Nervous System Circuits BIO 110 PDF

Professor Lindboom-Broberg (LB) Nervous System Circuits Sensory vs. Motor Somatic vs. Autonomic Neuronal Circuit Components Nervous System Circuit Functional divisions of the nervous system  Sensory (afferent) division Carries sensory information from receptors to CNS...

Professor Lindboom-Broberg (LB) Nervous System Circuits Sensory vs. Motor Somatic vs. Autonomic Neuronal Circuit Components Nervous System Circuit Functional divisions of the nervous system  Sensory (afferent) division Carries sensory information from receptors to CNS – Position, touch, pressure, pain, temperature – Smell, taste, sight, balance, hearing  Motor (efferent) division Carries motor commands from CNS to effector organs – Skeletal, smooth, cardiac muscle, glands, adipose, etc. Nervous System Circuit Functional divisions of the nervous system  Somatic Nervous System Controls voluntary movements – Skeletal muscle  Autonomic Nervous System Controls involuntary movements – Cardiac muscle – Smooth muscle – Glands (endocrine / exocrine) – Adipose tissue – Etc. Nervous System Circuit Three functional classes of neurons 1. ​Sensory neurons Bring sensations into central nervous system Sensory axons known as afferent fibers 2. ​Interneurons Relay signals from sensory to motor neurons 3. ​Motor neurons Transport signal to effector Motor axons known as efferent fibers Nervous System Circuit Sensory receptors  Receptors of sensory neurons that detect stimuli Interoceptors (intero-, inside) – Monitor internal organs/systems – Detect distension (stretch), deep pressure, pain Proprioceptors – Monitor position/movement of skeletal muscles/joints Exteroceptors (extero, outside) – Monitor external environment – Touch, temperature, pressure – Special senses Nervous System Circuit Sensory neurons  Ganglion: Collection of neuron cell bodies in PNS  Somatic sensory neurons: External monitor and body position Propioceptors Exteroreceptors  Visceral (autonomic) sensory neurons: Internal monitor Interoceptors Nervous System Circuit Interneurons  Interneurons: Located in CNS Usually between sensory and motor neurons Receive information from PNS and CNS Also responsible for higher functions (e.g., memory, learning) Nervous System Circuit Motor neurons  Somatic motor neurons: Transport signal to skeletal muscles  Visceral (autonomic) motor neurons: Transport signal to cardiac muscle, smooth muscle, glands, adipose, etc  Effectors Muscle (skeletal, cardiac, smooth) Glands Adipose Etc Nervous System Circuit  Professor Lindboom-Broberg (LB) Sensation vs Perception Sensation & Perception Receptors Somatic Pathways Nervous System Disorders Sensation Sensory terminology  Sensory receptors Specialized cells or cell processes that alert CNS about conditions in/out of your body Two categories 1. General senses o Senses without a specialized organ o Temperature, pain, touch, pressure, vibration, proprioception (position); receptors distributed throughout body 2. Special Senses o Senses that have receptors located within specialized organs o Vision, hearing, smell, taste, balance (vestibular) Sensation Sensory terminology  Sensation: Signal carried by a sensory pathway Requires… – Receptor activation – Sensory neuron activation  AP  Perception: Conscious awareness of a sensation Requires… – Sensory signal reaches cortex Perception – Signal is processed – Signal is sent to be made ‘aware’ of  Can you have a sensation without a perception? n ati o s S en  Professor Lindboom-Broberg (LB) Receptors Nociceptors Thermoreceptor Chemoreceptors Mechanoreceptors Receptor Physiology Receptors Nociceptors  Pain receptors (noci, to do harm)  Free nerve endings (simplest) Large receptive fields and broad sensitivity Respond to pressure, temperature, chemical, trauma, etc  Do not adapt quickly (i.e. pain signal persists) Receptors Thermoreceptors  Temperature receptors  Free nerve endings in dermis, skeletal muscles, liver, hypothalamus  Cold receptors 3-4X more numerous than warm receptors  No structural differences between warm and cold thermoreceptors Receptors Chemoreceptors  Respond to chemical concentrations Substances dissolved in body fluids (interstitial fluid, blood, CSF) Ex: O2, CO2, glucose, etc. Receptors Mechanoreceptors  Respond to mechanical distortion of their plasma membranes Mechanically-gated ion channels Respond to stretching, compression, twisting, etc.  Three types of mechanoreceptors Receptors Mechanoreceptors  Three types ​Proprioceptors – Position of joints/muscles – Ex: muscle spindle, golgi tendon organ ​Baroreceptors – Pressure changes – Ex: blood vessels, digestive, respiratory, and urinary tracts Tactile receptors – Touch (shape/texture), pressure, and vibration – Ex: skin Receptors Sensory terminology  Receptive field: Area monitored by single receptor cell Two-point discrimination test – Large receptor field = decreased detail – Small receptor field = increased detail Receptors Receptor response to stimulation  Stimulus = receptor AP What defines a stimulus? – The presence of a change? – The change itself? – BOTH!  