Unit Nine: The Nervous System: A Unit Nine: The Nervous System: A. General Principles and Sensory Physiology Chapter 46: Sensory Receptors, Neuronal Circuits for Processing Information Guyton and Hall, Textbook of Medical Physiology, 12th edition

Types of Sensory Receptors and Their Stimuli Mechanoreceptors- detect mechanical compression or stretching Thermoreceptors- detect changes in temperature Nociceptors- pain receptors (damage to tissues) Electromagnetic receptors- detect light on the retina Chemoreceptors- detect taste, smell, oxygen level, osmolality, etc. (See Table 46.1 in the text)

Types of Sensory Receptors and Their Stimuli Differential Sensitivity of Receptors Each receptor type is highly sensitive to one type of stimulus for which it is designed Non-responsiveness to other types of sensory stimuli Pain receptors do not respond to usual touch or pressure but will become active when the stimuli become severe enough to damage the tissues

Types of Sensory Receptors and Their Stimuli Modality of Sensation- The “Labeled Line” Principle- the specificity of nerve fibers for transmitting only one modality of sensation Each receptor type is highly sensitive to one type of stimulus for which it is designed Non-responsiveness to other types of sensory stimuli Pain receptors do not respond to usual touch or pressure but will become active when the stimuli become severe enough to damage the tissues

Fig. 46.1 Several types of somatic sensory nerve endings

Transduction of Sensory Stimuli into Nerve Impulses Local Electrical Currents at Nerve Endings— Receptor Potentials Mechanisms of receptor potentials Mechanical deformation of the receptor which stretches the membrane and opens channels Application of a chemical to the membrane Change of the temperature of the membrane Effects of electromagnetic radiation

Transduction of Sensory Stimuli into Nerve Impulses Local Electrical Currents at Nerve Endings— Receptor Potentials b. Maximum receptor potential amplitude (100 mV) Relation of the receptor potential to APs –the more the receptor potential rises above the threshold level, the greater becomes the AP frequency

Transduction of Sensory Stimuli into Nerve Impulses Fig. 46.2 Typical relation between receptor potential and action potentials when the receptor potential rises above threshold

Transduction of Sensory Stimuli into Nerve Impulses Receptor Potential of the Pacinian Corpuscle Fig. 46.3 Excitation of a sensory nerve fiber by a receptor potential produced in a Pacinian corpuscle

Transduction of Sensory Stimuli into Nerve Impulses Relation Between Stimulus Intensity and the Receptor Potential Fig. 46.4 Relation of amplitude of receptor potential to strength of a mechanical stimulus applied to a Pacinian corpuscle

Transduction of Sensory Stimuli into Nerve Impulses Adaptation of Receptors Fig. 46.5 Adaptation of different types of receptors, showing rapid adaptation of some receptors and slow adaptation of others

Transduction of Sensory Stimuli into Nerve Impulses Mechanism of Receptor Adaptation- different for each type of receptor In the mechanoreceptor the initial compression causes the receptor potential which disappears within a fraction of a second even though the compression continues Accommodation- slower adaptation and occurs in the nerve fiber itself; the tip of the nerve gradually becomes “accommodated” to the stimulus

Transduction of Sensory Stimuli into Nerve Impulses Tonic Receptors (Slow Adapting) Continue to transmit impulses as long as the stimulus is present; include the following: Macula receptors in the vestibular apparatus Pain receptors Baroreceptors of the arterial tree Chemoreceptors of the carotid and aortic bodies

Transduction of Sensory Stimuli into Nerve Impulses Phasic Receptors (Rapidly Adapting)- also called “rate receptors,” and “movement receptors” Stimulated only when the stimulus strength changes React strongly while a change is actually taking place Cannot be used to transmit a continuous signal

Transduction of Sensory Stimuli into Nerve Impulses Importance of Phasic Receptors- have a predictive function General Classification of Nerve Fibers Type A- large and medium sized myelinated fibers of spinal nerves (alpha, beta, gamma, delta) Type C- small, unmyelinated fibers that conduct at low velocities

Transduction of Sensory Stimuli into Nerve Impulses Fig. 46.6 Physiologic classification and functions of nerve fibers

Transmission of Signals of Different Intensity in Nerve Tracts Spatial Summation- increased signal strength by using progressively larger numbers of fibers; stronger signals spread to more and more fibers (Fig. 46.7) Temporal Summation- increased signal strength by increasing the frequency of nerve impulses in each fiber (Fig. 46.8)

Fig. 46.7 Pattern of stimulation of pain fibers in a nerve leading from an area of skin pricked by a pin (spatial summation) Fig. 46.8 Translation of signal strength into a frequency modulated series of nerve impulses (temporal summation)

