Sensory Pathways and Somatic Nervous System

Neural Integration: Sensory Pathways and the Somatic Nervous System

• Chapter focuses on neural integration, sensory pathways, and the somatic nervous system.

Introduction to Sensory Pathways

• Focus is on general senses
• Sensory receptors, processing, as well as conscious and subconscious motor functions are examined.

Sensory Information

• The nervous system's afferent division includes receptors, sensory neurons, and sensory pathways.
• The efferent division includes nuclei, motor tracts, and motor neurons.

sensoryReceptors

• Sensory receptors are specialized cells that monitor specific conditions either inside the body or in the external environment.
• When stimulated, a receptor passes information to the CNS.
• The information passes in the form of action potentials along the axon of a sensory neuron.
• The Somatic Nervous System (SNS) consists of motor neurons and pathways that control skeletal muscles.

General Senses

• General senses describe sensitivity to temperature, pain, touch, pressure, vibration, and proprioception.

Sensation vs. Perception

• Sensation is the arriving information from senses.
• Perception is the conscious awareness of a sensation.

Special Senses

• Special senses include olfaction (smell), vision (sight), gustation (taste), equilibrium (balance), and hearing.

Special Sensory Receptors

• The special senses are provided by special sensory receptors.
• Special sensory receptors are located in sense organs, such as the eye or ear.
• Special sensory receptors are protected by surrounding tissues.

Detection of Stimuli

• Each receptor has a characteristic sensitivity called receptor specificity.
• A receptive field is the area monitored by a single receptor cell.
• The larger the receptive field, the more difficult it is to localize a stimulus.

Interpretation of Sensory Information

• An arriving stimulus reaches cortical neurons via a labeled line.
• Stimuli (or modalities) can take the form of physical force like pressure, dissolved chemical, sound, or light.
• Sensations like taste, hearing, equilibrium, and vision are provided by specialized receptor cells.
• Specialized receptor cells communicate with sensory neurons across chemical synapses.

Adaptation

• Adaptation is a reduction in sensitivity to a constant stimulus.
• The nervous system quickly adapts to stimuli that are painless and constant.
• 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.
• Perception of a stimulus nature depends on the path it takes inside the CNS.

Tonic vs. Phasic Receptors

• Tonic receptors are always active, show little peripheral adaptation, and are slow-adapting receptors.
• Tonic receptors remind you of an injury long after the initial damage has occurred.
• Phasic receptors are normally inactive but become active for a short time whenever a change occurs.
• Phasic receptors provide information about the intensity and rate of change of a stimulus and are fast-adapting receptors.

Classifying Sensory Receptors

• Exteroceptors provide information about the external environment.
• Proprioceptors report the positions of skeletal muscles and joints.
• Interoceptors monitor visceral organs and functions.

Proprioceptors

• Proprioceptors provide a purely somatic sensation.
• There are no proprioceptors in the visceral organs of the thoracic and abdominopelvic cavities.

General Sensory Receptors

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

Nociceptors

• Nociceptors (pain receptors) are common in the superficial portions of the skin, joint capsules, within the periosteum of bones, and around the walls of blood vessels.
• Nociceptors may be sensitive to temperature extremes, mechanical damage, or dissolved chemicals released by injured cells.
• Nociceptors are free nerve endings with large receptive fields.
• 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.
• Type C fibers carry sensations of slow pain, or burning and aching pain.
  • They cause a generalized activation of the reticular formation and thalamus.
• Pain becomes apparent, awareness of the pain is generalized to broad area.

Thermoreceptors

• Thermoreceptors are also called temperature receptors.
• Thermoreceptors are conducted along the same pathways that carry pain sensations.
• Thermoreceptors are free nerve endings located in the dermis, skeletal muscles, the liver, and the hypothalamus.

Mechanoreceptors

• Mechanoreceptors are sensitive to stimuli that distort their plasma membranes.
• These receptors contain mechanically gated ion channels whose gates open or close in response to stretching, compression, twisting, or other distortions of the membrane.
• Three Classes of Mechanoreceptors: -Tactile receptors -Baroreceptors -Proprioceptors

Tactile Receptors

• Tactile receptors respond to touch, pressure, and vibration.
• Fine touch and pressure receptors are extremely sensitive, have a relatively narrow receptive field, and provide detailed information about a source of stimulation.
• Fine touch and pressure receptors include exact location, shape, size, texture, and movement.
• Crude touch and pressure receptors have relatively large receptive fields, provide poor localization, and give little information about the stimulus.

Baroreceptors

• Baroreceptors monitor changes in pressure.
• Baroreceptors consist of free nerve endings that branch within elastic tissues, such as in the wall of a distensible organ.
• Baroreceptors respond immediately to a change in pressure but adapt rapidly.

Proprioceptors

• Proprioceptors monitor the position of joints, tension in tendons and ligaments, and the state of muscular contraction.
• There are three major groups of Proprioceptors: -Muscle spindles -Golgi tendon organs
  • Receptors in joint capsules

Postural Reflexes

• Postural reflexes demonstrate stretch reflexes.
• Postural reflexes maintain normal upright posture.
• Stretched muscle responds by contracting and automatically maintains balance.

Chemoreceptors

• Chemoreceptors respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid.
• Chemoreceptors' receptors' peripheral or central adaptation may occur.
• Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood are located in carotid bodies and aortic bodies.
  • Carotid bodies are located near internal carotid arteries sides of the neck
  • Aortic bodies are located between the major branches of the aortic arch.

Sensory Pathways

• Feeling Pain (Lateral Spinothalamic Tract) can result in referred pain.
• 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.

Visceral Sensory Pathways

• Interoceptors include nociceptors, thermoreceptors, tactile receptors, baroreceptors, and chemoreceptors.

Somatic Motor Pathways

• The Somatic Nervous System (SNS) -This is also called the somatic motor system -The SNS controls contractions of skeletal muscles.
• The Autonomic Nervous System (ANS) -The ANS goes by the name visceral motor system -The ANS controls visceral effectors, such as smooth muscle, cardiac muscle, and glands.