FUNDAMENTALS OF ANATOMY AND PHYSIOLOGY Mrs. Willie Grant, Instructor wgrant4@alamo.edu (210) 485-2870

An Introduction to Sensory Pathways and the Somatic Nervous System Learning Outcomes 15-1 Specify the components of the afferent and efferent divisions of the nervous system, and explain what is meant by the somatic nervous system. 15-2 Explain why receptors respond to specific stimuli, and how the organization of a receptor affects its sensitivity. 15-3 Identify the receptors for the general senses, and describe how they function. 15-4 Identify the major sensory pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body. 15-5 Describe the components, processes, and functions of the somatic motor pathways, and the levels of information processing involved in motor control.

15-1 Sensory Information Sensory Receptors 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 Pathways Deliver somatic and visceral sensory information to their final destinations inside the CNS using: Nerves Nuclei Tracts 1 Specialized cells that monitor specific conditions in the body or the external environment are called ____________.

15-1 Sensory Information Somatic Motor Portion of the Efferent Division Controls peripheral effectors Somatic Motor Commands Travel from motor centers in the brain along somatic motor pathways of: Motor nuclei Tracts Nerves Somatic Nervous System (SNS) Motor neurons and pathways that control skeletal muscles

15-2 Sensory Receptors The Detection of Stimuli Receptor specificity—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

Adaptation is the reduction in sensitivity of a constant stimulus 15-2 Sensory Receptors Adaptation is the reduction in sensitivity of a constant stimulus Your nervous system quickly adapts to stimuli that are painless and constant Tonic receptors (always active, slow-adapting receptors that show little peripheral adaptation) Remind you of an injury long after the initial damage has occurred Phasic receptors (normally inactive, fast-adapting receptors that become active whenever a change occurs) Provide information about the intensity and rate of change of a stimulus 2 A reduction in sensitivity in the presence of a constant stimulus is __________________.

15-2 Sensory Receptors General Senses Describe our sensitivity to: Temperature Pressure Pain Vibration Touch Proprioception Sensation The arriving information from these senses Perception Conscious awareness of a sensation

15-2 Sensory Receptors Special Senses Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing The Special Senses Are provided by special sensory receptors located in sense organs

15-2 Sensory Receptors The Interpretation of Sensory Information Arriving stimulus reaches cortical neurons via labeled line (link between peripheral receptor and cortical neuron) Takes many forms (modalities) 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

15-3 Classifying Sensory Receptors Exteroceptors provide information about the external environment Proprioceptors report the positions of skeletal muscles and joints and 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). Interoceptors monitor visceral organs and functions General Sensory Receptors (divided by nature of the stimulus) Nociceptors (pain) Thermoreceptors (temperature) Mechanoreceptors (physical distortion) Chemoreceptors (chemical concentration) 3 Receptors that monitor visceral organs and functions are simply classified as ______. _________

15-3 Classifying Sensory Receptors 1 Nociceptors (Pain Receptors) Are common in the superficial portions of the skin, joint capsules, periostea of bones and around walls of blood vessels May be sensitive to: Temperature extremes, mechanical damage, dissolved chemicals, such as chemicals released by injured cells Are free nerve endings with large receptive fields. Two types of axons - Type A (myelinated for fast pain or prickling pain) Type C fibers (slow pain or burning and aching pain) 2 Thermoreceptors Also called temperature receptors Are free nerve endings located in: The dermis, Skeletal muscles, the liver, and the hypothalamus Temperature sensations carry pain sensations sent to: The reticular formation, the thalamus, and the primary sensory cortex

15-3 Classifying Sensory Receptors 3 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, and Other distortions Three Classes of Mechanoreceptors Tactile receptors (provide the sensations of touch, pressure, vibration, shape, texture) Pressure sensations indicate degree of mechanical distortion Vibration sensations indicate pulsing or oscillating pressure Baroreceptors (detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts) Proprioceptors (monitor the positions of the joints and muscles) The most structurally and functionally complex of general sensory receptors

15-3 Classifying Sensory Receptors Tactile Receptors Fine touch and pressure receptors Are extremely sensitive. Have a relatively narrow receptive field. Provide detailed information about a source of stimulation including 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.

15-3 Classifying Sensory Receptors Six Types of Tactile Receptors in the Skin 1 Free nerve endings which are sensitive to touch and pressure situated between epidermal cells. The nerve endings providing touch sensations are tonic receptors with small receptive fields. 2 Root hair plexus nerve endings monitor distortions and movements across the body surface whereever hairs are located 3 Tactile discs (Merkel discs) are fine touch and pressure receptors that are extremely sensitive to tonic receptors and have very small receptive fields.

15-3 Classifying Sensory Receptors Six Types of Tactile Receptors in the Skin 4 Tactile corpuscles (Meissner’s corpuscles) perceive sensations of fine touch, pressure and low-frequency vibration. They adapt to stimulation with 1 second after contact. Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia 5 Lamellated corpuscles (Pacinian corpuscles) are sensitive to deep pressure. They are fast-adapting and are most sensitive to pulsing or high frequency vibrating stimuli 6 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

4 Fine touch and pressure receptors are called __________. Figure 15-3 Tactile Receptors in the Skin 4 Fine touch and pressure receptors are called __________. 5 Temperature receptors are called ___________________.