Transmission and Processing of Signals in Neuronal Pools Relaying of Signals Through Neuronal Pools- organization of neurons for relaying signals The neuron area stimulated by each incoming nerve fiber is the stimulatory field Fig. 46.9 Basic organization of a neuronal pool

Transmission and Processing of Signals in Neuronal Pools Threshold and Sub-threshold Stimuli The discharge of a single excitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron b. Instead, large numbers of input terminals must discharge on the same neuron either at the same time or in rapid succession to cause excitation

Transmission and Processing of Signals in Neuronal Pools Threshold and Sub-threshold Stimuli Fig. 46.10 “Discharge” and “Facilitated” zones of a neuronal pool

Transmission and Processing of Signals in Neuronal Pools Inhibition of a Neuronal Pool Some incoming fibers inhibit neurons, rather than excite them This is the opposite of “facilitation” and is called the “inhibitory zone”

Transmission and Processing of Signals in Neuronal Pools Divergence of Signals Amplifying-an input signal spreads to an increasing number of neurons as it passes through successive orders of neurons in its path b. Divergence in multiple tracts- the signal is transmitted into two directions from the pool; information transmitted up the dorsal column from the spinal cord takes two courses (a) into the cerebellum, and (2) on through the lower regions of the brain to the thalamus and cerebral cortex

Transmission and Processing of Signals in Neuronal Pools Divergence of Signals Fig. 46.11 Divergence in neuronal pathways. A: Divergence within a pathway to cause amplification of the signal, B: Divergence into multiple tracts to transmit the signal to separate areas.

Transmission and Processing of Signals in Neuronal Pools Convergence of Signals- signals from multiple inputs uniting to excite a single neuron Convergence from a single source- multiple terminals from a single incoming fiber tract terminate on the same neuron Convergence from input signals (excitatory or inhibitory) from multiple sources Allows the summation of information from different sources

Transmission and Processing of Signals in Neuronal Pools Convergence of Signals- signals from multiple inputs uniting to excite a single neuron Fig. 46.12 Convergence of multiple input fibers onto a single neuron

Transmission and Processing of Signals in Neuronal Pools Neuronal Circuit with both Excitatory and Inhibitory Output Signals Sometimes an incoming signal causes an excitatory signal going in one direction and an inhibitory signal going elsewhere Reciprocal inhibition circuit in some reflexes

Transmission and Processing of Signals in Neuronal Pools Neuronal Circuit with both Excitatory and Inhibitory Output Signals Fig. 46.13 Inhibitory circuit. Neuron 2 is an inhibitory neuron

Transmission and Processing of Signals in Neuronal Pools Prolongation of a Signal by a Neuronal Pool- “Afterdischarge” Synaptic Afterdischarge- when excitatory synapses discharge on a dendrite or on the soma, a postsynaptic electrical potential develops in the neuron and lasts for msec As long as the potential lasts, it will continue to excite the neuron, causing it to transmit a continuous train or output impulses

Transmission and Processing of Signals in Neuronal Pools Reverberatory (Oscillatory) Circuit Caused by positive feedback within the neuronal circuit that feeds back to re-excite the input of the same circuit

Transmission and Processing of Signals in Neuronal Pools Reverberatory (Oscillatory) Circuit Fig. 46.14 Reverberatory circuits of increasing complexity

Transmission and Processing of Signals in Neuronal Pools Characteristics of Signal Propagation from a Reverberatory Circuit Fig. 46.15 Typical pattern of the output signal from a reverberatory circuit after a single input stimulus showing the effects of facilitation and inhibition

Transmission and Processing of Signals in Neuronal Pools Continuous Signal Output from Some Neuronal Circuits Continuous discharge caused by intrinsic neuronal excitability Continuous signals emitted from reverberating circuits

Transmission and Processing of Signals in Neuronal Pools Continuous Signal Output from Some Neuronal Circuits Fig. 46.16 Continuous output from either a reverberating circuit or a pool of intrinsic discharging neurons. Also showing the effects of excitatory or inhibitory input signals

Transmission and Processing of Signals in Neuronal Pools Rhythmical Signal Output Fig. 46.17 Rhythmical output of summated nerve impulses

Instability and Stability of Neuronal Circuits Inhibitory Circuits as a Mechanism for Stabilizing Nervous System Function Synaptic Fatigue as a Means of Stabilizing the Nervous System Automatic Short-Term Adjustment of Pathway Sensitivity by the Fatigue Mechanism Long-Term Changes in Synaptic Sensitivity by Automatic Down Regulation or Up Regulation of Synaptic Receptors

Fig. 46.19 Successive flexor reflexes showing fatigue of conduction through the reflex pathway