Sensory and Motor Pathways

Sensory and Motor Pathways

• Sensory pathways are neuron series relaying sensory information from sensory receptors to the CNS.
• Motor pathways are neuron series relaying motor commands from the CNS to effectors.

Sensory or Afferent Division

• Sensory pathways relay information from sensory receptors to the CNS.
• Special senses include vision, hearing, equilibrium, taste, and smell (covered in Ch. 17).
• General senses include touch, temperature, vibration, pressure, stretch, and pain.
  • Somatic sensory pathways monitor skeletal muscles, joints, and skin surface.
  • Visceral sensory pathways monitor internal organs.

Motor or Efferent Division

• Motor pathways relay motor commands from the CNS to effectors.
• Motor pathways for the Somatic Nervous System (SNS) control skeletal muscle.
• Motor pathways for the Autonomic Nervous System (ANS) control smooth muscle, cardiac muscle, glands, and adipose tissue (covered in Ch. 16).

Sensory Reception

• Sensory receptors monitor conditions inside the body or in the external environment.
• Sensory receptors, structurally are either:
  • Free nerve endings: cell processes (dendrites) of sensory neurons.
  • Encapsulated nerve endings: cell processes of sensory neurons in connective tissue.
  • Receptor cells: specialized cells monitored by sensory neurons.
• A stimulated sensory receptor generates a signal (action potential) that travels along the axon of a sensory neuron towards the CNS.
• General senses are provided by general sensory receptors located throughout the body (skin, skeletal muscles, tendons, visceral organs).
• Special senses are provided by special sensory receptors located in sense organs like the eye, ear, nose, and tongue.

Detection of Stimuli

• Receptor specificity means each receptor has a characteristic sensitivity (e.g., mechanical or chemical stimuli).
• Receptive field is the area monitored by a single sensory receptor
  • The larger the receptive field, the harder it is to localize a stimulus.
• Transduction converts an arriving stimulus into an action potential by a sensory receptor.

Adaptation

• Adaptation is a reduction in sensitivity to a constant but painless stimulus.
  • Receptors respond strongly at first, then activity gradually declines.
• Peripheral adaptation occurs in the PNS when the level of receptor activity changes.
  • Phasic receptors (fast-adapting) are normally inactive, but respond strongly to a change before activity decreases (e.g., noticing a smell initially then not).
  • Tonic receptors (slow-adapting) are always active and show little peripheral adaptation (e.g., pain receptors).
• Central adaptation involves inhibition of nuclei along sensory pathways
  • For example, smelling perfume when applied but barely noticing 30 minutes later.

Sensation vs. Perception

• Sensation is the arrival of sensory information at the primary somatosensory cortex.
• Perception is conscious awareness of a sensation, produced when information is sent from the primary somatosensory cortex to the somatosensory association area.

Sensory Receptors

• Modalities refer to the types of stimuli (e.g., touch, smell, pressure, light).
  • The CNS interprets stimuli based on the labeled line it takes.
  • A labeled line is the pathway between a sensory receptor and a neuron in the cerebral cortex, carrying information about only one modality.
• Different strengths, durations, or variations are perceived as the CNS interprets the frequency and pattern of action potentials, not the labeled line over which it travels.
• General Sensory Receptors are classified by stimulus location
  • Exteroceptors provide information about the external environment.
  • Proprioceptors report positions of skeletal muscles and joints.
  • Interoceptors monitor visceral organs and functions.
• Receptors can be classified by the nature of the stimulus:

1. Nociceptors (pain)
   • Common in skin, joint capsules, periosteum, and blood vessel walls.
   • Sensitive to temperature extremes, mechanical damage, and chemicals released by injured cells.
   • Large receptor fields make it difficult to pinpoint the source of pain.

• Free nerve endings are the dendrites of sensory neurons.
• Two types of axons carry information:

1. Type A fibers (fastest) - Carry sensations of fast pain (prickling) - Stimulation triggers withdrawal reflex.
2. Type C fibers (slowest) - Carry sensations of slow pain (burning, aching) - Cause generalized activation of reticular formation and thalamus, allowing general awareness.
3. Thermoreceptors (temperature)

• Called temperature receptors
• Free nerve endings in dermis, skeletal muscles, liver, and hypothalamus.
• Travel the same pathways as pain.

3. Chemoreceptors (chemical concentrations)

• Sensitive to chemicals dissolved in surrounding fluids with chemically gated ion channels responding to water and lipid-soluble chemicals.
• Monitor pH, carbon dioxide, and oxygen levels in arterial blood and are located in:

1. Carotid bodies: near internal carotid arteries.
2. Aortic bodies: between major branches of aortic arch.
3. Medulla oblongata.

• Olfactory and gustatory receptors are also chemoreceptors.