Two categories of responses Tonic receptors Phasic receptors Receptors Receptor response to stimulation  Tonic receptors Slow-adapting; always active Action potential frequency reflects level of stimulation Increased stimulus = increased action potential frequency Receptors Receptor response to stimulation  Phasic receptors Fast-adapting; normally inactive Become active for short time when a change occurs in what they are monitoring Receptors Adaptation  Reduction in sensitivity with constant stimulus  Two types ​Peripheral adaptation – Receptor threshold increases; activity declines – Especially characteristic of phasic receptors – Example: temperature—not noticed much unless it changes Central adaptation – Nuclei along a sensory pathway are inhibited; signal lessened – Example: new smell—once it is initially detected, awareness almost stops even though neurons are still sending signals  Professor Lindboom-Broberg (LB) Pathways Sensory Pathways Motor Pathways Motor Control Pathways Representation  Sensations are created by receptors  Perceptions are created by CNS Require CNS processing center Each body part needs a dedicated processing center Pathways Somatotopy  Somatic, relating to the body  -topy, position  Functional map of the body in the brain Sensory Motor Somatotopy Somatotopy Pathways Homunculus (“little human”)  Somatotope showing the relative size of somatosensory or motor cortex devoted to any specific body area Sensory Motor Somatotopy Somatotopy Pathways Somatic sensory pathways  Sensory information must be processed in the cerebrum  3-Neuron Pathway Neuron 1: Receptor  spinal cord or brainstem Neuron 2: Spinal cord or brainstem  thalamus Neuron 3: Thalamus  cerebral cortex  Lateralization Info from spinal nerves – Right side receptor  Left cortex – Left side receptor  Right cortex Info from cranial nerves – Right side receptor  Right cortex – Left side receptor  Left cortex ** There are exceptions Sensory Somatotopy Pathways Somatic sensory pathways  Sensory information must also be sent to cerebellum  2-Neuron Pathway Neuron 1: Receptor  spinal cord Neuron 2: spinal cord  cerebellum  Lateralization Both sides go to both sides Right  left & right cerebellar hemispheres Left  right & left cerebellar hemispheres Sensory Somatotopy Pathways Somatic motor pathways  Motor responses (effects) must be sent to a skeletal muscle (effector)  2-Neuron Pathway Upper motor neuron – Motor cortex  midbrain/spinal cord Lower motor neuron – Midbrain/spinal cord  effector  Control Motor cortex activates upper & lower Reflexes can activate lower  Professor Lindboom-Broberg (LB) Somatic Motor Control A look at the complexity of somatic motor control Somatic Motor Control Preparing for movement 1. Decision to move “I should make X movement” Location: Frontal Lobe High order thinking, consideration, reflection, personality, etc Somatic Motor Control Preparing for movement 2. Frontal lobes sends signal to premotor cortex “This is what I want to do. What preprogrammed movements do we already have setup (if any)?” Location: Premotor cortex (frontal lobe) – Anterior of precentral gyrus Somatic Motor Control Preparing for movement 3. Premotor cortex sends signal to basal nuclei & cerebellum “Heads up! We’re going to be doing X movement.” Location: Basal nuclei, cerebellum Preparation for required complementary movements Somatic Motor Control Preparing for movement 4a. Premotor sends signal to primary motor cortex Preprogrammed movement command is itemized Location: Precentral gyrus (frontal lobe) Raw motor commands generated Somatic Motor Control Preparing for movement 4b. Basal nuclei & cerebellum send signal to primary motor cortex Coordinating and balance movements are itemized – Some commands altered; others added Location: Precentral gyrus (frontal lobe) Raw motor commands generated Somatic Motor Control Preparing for movement 5. Motor commands sent All motor commands sent to effectors – Most from primary motor cortex – Some from basal nuclei Location: Effectors  Professor Lindboom-Broberg (LB) Nervous System Disorders What do we know? How can it go wrong? How do symptoms present? Nervous System Disorders Referred pain  Sensation of pain in a part of the body other than its actual source Example: pain of a heart attack felt in left arm  Strong visceral pain sensations arriving at a spinal cord segment can stimulate interneurons in spinothalamic pathway Stimulates primary sensory cortex Pain perceived pain as coming from corresponding part of body surface (see sensory homunculus) Nervous System Disorders Parkinson’s disease  Normal Substantia nigra (midbrain) secretes dopamine on basal ganglia controlling movement Dompamine excites motor neurons and quiets inhibiting neurons, smoothing out motor movements  Disease Substantia nigra neurons degenerate Basal ganglia control is spotty Movements become shaky – Too much inhibition – Like driving a car with a pulsing brake Every movement must be voluntarily controlled through intense effort and concentration Nervous System Disorders Rabies  Virus  Bite from rabid animal injects rabies virus  Virus “hides” by entering axon terminals  Virus carried by axons back into CNS through retrograde flow Affects various mental processes Notable invasion of salivary glands Nervous System Disorders Amyotrophic Lateral Sclerosis (ALS)  Progressive, degenerative disorder affecting motor neurons in spinal cord, brainstem, cerebrum Destroys upper/lower motor neurons Associated skeletal muscles atrophy  Likely defect in axonal transport  Sporadic (most common) and inherited  Also called Lou Gehrig’s disease (famous Yankees player who died from it); Stephen Hawking, noted physicist, also afflicted with ALS Nervous System Disorders Alzheimer’s disease (AD)  Progressive disorder causing loss of higher-order cerebral functions  Most common cause of senile dementia, or senility  AD patients have intracellular and extracellular abnormalities in hippocampus (affects memory processing)  Symptoms may appear at age 50–60 or later; also affects younger people  Estimated 2 million people affected in United States (~15 percent of those over 65, ~50 percent of those over 85); ~100,000 deaths/year Nervous System Disorders Multiple sclerosis (sklerosis, hardness) (MS)  CNS disease characterized by automimmune-driven demyelination of axons of optic nerve, brain, spinal cord  Common signs/symptoms include impairment of vision, speech, balance, general motor coordination Time between incidents and degree of recovery vary – In about one-third of patients, disease progresses – More functional impairment occurs with each incident Onset often occurs at age 30–40 years; 1.5 times more common in women