Sensory Pathways and Somatic Nervous System

Sensory Pathways
- Pathways that relay sensory information from receptors to Central Nervous System (CNS).
- Include General Senses such as temperature and pain.
Motor Pathways
- Associated with the Somatic Nervous System (SNS), which controls the contractions of skeletal muscles.

Sensory Pathways
- Defined as a series of neurons that relay sensory information from receptors to the CNS.
Sensory Receptors
- Specialized cells or processes that monitor specific conditions within the body or external environment.
- When stimulated, receptors generate action potentials sent along sensory pathways.

Afferent Division:
- Includes somatic and visceral sensory pathways.
Efferent Division:
- Includes the somatic motor portion controlling peripheral effectors.
- Commands travel from motor centers in the brain along somatic motor pathways.

Depolarization of Sensory Receptor
- A stimulus produces a graded change in the membrane potential of a receptor cell.
Action Potential Generation
- If the membrane potential reaches threshold, action potentials develop.
Propagation
- Axons of sensory neurons carry information about the type of stimulus (e.g., touch, pressure, temperature) to the CNS.
CNS Processing
- Occurs at every relay synapse with information possibly distributed to various nuclei and centers in spinal cord and brain.
Immediate Involuntary Response
- Processing centers may direct a reflex response before sensations reach the cerebral cortex.
Voluntary Response
- Not immediate; can enhance or supplement involuntary reflexes.
Perception
- Only about 1% of arriving sensations are relayed to the primary somatosensory cortex.

Definitions:
- Sensation: Arriving information.
- Perception: Conscious awareness of a sensation.
General Senses:
- Sensitivity to temperature, pain, touch, pressure, vibration, and proprioception (body position).
Special Senses:
- Include olfaction (smell), gustation (taste), vision (sight), equilibrium (balance), and hearing.
- Special sensory receptors located in sense organs (e.g., eyes, ears) and protected by surrounding tissues.

Receptor Specificity:
- Each receptor has a characteristic sensitivity.
Receptive Field:
- Area monitored by a single receptor cell; larger fields make localization of stimuli more difficult.
Transduction:
- Conversion of stimulus into an action potential by a sensory receptor.

Labeled Line:
- Each carries information about one modality (e.g., touch or light).
- Frequency and pattern of action potentials convey strength, duration, and variation of a stimulus.
Perception of Stimulus
- Depends on the CNS pathway taken.
Adaptation:
- Reduction of receptor sensitivity due to constant stimuli (includes peripheral and central adaptation).
Types of Receptors:
- Tonic Receptors: Always active, slow-adapting (e.g., pain receptors).
- Phasic Receptors: Normally inactive, provide information about intensity and rate of change of stimulus, fast-adapting.

Types by Stimulus:
- Nociceptors: Pain receptors.
- Thermoreceptors: Temperature receptors.
- Mechanoreceptors: Sensitive to physical distortion.
- Chemoreceptors: Sensitive to chemical concentration.
Nociceptors:
- Free nerve endings; common in skin and joints.
Thermoreceptors:
- Free nerve endings located in dermis, muscles, and hypothalamus.
Mechanoreceptors:
- Sensitive to physical stimuli; three classes are tactile receptors, baroreceptors, and proprioceptors.

Types of Tactile Receptors:
- Free Nerve Endings: Sensitive to touch and pressure.
- Root Hair Plexuses: Detect hair movement.
- Tactile Discs: Sensitive to fine touch and texture.
- Bulbous Corpuscles: Detect pressure.
- Lamellar Corpuscles: Sensitive to deep pressure and vibration.
- Tactile Corpuscles: Sensitive to fine touch and low-frequency vibration.

Function: Monitor positions of joints, tendons, and skeletal muscles.
Types:
- Muscle Spindles: Monitor muscle length.
- Golgi Tendon Organs: Monitor tension in skeletal muscles.
- Joint Capsule Receptors: Detect pressure and movement.

Function: Respond to water- and lipid-soluble substances and monitor pH, CO2, and O2 levels in blood.
Locations: Carotid bodies and aortic bodies.

Neurons in Sensory Pathways:
- First-order Neuron: Delivers sensations to CNS.
- Second-order Neuron: Interneuron that receives information from the first-order neuron; may cross to the opposite side of CNS.
- Third-order Neuron: Located in thalamus; conveys information to the primary somatosensory cortex.
Sensor Pathways:
- Somatic Sensory Pathways: Carry information from skin and muscles to CNS.
- Major Pathways: Spinothalamic pathway, posterior column pathway, and spinocerebellar pathway.

Spinothalamic Pathway:
- Carries sensations of crude touch, pressure, pain, and temperature.
- First-order neurons synapse in posterior horns; second-order neurons cross to opposite side before ascending.
Posterior Column Pathway:
- Carries sensations of fine touch, vibration, pressure, and proprioception.
- Involves gracile and cuneate fasciculi and synapses in thalamus.
Spinocerebellar Pathway:
- Conveys information about muscle, tendon, and joint position to cerebellum; does not reach awareness.

Function: Collect visceral sensory information from within thoracic and abdominopelvic cavities via interoceptors.
Cranial Nerves: V, VII, IX, and X carry sensory information from structures like the mouth and throat.
Solitary Nucleus: Major processing center for visceral sensory information located in medulla oblongata.

SNS Function: Controls contractions of skeletal muscles.
Motor Pathways: Involve at least two motor neurons: upper and lower motor neurons.
Upper Motor Neuron: Body in CNS processing center, synapses on lower motor neuron.
Lower Motor Neuron: Body in brainstem/spinal cord; triggers muscle contraction.

Corticospinal Pathway: Provides voluntary control; begins in primary motor cortex.
Medial Pathway: Controls gross movements of trunk and proximal limbs.
Lateral Pathway: Controls precise movements of distal limbs.

Basal Nuclei: Provide background movement patterns; modulate corticospinal instructions.
Cerebellum: Monitors proprioceptive, visual and balance sensations; improves motor control through practice.

Sensory and motor pathways are integral to the functioning of the nervous system, coordinating sensory inputs and motor outputs for an integrated response in the body.