and Aortic Sinus Provide information on blood pressure to cardiovascular and respiratory control centers Baroreceptors of Lungs Provide information on lung stretching to respiratory rhythmicity centers for control of respiratory rate Baroreceptors of Digestive Tract Provide information on volume of tract segments, trigger reflex movement of materials along tract Figure 14.8.1 Baroreceptors and chemoreceptors initiate important autonomic reflexes involving visceral sensory pathways Baroreceptors of Bladder Wall Baroreceptors of Colon Provide information on volume of fecal material in colon, trigger defecation reflex Provide information on volume of urinary bladder, trigger urination reflex 28

Mechanoreceptors: C. Proprioceptors Monitor the position of joints and muscles Tonic receptors-Do not adapt to constant stimulation Most proprioceptive information is processed at a subconscious level 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

Somatic sensory pathways Somatic sensory pathways relay information from skin and musculature receptors to CNS. Neural pathway involves First-order neuron From receptor to synapse in spinal cord posterior gray horn Second-order neuron From posterior gray horn, crosses spinal cord and reaches thalamus Third-order neuron If conscious perception occurs From thalamus to primary sensory cortex Right side of cerebral hemisphere receives sensory information from left side of body due to decussation along the pathway

Three major somatic sensory pathways The spinothalamic pathway The posterior column pathway The spinocerebellar pathway Posterior column pathway Fasciculus gracilis Fasciculus cuneatus Dorsal root Dorsal root ganglion Spinocerebellar tracts Posterior spinocerebellar tract Anterior spinocerebellar tract Spinothalamic tracts Ventral root Lateral spinothalamic tract Anterior spinothalamic tract

Sensory homunculus (“little man”) is a functional map of somatic sensations from body parts to discrete areas in cerebral cortex Proportions very different from individual 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

Figure 15-5 Somatic Sensory Pathways POSTERIOR COLUMN PATHWAY Ventral nuclei in thalamus Midbrain Nucleus gracilis and nucleus cuneatus Medial lemniscus Medulla oblongata Fasciculus gracilis and fasciculus cuneatus Dorsal root ganglion Fine-touch, vibration, pressure, and proprioception sensations from right side of body 33

Most somatic sensory information is relayed to the thalamus for processing A small fraction of the arriving information is projected to the cerebral cortex and reaches our awareness

15-4 Sensory Pathways Visceral Sensory Pathways Collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities These interoceptors are not as numerous as in somatic tissues Interoceptors include: Nociceptors Thermoreceptors Tactile receptors Baroreceptors Chemoreceptors

Cranial Nerves V, VII, IX, and X Carry visceral sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands Solitary nucleus Large nucleus in the medulla oblongata Major processing and sorting center for visceral sensory information Extensive connections with the various cardiovascular and respiratory centers, reticular formation