Chapter 15: Sensory Pathways & the Somatic Nervous System

Homeostasis Sensory & motor pathways provide routes for input into brain & spinal cord & for output to targeted organs for responses Each piece of incoming information is combined with others arriving & those previously stored in process known as integration

Motor Pathway (involuntary) Figure 15-1 An Overview of Events Occurring Along the Sensory and Motor Pathways. Immediate Involuntary Response Motor Pathway (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 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 (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.

Sensation Conscious or subconscious awareness of changes in external or internal environment When they reach cerebral cortex, have conscious awareness & can precisely locate & identify specific sensation Perception is conscious interpretation of sensation which is a function of the cortex

Sensory Modalities Unique type of sensation Sensory neuron carries information for only 1 modality 2 main groups of modalities General Senses Special Senses Somatic Tactile Thermal Pain Proprioception Visceral Pressure Stretch Chemicals Nausea Hunger Smell Taste Vision Hearing Equilibrium

Process of Sensation Sensory receptor responds to 1 particular kind of stimulus Specialized cell or dendrites of sensory neuron Stimulus must occur in receptor’s receptive field

Figure 15-2 Receptors and Receptive Fields.

Process of Sensation Sensory receptor responds to 1 particular kind of stimulus Specialized cell or dendrites of sensory neuron Stimulus must occur in receptor’s receptive field Receptor transduces (converts) energy in stimulus into graded potential If graded potential reaches threshold then an action potential is generated Particular region of CNS receives & integrates sensory nerve impulse

Sensory Receptors Types are grouped into different classes by structural or functional characteristics Microscopic structure Location of receptors & origin of activating stimuli Type of stimulus detected Produce 2 kinds of graded potentials Generator potential If large enough to reach threshold, then generate an action potential Free nerve endings, encapsulated nerve endings, olfactory receptors Receptor potential Trigger release of neurotransmitter via exocytosis Neurotransmitter synapses on 1st order neuron May trigger action potential

Microscopic Structure Free nerve endings Bare dendrites Pain, temperature, tickle, itch, some touch Encapsulated nerve endings Dendrite enclosed in CT capsule Pressure, vibration, some touch Separate cells Special senses Hair cells Photoreceptors Gustatory receptors

Receptor Location Exteroceptors Interoceptors Proprioceptors Located at or near the external surface of the body Stimuli originate outside the body Hearing, vision, smell, taste, touch, pressure, vibration, temperature, pain Interoceptors Aka visceroceptors Located in blood vessels, organs, muscles, nervous system Stimuli originate inside body Sometimes with very strong stimuli may be perceived as pain or pressure Proprioceptors Located in muscles, joints, tendons, inner ear Information about body position, muscle length & tension, jt position, movement, equilibrium

Type of Stimulus Mechanoreceptors Thermoreceptors Nociceptors Mechanical stimuli such as deformation, stretching, bending of cells Thermoreceptors Changes in temperature Nociceptors Painful stimuli Photoreceptors detect light Chemoreceptors Detects chemicals Osmoreceptors Detect osmotic pressure

Figure 15-3 Tonic and Phasic Sensory Receptors. 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. Increased Stimulus Normal Normal 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.

Adaptation Generator potential or receptor potential decreases in amplitude during constant stimulus Perception may fade or disappear even though stimulus persists Rapidly adapting Signals changes in stimulus Pressure, touch, smell Slowly adapting Triggers nerve impulses as long as stimulus persists

Tactile Sensation Hair Tactile disc Tactile corpuscle Free nerve ending Root hair plexus Ruffini corpuscle Lamellated corpuscle Sensory nerve

Rapidly Adapting Touch Receptors Root hair plexus b Tactile corpuscle d Epidermis Capsule Dendrites Dermis Sensory nerve fiber LM × 330

Slowly Adapting Touch Receptors Dendrites Ruffini corpuscle f Capsule Sensory nerve fiber Collagen fibers Merkel cell Tactile discs c Nerve terminal (dendrite) Tactile disc Afferent nerve fiber

Pressure/Vibration Receptors Dermis Dendritic process Acceesory cells (specialized fibroblasts) Concentric layers (lamellae) of collagen fibers separated by fluid Lamellated corpuscle (cross section) LM × 125 e Lamellated corpuscle

Free Nerve Endings Used in various modalities Detects Itch Tickle Stimulated by chemicals Tickle Temperature Cold 10-40º C Warm 32-48º C Pain Everywhere except brain Activated by intense thermal, mechanical, or chemical stimuli Fast pain Slow pain Free nerve endings a

Referred Pain Heart Liver and gallbladder Stomach Small intestine Ureters Appendix Colon

