Chapter 15: Afferent Pathway

Chapter 15 Afferent Pathway · carries sensory into from PNS to CNS · begins with sensory receptors · includes somatic and visceral sensory pathways: ↳ somatic sensory → somatosensory cortex ↳ visceral sensory → brainstem & diencephalon (reflex centers) Efferent Pathway · carries motor commands from the CNS to the PNS · somatic motor pathways → motor commands to skeletal muscles · autonomic pathways → includes sympathetic and parasympathetic NS General Sensory Receptors · Exteroreceptors: provides information about the external environment · Proprioreceptors: report the positions and movements of skeletal muscles and joints · Interoreceptors: monitor visceral organs and functions · Sensation: the arriving information (stimulus) · Perception: the conscious awareness of a sensation (must reach somatosensory cortex) · Transduction: when a stimulus is converted to an action potential by a sensory receptor Sensory Reception · Receptor specificity ↳ most receptors are specific and detect a specific stimulus ↳ free nerve endings can be stimulated by many different stimuli (no specificity) · Receptive Field ↳ the area monitored by a single receptor ↳ smaller receptive field = better localization of stimuli Tonic receptors: receptors that are always active with no adaptability or numbness Phasic receptors: receptors that are usually not active, but notices a change in a stimulus Adaptation: a reduction in receptor sensitivity in the presence of a constant stimulus Classifications of Sensory Receptors ① Nociceptors (pain) ② Thermoreceptors (temperature) ③ Mechanoreceptors (physical distortion) · Tactile receptors · Baroreceptors · Proprioreceptors ④ Chemoreceptors (chemical concentration) Nociceptors · pain receptors · free nerve endings with large receptive fields · different nociceptors may be sensitive to chemical stimuli, mechanical stimuli, or temperature · nociceptors are tonic receptors, but pain perception can be modified · Sensory neurons release neurotransmitters substance P and glutamate somatosensorycortex NOW - V Crelaystation) - ↑ ↑ > ⑧ ↑ "ij nociceptor - nociceptor → A or C fibers (1st order neuron) → spinal nerve → dorsal root & ganglion → neurotransmitters → 2nd order neuron decussates → ascends via spinothalmic tract → synapse with 3rd order neuron → somatosensory cortex (awareness and localization) - efferent pathway is impacted by neuromodulators (endorphins, enkephalins) that modulate pain perception Thermoreceptors · temperature receptors · non-specialized, free nerve endings · phasic receptors (sensitive to change) · temperature sensations conducted along same pathways as pain sensation Mechanoreceptors · mechanically gated ion channels in plasma membranes open and close in response to physical distortion · includes tactile receptors, baroreceptors, and proprioreceptors ① Tactile receptors · sense touch, pressure, vibration · fine vs. crude touch and pressure receptors · free nerve endings (see pic) * ② Baroreceptors: · monitor change in pressure · free nerve endings in walls of distensible organs · detection of blood pressure ③ Proprioreceptors: · position sense · a somatic sensation · continuously sending information to the CNS Chemoreceptors · specialized nerve cells that detect small changes in the concentration of specific chemicals or compounds · exhibit peripheral adaptation · found in general senses, we do not detect the changes eg. PH and CO2 monitoring 1st, 2nd & 3rd Order Neurons · first-order neuron: delivers sensations directly to the CNS · second-order neuron: an interneuron in the spinal cord or brainstem that passes the information to the third-order neuron · third-order neuron: passes the signal from the thalamus for awareness/perception of sensation · decussation: the point where the second order neuron crosses to opposite side of CNS Sensory Pathways · somatic sensory pathways carry sensory information from skin and muscles of body wall, head, neck, and limbs to the CNS · Spinothalamic Pathway ↳ conscious sensation of pain, temperature, crude touch ↳ 1st order neurons synapse with 2nd order in posterior horn → decussate before ascending to thalamus → 3rd order neuron to somatosensory cortex · Posterior Column Pathway (DCML pathway) ↳ sensation of fine touch, pressure, vibration, and proprioception ↳ ascends on same side to medulla → synapses with 2nd order neuron and decussates → ends at primary somatosensory cortex · Spinocerebellar Pathway ↳ delivers proprioceptive information from skeletal muscles, tendons, and joints to cerebellum (purkinje cells) does not go to the somatosensory cortex Homunculus · sensory vs. motor homunculus · cortical map · sensory homunculus ↳ proportions based on amount of sensation · motor homunculus ↳ proportions based on amount of fine movement Pain Gate Theory Visceral Sensory Pathways · information recieved from interoceptors of visceral organs and tissues · first-order neurons → second-order neurons in spinothalamic pathway → solitary nucleus of medulla oblongata → specific centers (respiratory center, cardiovascular center, reticular formation) · no third-order neurons ↳ no conscious awareness Somatic NS & Somatic Motor Pathways · somatic nervous system (SNS): allows for control of skeletal muscles · somatic motor pathways: consists of motor nuclei, tracts, and nerves ↳ always involve at least 2 motor neurons -upper motor neuron (in CNS) -lower motor neuron (in brainstem or spinal cord) · only the axon of the lower neuron extends outside CNS · motor commands travel by motor pathways: ↳ corticospinal pathway ↳ medial pathway & lateral pathway Corticospinal Pathway · pyramidal system · conscious/voluntary control over skeletal muscles · 3 pairs of descending tracts: ↳ corticobulbar tract ↳ lateral corticospinal tract ↳ anterior corticospinal tract Medial & Lateral Pathways · extrapyramidal system; subconscious movement · motor commands from the cerebrum, diencephalon, and brainstem · Lateral pathway · Medial pathway ↳ muscle tone and gross movements of neck, trunk and proximal limb muscles (primary goal is to maintain posture and balance) ↳ vestibulospinal tracts ↳ muscle tone in distal limb muscles and precise movements ↳ descend in rubrospinal tracts Basal Nuclei & Cerebrum · responsible for coordination and feedback control over muscle contractions · Basal nuclei ↳ adjust or establish patterns of movement ↳ provide background patterns of movement involved in voluntary motor activities ↳ interneurons in basal nuclei: -stimulate neurons by releasing acetylcholihe (Ach) -inhibit neurons by releasing gamma aminobutyric acid (GABA) · Cerebellum ↳ monitors proprioceptive (position) sensations, visual information, and vestibular (balance) sensations from internal ear ↳ fine tuning of complex movements improves with practice ↳ allows for efficient, smooth, precisely controlled movements