4. Mechanoreceptors (physical distortion)
   • Sensitive to stimuli that distort plasma membranes.

• Contain mechanically gated ion channels that open in response to stretching, compression, twisting, or other distortions.
• Classes of mechanoreceptors include:

1. Tactile receptors - Commonly found in the skin and may be found in other areas of the body. - Provide sensations of touch, pressure, and vibration. - May detect fine or crude touch, pressure.
   • Fine Touch and Pressure Receptors have narrow receptive fields for excellent localization and give detailed information shape, size, texture, and movement. - Crude Touch and Pressure Receptors have large receptive fields for poor localization and don't provide much detail -There are 6 types of tactile receptors 1. Free Nerve Endings
     • Located between cells of the epidermis for the sensation touch and/or pressure
   2. Root Hair Plexus
      • Monitors distortions to the hair follicle caused by hair movements
      • Located in the dermis, wrapped around a hair follicle
   3. Tactile Discs (Merkel discs)
      • Sensitive to shape and texture
      • It is formed by a Tactile cell that associates with a free nerve ending
      • Located in the stratum basale of the epidermis
   4. Tactile corpuscles (Meissner's corpuscles)
      • Perceive sensations of fine touch, pressure, and low-frequency vibration
      • Located in the dermal papillae of the skin of the eyelids, lips, fingertips, nipples, and external genitalia.
   5. Lamellar corpuscles (Pacinian corpuscles)
      • Sensitive to deep pressure and vibrating stimuli
      • Located in the reticular dermis, subcutaneous layer, and within some visceral organs E. Bulbous corpuscles (Ruffini corpuscle) -Sensitive to pressure and distortion of the skin -Located in the reticular (deep) dermis
2. Baroreceptors -Consist of free nerve endings that branch within elastic tissues -Detect pressure changes in distensible organs:
   • walls of blood vessels (aorta & common carotid arteries) -portions of the digestive, respiratory, and urinary tracts
3. Proprioceptors

• Three Major Groups of Proprioceptors - Muscle Spindles - Monitors the state of a muscular contraction -Monitor skeletal muscle length and trigger stretch reflexes - Receptors in Joint Capsules - Monitor the position of the joint - Detects pressure, tension, and movement at a joint - Golgi Tendon Organs -Monitor external tension developed within a tendon during a muscle contraction - Not in visceral organs

Sensory Pathways

• Sensory pathways consist of a series of three neurons:
• First-order neuron: sensory neuron from the sensory receptor to the spinal cord or brain stem.
• Second-order neuron: interneuron extending from the spinal cord or brain stem to the thalamus.
• Third-order neuron: interneuron extending from the thalamus to the cerebral cortex (for awareness).

Somatic Sensory Pathways

• Most somatic sensory information relays to the thalamus for processing; only a small fraction (1%) reaches our awareness.
• Somatic sensory pathways carry information from the skin, muscles, and tendons of the body wall, head, neck, and limbs to the CNS.
• Major pathways:

1. Spinothalamic pathway
   • Provides sensations of poorly localized ("crude") touch, pressure, pain, and temperature.
   • Pathway: 1st order neuron (peripheral receptor to spinal cord), 2nd order neuron (spinal cord to thalamus), 3rd order neuron (thalamus to primary somatosensory cortex).
   • Abnormalities can cause pain perceived in locations where they were not produced. - Phantom limb syndrome: continued pain in amputated limb, with sensory and interneurons remaining active. - Referred pain: strong visceral sensations stimulate spinothalamic interneurons, causing visceral pain to manifest elsewhere.
2. Posterior column pathway
   • Carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception.
   • Pathway: 1st order neuron (peripheral receptor to medulla oblongata), 2nd order neuron (medulla oblongata to thalamus), 3rd order neuron (thalamus to primary somatosensory cortex).
3. Spinocerebellar pathway

• Cerebellum receives proprioceptive information about the position of muscles, tendons and joints. The pathway skips the primary somatosensory cortex.
• Sensory homunculus: functional map of the primary somatosensory cortex
  • The map looks distorted because the area of sensory cortex is proportional to the density of sensory neurons

Visceral Sensory Pathways

• Visceral sensory information is collected by interoceptors monitoring visceral tissues and organs in the thoracic and abdominopelvic cavities.
• Interoceptors may be nociceptors, thermoreceptors, tactile receptors, baroreceptors, or chemoreceptors.
• Visceral sensations are delivered to the solitary nucleus of the medulla oblongata.
• Major processing and sorting center for visceral sensory information
• Extensive connections with cardiovascular and respiratory centers

The Somatic Nervous System (SNS)

• The Somatic Nervous System (SNS) controls skeletal muscle with somatic motor pathways consisting of motor nuclei, tracts, and nerves that carry somatic motor commands from the brain.
• At least two motor neurons are always involved:
• Upper motor neuron: cell body in CNS processing center, synapses on lower motor neuron, its activity may facilitate or inhibit the lower motor neuron.
• Lower motor neuron: cell body in nucleus of brainstem or spinal cord, axon extends outside CNS, innervates a single "motor unit" in skeletal muscle to trigger contraction.
• Damage to a lower motor neuron eliminates voluntary and reflexive control over the innervated motor unit
• Carries conscious & subconscious (reflexive) motor commands to skeletal muscles
• Three major motor pathways:

1. Corticospinal pathway
   • Carries voluntary somatic motor commands to provide voluntary control over skeletal muscles to the spinal cord and then skeletal muscle. - Axons descend into brainstem and spinal cord and synapse on lower motor neurons that control skeletal muscles
     • Pyramidal system: upper motor neuron is a pyramidal cell of the primary motor cortex, and decussation (cross-over) occurs at the pyramids.
   • Motor homunculus: functional map of the primary motor cortex (distorted human for degree of motor control available).
2. Medial pathway
   • Carries subconscious somatic motor commands to provide adjustments to skeletal muscles.

• Helps control gross movements of trunk/proximal limb muscles.

3. Lateral pathway
   • Carries subconscious somatic motor commands to provide adjustments to skeletal muscles
   • Helps control distal limb muscles for precise movements.

• The medial and lateral pathways operate subconsciously to modify and direct skeletal muscle contractions by stimulating, facilitating, or inhibiting lower motor neurons controlled by the corticospinal pathway