Part 2: Pathways (Sensory & Motor)

Motor Pathways (Descending)

Pathways that control effectors such as skeletal muscles; transmit commands from brain to body.

Composition of Motor Pathways

Include at least two neurons: upper motor neuron and lower motor neuron.

Upper Motor Neuron

Located in motor cortex, cerebral nucleus, or brainstem nucleus; sends signal to lower motor neuron.

Lower Motor Neuron

Located in cranial nerve nucleus or spinal cord anterior horn; directly innervates skeletal muscle.

Function of Motor Pathways

Carry motor commands for voluntary and involuntary movement.

Somatic Motor Pathways (Corticospinal Tracts)

Transmit voluntary control signals from motor cortex to skeletal muscles.

Autonomic Motor Pathways

Regulate smooth muscle, cardiac muscle, and gland activity.

Sensory Pathways (Ascending)

Carry sensory information such as touch, temperature, pain, and proprioception to the brain.

Somatosensory Pathways

Transmit sensory input from skin, muscles, and joints to the brain.

Viscerosensory Pathways

Transmit sensory input from internal organs (viscera) to the brain.

Number of Neurons in Sensory Pathway

Typically use a three-neuron chain: primary, secondary, and tertiary neurons.

Primary (1st Order) Neuron

Has peripheral ending in receptor, cell body in posterior root ganglion, and axon leading to secondary neuron.

Secondary (2nd Order) Neuron

An interneuron in the spinal cord or brainstem; transmits signals to tertiary neuron or cerebellum.

Tertiary (3rd Order) Neuron

Located in thalamus; sends information to somatosensory cortex of parietal lobe.

Anterolateral Pathway

Sensory pathway for crude touch, pressure, pain, and temperature using three neurons.

Primary Neuron in Anterolateral Pathway

Relays signal from skin to spinal cord; axon travels in spinal nerve and posterior root.

Secondary Neuron in Anterolateral Pathway

Relays signal from spinal cord to thalamus; axon crosses over (decussates) and ascends contralaterally.

Tertiary Neuron in Anterolateral Pathway

Relays signal from thalamus to primary somatosensory cortex for conscious perception.

Proprioception

Sense of body position and movement from muscles, tendons, and joints.

Sensation

Awareness of changes in internal or external environments.

Sensory Receptors

Specialized structures that detect stimuli and convert them to electrical impulses.

Mechanoreceptors

Detect mechanical force such as pressure, vibration, stretch, and touch.

Thermoreceptors

Detect temperature changes (hot or cold).

Photoreceptors

Detect light (found in the retina of the eye).

Chemoreceptors

Detect chemicals in solution; monitor blood, interstitial fluid, taste, and smell.

Nociceptors

Detect tissue damage or stimuli that result in pain.

Classification of Receptors by Location

Grouped as exteroceptors, interoceptors, and proprioceptors.

Exteroceptors

Located on body surface; respond to external stimuli like touch, pain, pressure, and temperature; include special senses.

Interoceptors

Respond to internal stimuli from viscera such as hunger, thirst, and internal pain.

Proprioceptors

Located in muscles, tendons, and joints; provide awareness of body position and movement.

Classification of Receptors by Complexity

Divided into simple receptors and complex sense organs.

Simple Receptors

Detect general senses like tactile sensations, temperature, and pain.

Free (Naked) Nerve Endings

Unencapsulated simple receptors found throughout body, especially in skin and connective tissue.

Examples of Free Nerve Endings

Include Merkel discs and hair follicle receptors.

Encapsulated Nerve Endings

Receptors enclosed by connective tissue; detect pressure, touch, and vibration.

Meissner’s Corpuscles (Tactile Corpuscles)

Found in superficial dermis; responsible for discriminative (fine) touch.

Lamellar (Pacinian) Corpuscles

Located deep in dermis; detect deep pressure and vibration when first applied.

Ruffini’s Corpuscles

Found in dermis, subcutaneous tissue, and joint capsules; detect continuous pressure and stretch.

Muscle Spindles

Modified muscle fibers that detect muscle stretch and help regulate muscle tone.

Golgi Tendon Organs

Proprioceptors located in tendons; monitor tension and help prevent muscle overcontraction.

Proprioceptive Receptor Function

Provide feedback about muscle position and initiate spinal reflexes for protection.

Dermatomes

Segments of skin supplied by specific spinal nerves; useful for localizing nerve or spinal damage.

Dermatome Example

T10 dermatome corresponds to a horizontal ring of skin around the umbilicus.

Referred Visceral Pain

Pain from internal organs perceived as originating from a dermatome (e.g., appendicitis pain at T10 region).

Clinical View: Shingles

Reactivation of varicella-zoster virus in posterior root ganglia causing rash, blisters, and pain along a dermatome.

Treatment of Shingles

Antiviral medication reduces severity; vaccination prevents or lessens disease.