15-3 Classifying Sensory Receptors Baroreceptors—Monitor change in pressure Consist of free nerve endings that branch within elastic tissues within walls of distensible organs (blood vessels). Respond immediately to a change in pressure, but adapt rapidly. Proprioceptors Monitor position of the joints, tension in the tendons and ligaments as well as the state of muscular contraction Three Major Groups of Proprioceptors Muscle spindles (monitor skeletal muscle length and trigger stretch reflexes) Golgi tendon organs (receptors in tendon that monitors the external tension during muscle contraction) Receptors in joint capsules (receptors that detect pressure, tension, and movement at the joint). 6 Proprioceptors that monitor skeletal muscle length and trigger stretch reflexes are the _____________.

15-3 Classifying Sensory Receptors Chemoreceptors Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood are located in: Carotid bodies (within carotid arteries) and Aortic bodies (between branches of the aortic arch)

Somatic Sensory Pathways Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs Three major somatic sensory pathways The spinothalamic pathway (provides conscious sensations of poorly localized (“crude”) touch, pressure, pain and temperature) The posterior column pathway (carries sensations of highly localized (“fine”) touch, pressure, vibration and proprioception). The spinocerebellar pathway (sends proprioceptive information about the position of skeletal muscles, tendons, and joints to the cerebellum). 7 Conscious sensations of poorly localized “crude” touch, pressure, pain and temperature are provided by the __________.

Feeling Pain (Lateral Spinothalamic Tract) 15-4 Sensory Pathways Feeling Pain (Lateral Spinothalamic Tract) Referred pain (Pain felt at a site other than where the cause is situated) The pain of a heart attack is frequently felt in the left arm The pain of appendicitis is generally felt first in the area around the navel and then in the right, lower quadrant 8 Where is the pain of a heart attack felt? 9 Which visceral organ has the broadest area for referred pain?

The Somatic Nervous System (SNS)—the somatic motor system 15-4 Sensory Pathways Sensory Information 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 The Somatic Nervous System (SNS)—the somatic motor system Controls contractions of skeletal muscles The Autonomic Nervous System (ANS)—the visceral motor system Controls visceral effectors, such as smooth muscle, cardiac muscle, and glands 10 What is ALS?

Levels of Processing and Motor Control All sensory and motor pathways involve a series of synapses, one after the other. Along the way, the information is distributed to processing centers operating at the subconscious level. Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over the short term. Voluntary responses are more complex and require more time to prepare and execute. Cranial and spinal reflexes control the most basic motor activities. Integrative centers in the brain perform more elaborate processing. As you move from the medulla oblongata to the cerebral cortex, the motor patterns become more complex and variable. The primary motor cortex of the cerebral hemispheres directs the most complex and variable motor activities.

Clinical Case Living with Cerebral Palsy Name the principal descending motor pathways that carry information that would be Affected by the brain damage causing Michael’s cerebral palsy. Besides aspiration pneumonia, what other difficulties could result from poor motor Control of the muscles used in swallowing?

Figure 15-5 Somatic Sensory Pathways (Part 4 of 6) POSTERIOR COLUMN PATHWAY SPINOCEREBELLAR PATHWAY The posterior column pathway carries sensations of highly localized (”fine”) touch, pressure, vibration, and proprioception. This pathway, also known as the dorsal column-medial lemniscus, begins at a peripheral receptor and ends at the primary sensory cortex of the cerebral hemispheres. The cerebellum receives proprioceptive information about the position of skeletal muscles, tendons, and joints along the spinocerebellar pathway. The posterior spinocerebellar tracts contain axons that do not cross over to the opposite side of the spinal cord. These axons reach the cerebellar cortex via the inferior cerebellar peduncle of that side. The anterior spinocerebellar tracts are dominated by axons that have crossed over to the opposite site of the spinal cord. Ventral nuclei in thalamus PONS Midbrain Cerebellum Nucleus gracilis and nucleus cuneatus Medial lemniscus Medulla oblongata Medulla oblongata Spinocerebellar pathway Fasciculus gracilis and fasciculus cuneatus Posterior spinocerebellar tract Spinal cord Anterior spinocerebellar tract Dorsal root ganglion Fine-touch, vibration, pressure, and proprioception sensations from right side of body Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsules 31

Figure 15-5 Somatic Sensory Pathways (Part 1 of 6) SPINOTHALAMIC PATHWAY The spinothalamic pathway provides conscious sensations of poorly localized (“crude”) touch, pressure, pain, and temperature. In this pathway, the axons of first-order neurons enter the spinal cord and synapse on second-order neurons within the posterior gray horns. The axons of these interneurons cross to the opposite side of the spinal cord before ascending to the thalamus. The third-order neuron synapses in the primary sensory cortex. KEY Axon of first- order neuron Second-order neuron Third-order neuron A Sensory Homunculus A sensory homunculus (“little human”) is a functional map of the primary sensory cortex. The proportions are very different from those of any individual because the area of sensory cortex devoted to a particular body region is proportional to the number of sensory receptors it contains. Midbrain Midbrain The anterior spinothalamic tracts of the spinothalamic pathway carry crude touch and pressure sensations. The lateral spinothalamic tracts of the spinothalamic pathway carry pain and temperature sensations. Medulla oblongata Medulla oblongata Anterior spinothalamic tract Lateral spinothalamic tract Spinal cord Crude touch and pressure sensations from right side of body Pain and temperature sensations from right side of body 32