-
Labeled Line
Neuronal pathway from receptor to cortex is the "labeled line"
-
Rapidly-Adapting (phasic) Receptors
- Reduce firing rate during constant stimulus
- Fire rapidly at the beginning and end of the stimulation
- Conveys information about changes in stimulus intensity
- Examples: Pacinian Corpuscle, Thermoreceptor, Meissner's Corpuscle
-
Slowly-Adapting (tonic) Receptors
- Fire at some level all the time
- Continue firing during constant stimulation
- Conveys information about the duration of the stimulus
- Examples: Pain receptors, Muscle spindle, Merkel disc
-
Nociceptors (pain receptors)
- Slow-adapting
- Free-nerve endings
- LARGE receptive fields
- Extreme temp, mechanical damage, chemicals
- Neurotransmitters: Glutamate, Substance P
- Type A: Myleinated, "fast pain", Primary Sensory Cortex
- Type C: Unmylinated, "slow pain", Thalamus and Reticular Formation
-
Thermoreceptors
- Free-nerve endings
- Phasic (fast-adapting)
- There are 3 to 4 times more cold than warm receptors
- Information ascends in Lateral Spinothalamic Tract (with pain information)
-
Mechanorecpetors
Mechanically-gated ion channels
- Tactile Receptors: touch, pressure, vibration
- Fine touch: precisely localized (posterior column pathway)
- Crude touch: poorly localized (anterior spinothalamic pathway)
-
Free-Nerve Endings
Tactile (touch and pressure) receptor
- Between epithelial cells (corneal epithelium)
- Touch and pressure
- Tonic (slow adapting)
- Small receptive fields
-
Root Hair Plexus
Tactile (touch and pressure) receptor
- Dendrites surround hair root
- Movement of hair
- Phasic (fast adapting)
-
Merkel's (Tactile) Discs
Tactile (touch and pressure) receptor
- Dendrites contact Merkel (epidermal) cells
- Fine touch and pressure - very sensitive
- Tonic (slow adapting)
- Small receptive field
-
Meissner's (Tactile) Corpuscles
Tactile (touch and pressure) receptor
- Papillary layer of dermis
- Capsule = modified Schwann Cells
- Fine touch and pressure, low freq. vibration
- Phasic (fast adapting)
-
Pacinian (Lamellated) Corpuscles
Tactile (touch and pressure) receptor
- Dermis, joint capsules, fasciae, serous membranes, some viscera
- CT capsule
- Deep pressure, high freq. vibration
- Phasic (fast adapting)
- Large size = large receptive field
-
Ruffini Corpuscles (End Organs)
Tactile (touch and pressure) receptor
- Retcular layer of skin
- Capsule covers dendrites innervation collagen fibers of skin
- Senses stretch, distortion of dermis
- Tonic (very slow adapting)
-
Baroreceptors (Mechanoreceptors)
- Monitors pressure (stretch due to fluid)
- Free nerve endings with elastic tissue
- Phasic (fast adapting): responds to change
- Locations: digestive tract, urinary bladder, carotid sinus, aortic sinus, lungs
-
Proprioceptors
Do not adapt to constant stimulation
- Muscle Spindles
- Golgi tendon organs: free nerve endings, sense tension in tendons
- Joint capsule receptors: free nerve endings, pressure, tension, movement of joint
-
Chemoreceptors
- Respond to water and lipid-soluble substances
- Adapt rapidly
- Send info to brain stem (not sensory cortex)
-
Organization of Sensory Pathways
(1st, 2nd, 3rd order neurons)
- First-Order Neuron
- Connected to receptor
- Cell body in dorsal root ganglion or cranial nerve ganglion
- Synapses with 2nd-order (interneuron) in brain stem or cord
- Second-Order Neuron
- Crosses over to contralateral side
- Usually synapses with 3rd-order neuron in Thalamus
- Third-Order Neuron
- Synapses in post-central gyrus (primary sensory cortex)
- Does not cross over
-
Posterior Column Pathway (Ascending)
- Sensations: Fine (highly localized) touch, pressure, vibration, conscious proprioception
- Tracts: Fasciculus gracilis (lower half of body), Fasciculus cuneatus (upper half of body)
- 1st order neuron: Receptor = synpase in nucleus gracilis or cuneatus in medulla
- 2nd order neuron: Crosses over to other side in medulla, enter medial lemniscus, synapse in thalamus
- 3rd order neuron: Thalamus = somatosensory cortex, thalamus processes info according to stimulus type and location
-
Anterior and Lateral Spinothalamic Pathways (Ascending)
Anterior Spinothalamic Pathway: Crude touch and pressure (poorly localized)
Lateral Spinothalamic Pathway: Pain and Temp.
- BOTH
- 2nd order neurons: Cross over in spinal cord, synapse in thalamus
- 3rd order neurons: Synapse in primary sensory cortex
-
Spinocerebellar Pathway (Ascending)
- Sensation
- Unconscious Proprioception (cerebellum)
- Input from Golgi tendon organs, muscle spindles, joint capsules
- Pathway
- 1st order neurons: Synapse in dorsal horn
- 2nd order neurons: Synapse with Purkinje cells in cerebellum
- 3rd order neurons: NONE
-
Motor Pathways (Descending Tracts)
- Upper Motor Neuron
- Cell body in CNS
- Synapses with lower motor neurons (Excitatory or Inhibitory)
- Stays in CNS
- Lower Motor Neuron
- Cell body in brain stem or cord
- Innervates skeletal muscle fibers of a motor unit (always excitatory at a synapse)
- Called "Final Common Pathway"
-
Corticospinal Pathway (Descending)
- AKA "Pyrimdal System" or "Direct Pathways"
- Conscious control of skeletal muscle
- 1. Corticobulbar Tract
- 2. Lateral Corticospinal Tract
- 3. Anterior Corticospinal Tract
-
Corticobulbar Tract (Descending)
- "bulbar" = brain stem
- UMN cell bodies in primary motor cortex
- Synapse in cranial nerve nuclei
- Conscious control of skeletal muscle
- Examples: eye, jaw, facial muscle movements
-
Lateral Corticospinal Tract (Descending)
- 85% of motor corticospinal pathway neurons
- UMN in primary motor cortex (cross over at decussation of pyramids in medulla)
- Synapse on LMN in cord
- Conscious control of skeletal muscles
-
Anterior Corticospinal Tract (Descending)
- 15% of corticospinal neurons
- UMN in primary motor cortex
- Synpases on LMN in cord
- Cross over in spinal cord (not medulla)
- Conscious control of skeletal muscle
|
|
