Sensory System represents the afferent arm of the nervous system and provides the brain with information about the internal and external environment.
The sensory system consists of
sensory receptors that receive stimuli from the environment
neuralpathways that conduct information from the receptors to the brain/spinalcord
parts of the brain that process information
All parts of the sensory system (hearing, vision, equilibrium, taste) travel to the thalamus except for smell, which goes directly to the olfactory cortex
Transduction
Conversion of one form of energy into another
The sensory system functions by:And the motor system is the inverse
Threshold by physical stimulus -> transduction by receptors -> conduction by neurons -> processing by the brain -> perception!
A device that transforms energy from one type to another.
Sensory receptors transfer stimulus energy into electrical signals known as:
Receptorpotentials (which upon reaching threshold trigger actionpotentials)
The receptor potential is a graded (proportional to stimulus intensity) response that may be depolarizing or hyperpolarizing & can work to trigger an AP within the same cell or adjacent cell via neurotransmitter release.
The receptor potential is propagated passively towards the neuron's trigger zone
Primary afferent neuron firing frequency reflects the magnitude of the receptor potential which in turn reflects the magnitude of the stimulus
Coding
The conversion of stimuli into the action potentials in the CNS and the AP frequencies
Key questions of coding:
What is it? (quality/modality)
How strong is it? (intensity)
How long is it? (Timing)
Where is it? (Localization)
The specific form of energy of a stimulus (ie. temperature, sound, pressure, light, pain, taste) is its modality
The modality to which the receptor responds to best is the
Adequatestimulus
Stimulus intensity is coded by the frequency of APs generated in the firstorder neuron
Stronger stimuli excite more receptors and afferents
Recruitment of sensory units
The typical response to a constant stimulus will decrease over time
Sensory adaptation
Receptor adaptation can be phasic/rapid (respond to change) or tonic/slow (track duration)
Labelledlinecode connects the peripheral receptor to the cortex via a series of neurons
Nerveterminals have 1st order neurons which send signals up to the 2nd order neurons in the spinalcord which sends them to the 3rd order neurons in the thalamus
Lateral inhibition involves the inhibitory neurons between 1st and 2nd order neurons that allow the body to "focus" on the are that is being stimulated by preventing the unnecessary activation of nearby neurons
Sensoryneurondisease (SND)
PNS diseases caused by degeneration of short & long peripheral axons and afferent sensory projections
Sensory processing disease examples:
Synesthesia
Prosopagnosia (face blindness)
Acetylcholine is released in the neuromuscularjunction to activate a motor neuron
Motorneurons are the final common pathway out of the CNS
Motor neurons send their axons to innervate the muscles which allows actionpotentials to travel down the motorneuron and branch into many terminals near their target
Motor pathways descend from brain through the pons, to the spinalcord, then out to the effectororgan
A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonalterminals
Innervation ratio
Number of fibers innervated by an axon
Almost all motor behaviors involve both voluntary and involuntaryactions
Actions become more involuntary as the actions are repeated
Three classes of motor movements:
Reflexes
Simplest
Largely involuntary
Spinal cord
Ex. sneezing, knee-jerk, flexor withdrawal
Three classes of motor movements:
RhythmicMotorPatterns
Starting and stopping are voluntary
Sequence of stereotyped, repetitive movements
Thalamus, basal ganglia, cerebellum
Ex. Walking, running
Three classes of motor movements:
Voluntarymovements
Goal-directed and purposeful
Largely learned and improve with practice
Cerebral cortex motor areas
Ex. Playing piano, writing
Motor control hierarchy: High
Pre-command level : Planning
Controls voluntary movements
Corticalassociation cortex (idea) leads to cerebellum, basal ganglia, and motor cortex/brain stem
Motor control hierarchy: Middle
Projection level : instructions
Controls rhythmic motor patterns
Cerebellum sends to spinal cord and motor cortex/brain stem, the basalganglia sends to the motor cortex/brain stem which sends to the spinalcord
Motor control hierarchy: Low
Segmental level : Central pattern generators
Controls reflexes
Spinalcord leads to movement
Interneurons can be inhibitory or excitatory and can function as switches to turn motor neurons "on" and "off" (also constitute 90% of spinal cord neurons)