Gamma efferent from CNS Proprioception Recognize body parts, know where head & limbs are located & how they are moving without looking at them Includes Muscle spindles Tendon organs Joint kinesthetic receptors Muscle spindle Extrafusal fiber Sensory region Intrafusal fiber Gamma efferent from CNS To CNS

Sensory Pathways and Ascending Tracts Dorsal root ganglion Dorsal root Ventral root Fasciculus gracilis Fasciculus cuneatus Posterior spinocerebellar tract Anterior spinocerebellar tract Lateral spinothalamic tract Anterior spinothalamic tract Spinothalamic pathway Spinocerebellar pathway Posterior column pathway

Sensory Pathway Uses set of 3 neurons 1st order 2nd order 3rd order From somatic receptors into brain or spinal cord 2nd order Conducts impulses from brain stem or cord to thalamus 3rd order Conducts impulses from thalamus to primary somatosensory cortex

Figure 15-6 Somatic Sensory Pathways (Part 3 of 4). POSTERIOR COLUMN PATHWAY The posterior column pathway carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception. This pathway is also known as the dorsal column-medial lemniscus pathway. It begins at a peripheral receptor and ends at the primary sensory cortex of the cerebral hemispheres. Ventral nuclei in thalamus Midbrain Nucleus gracilis and nucleus cuneatus Medial lemniscus Medulla oblongata Fasciculus gracilis and fasciculus cuneatus Dorsal root ganglion Spinal cord Fine-touch, vibration, pressure, and proprioception sensations from right side of body

Spinothalamic Pathways Midbrain Medulla oblongata Anterior spinothalamic tract Spinal cord The anterior spinothalamic tracts of the spinothalamic pathway carry crude touch and pressure sensations. Crude touch and pressure sensations from right side of body Midbrain Medulla oblongata Lateral spinothalamic tract Spinal cord The lateral spinothalamic tracts of the spinothalamic pathway carry pain and temperature sensations. Pain and temperature sensations from right side of body

Figure 15-6 Somatic Sensory Pathways (Part 4 of 4). SPINOCEREBELLAR PATHWAY 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 by the inferior cerebellar peduncle of that side. The anterior spinocerebellar tracts are dominated by axons that have crossed over to the opposite side of the spinal cord. PONS Cerebellum Medulla oblongata Spinocerebellar pathway Posterior spinocerebellar tract Anterior spinocerebellar tract Spinal cord Proprioceptive input from Golgi tendon organs, muscle spindles, and joint capsule receptors

Table 15-1 Principal Ascending (Sensory) Pathways (Part 1 of 3).

Table 15-1 Principal Ascending (Sensory) Pathways (Part 2 of 3).

Table 15-1 Principal Ascending (Sensory) Pathways (Part 3 of 3).

Descending (Motor) Tracts Lateral corticospinal tract Anterior corticospinal tract Rubrospinal tract Reticulospinal tract Tectospinal tract Vestibulospinal tract Corticospinal pathway Lateral pathway Medial pathway

Corticospinal Pathway Motor homunculus on primary motor cortex of left cerebral hemisphere KEY Axon of upper- motor neuron Lower-motor neuron Corticobulbar tract To skeletal muscles Midbrain Cerebral peduncle Motor nuclei of cranial nerves To skeletal muscles Medulla oblongata Decussation of pyramids Pyramids Lateral corticospinal tract Anterior corticospinal tract To skeletal muscles Spinal cord

Descending (Motor) Tracts Lateral corticospinal tract Anterior corticospinal tract Rubrospinal tract Reticulospinal tract Tectospinal tract Vestibulospinal tract Corticospinal pathway Lateral pathway Medial pathway

Table 15-2 Principal Descending (Motor) Pathways (Part 1 of 2).

Table 15-2 Principal Descending (Motor) Pathways (Part 2 of 2).

Learning & Memory Learning is the ability to acquire new information or skills through instruction or experience Memory is process by which information acquired through learning is stored & retrieved Produces structural & functional changes in brain Called plasticity Changes in individual neurons such as synthesizing different proteins or sprouting new dendrites Also involves changes in synaptic connections Parts of brain involved Association areas of frontal, parietal, occipital, & temporal lobes; hippocampus, amygdala, diencephalon

Figure 16-11 Memory Storage. Repetition promotes retention Long-Term Memory Sensory input Short-Term Memory Secondary Memory Tertiary Memory Consolidation Cerebral cortex (fact memory) Cerebral cortex and cerebellar cortex (skill memory) Temporary loss Permanent loss due to neural fatigue, shock, interference by other stimuli Permanent loss