• CNS=brain and spinal cord.
• PNS=afferent neurons or efferent neurons
• afferent neurons: input, from the somatic, special, or visceral senses. Usually pseudo-unipolar neurons that send signals to the CNS via interneurons, which are multipolar). 
• efferent neurons: send input to the somatic and autonomic muscle of both sympathetic and parasympathetic nervous system.

• Bipolar: have an axon and a dendrite coming off the cell body.
• Pseudo-Unipolar: subclass of bipolar neurons; axon and dendrite appear as a single extension.
• Multipolar: multiple projections from the cell body; one axon, everything else is dendrites.

Stimulus→Receptors on Afferent Neuron (1st Order Neuron)→Brain→Interneuron (2nd Order Neuron)→Thalamus→Interneuron (3rd Order Neuron)→Cortex

Dorsal Root Ganglion=where afferent neurons reach the CNS.

• Deals with afferent nervous system signals (info from periphery to CNS).
• Sensory Information (external): comes from somatic sensations (somesthetic/skin touch or proprioception (limb position) or special senses (vision, hearing, equilibrium, taste, smell)

• Have specialized nerve endings that detect a sensory stimulus.
• Must convert stimulus energy into electrical energy.
• Types: photoreceptors, chemoreceptors, thermoreceptors, mechanoreceptors.
• Stimulus causes a change in the membrane potential of the sensory receptor.
• Graded potential (caused by opening/closing of an ion channel); can lead to action potential if threshold is reached.
• Decrease amplitude of receptor potential over time with constant stimulus.
• This decreases our perception of a stimulus.

• Labeled Line Theory: says that there is a specific neural pathway for each sense.
• Sensory Unit: a single afferent neuron + associated receptors.
• Sensory Pathway: when all receptors/receptor fields are the same.
• Receptor Field: all axons of a particular nerve.
• The location of a stimulus is gauged by receptor fields; if multiple receptive fields are involved, the location is determined by which neuron induces the greatest number of action potenials.
• The intensity of a stimulus is gauged by the frequency of the action potential.
• A stronger stimulus activates more receptors (recruitment), either from the same afferent neuron or not.
• Two-Point Discrimination: ability to perceive 2 points on skin. 

• Rapidly Adapting:
• Pacinian Corpuscle
• Meissner's Corpuscle
• Hair Follicle
• Slowly Adapting:
• Free Nerve Endings
• Merkel's Disk
• Ruffini's Ending

• Warm Receptors
• Free Nerve Endings that sense between 30-43 degrees.
• Increase frequency with increase in temperature
• Cold Receptors
• Free Nerve Endings that sense between 35 and 20 degrees.

• Receptors: Mechanoreceptors, Thermoreceptors, Nocireceptors (for pain)
• Dorsal Column-Medial Lemniscal Pathway
• Somesthetic Sensations: associated with skin
• Proprioception: awareness of body's position in space.
• Spinothalamic Tract
• Pain
• Temperature
• Pain Perception
• Pain Response
• Visceral Pain (Referred Pain)
• Modulation of Main

• Transfers info from mechanoreceptors and prorioceptors to the CNS; crosses to the other side of the CNS in the medulla oblongata.
• 1st Order Neurons initiate in the periphery and enter the dorsal horn of the spinal cord.
• The collateral from the main axon may end up in the spinal cord
• This causes communication with interneurons, resulting reflexes.
• Main 1st Order Axon has ipsilateral ascension (ascends from the spinal cord to the brainstem, on the same side as the stimulus, in dorsal columns (tracts of white matter).
• 1st Order Neurons terminate in dorsal column nuclei, located in the medulla, where they form synapses with second order neurons.
• 2nd order neurons cross over to the other side of the medulla, via a tract called the medial lemniscus, and then ascend to the thalamus.
• In the thalamus, 2nd order neurons form synapses with 3rd order neurons, which transmit info to the somatosensory cortex.

• Transmits info from thermoreceptors and nocireceptors to the thalamus; crosses to the other side of the CNS within the spinal cord. 
• 1st Order Neurons initiate in periphery and enter the dorsal horn of the spinal cord.
• 1st order may ascend or descend a short distance (a few spinal segments) along Lissauer's tract--> Eventually form synapses with second order neurons in the dorsal horn.
• Second order cross over to the other side of the spinal cord (contralateral), ascends in the anterolateral quadrant of the spinal cord to the brainstem.
• 2nd order terminates in the thalamus, forming a synapse with 3rd order neurons that ascend to the somatosensory system.

Fast Pain: A Delta fibers; sharp pricking sensation; well localized.

Slow Pain: C Fibers; dull aching; poorly localized.

• A Delta or C; both terminate in spinal cord dorsal horn.
• The Neurotransmitter of C Fibers (and maybe A Delta) is Substance P.
• Specific Pathway: spinothalamic tract
• Non-Specific Pathway: to reticular formation, hypothalamus, limbic system.

pain originating in internal organs; sensation "referred" to body surface

ex. angina due to heart damage.

Mechanism: Nocireceptors in organs detect damage, 2nd order neurons from the visceral 1st order afferent also receive input from skin's 1st order afferents--> the brain interprets visceral input as input from somatic afferents, causing pain to be perceived in the skin. 

• The perception of pain varies depending on circumstances and past experiences.
• Gate-Control Theory: somatic signals of nonpainful sources can inhibit signals of pain at the spinal level.
• In unmodulated pain, Collaterals of nocireceptor afferents (C Fibers) inhibit the inhibitory interneurons, allowing pain signal transmission.
• In modulated pain, Collaterals of large diameter afferents (A Beta fibers) that branch from touch and pressure receptors excite the inhibitory neuron, decreasing pain signal transmission).

• Analgesia=blocking of pain
• Brain can block pain through this system.
• Stress activates signals in the periaqueductal gray matter (in the midbrain).
• The periaqueductal gray matter communicates with the nucleus raphe magnus (in the medulla) and the lateral reticular formation (extends the length of the brainstem).
• Neurons from these two areas go down to the dorsal horn and block communication between nocireceptor primary afferent neurons and second order neurons.
• They do this by releasing the neurotransmitter enkephalin, which binds to receptors on the second order neurons and the receptors of the nociceptive afferent neuron.
• This causes presynaptic inhibition (so no release of the pain neurotransmitter Substance P) and production of IPSPs on second order neuron.