Neural Integration and Sensory Pathways

Exam & Schedule

• Most people did well on the previous exam, including a question about action potentials being all or none.

• Remaining points distribution:

  • Last assignment: 15 points

  • Lab exam: Approximately 50 points

  • Final lecture exam: Approximately 50 points

• Two worksheets are available, one on neural integration and another as a general overview for the final exam.

Neural Integration and Sensory Pathways

• Neural integration involves combining various elements of the nervous system.

• Focus on sensory pathways:

  • Visceral senses (internal organs)

  • External senses (skin, muscles, tendons)

• Sensory signals travel along shared pathways to the central nervous system.

• Motor pathways are divided into:

  • Somatic nervous system (skeletal muscles)

  • Autonomic nervous system (cardiac muscle, smooth muscle, glands)

Sensory and Motor Pathway Events

• Arriving stimulus triggers action potentials.

• If stimulus is strong enough to reach the central nervous system's attention, it generates action potentials.

• Sensory signals report on:

  • Internal organs: pressure changes, chemical changes, pain (less than skin)

  • Skin/joints: body position, muscle/joint position, pain, pressure, temperature, light touch

• If stimulus reaches threshold, action potential is generated.

• Response may be a reflex or voluntary, such as skeletal muscle movement.

Sensory Receptors

• Specialized cells or dendrites of sensory neurons.

• Stimulated by various stimuli.

• Types of Senses:

  • General Senses

  • Special Senses

    • Sight (vision)

    • Taste (gustation)

    • Smell

    • Hearing

    • Balance

• Special senses receptors are located in specific body parts:

  • Visual receptors in the eyeball.

  • Hearing receptors in the inner ear.

  • Taste receptors in the tongue and throat.

  • Smell receptors in the nose.

• Organs containing special sense receptors are designed to get the stimulus to the receptors.

Protection of Special Sense Receptors

• Special sense receptors are often embedded in bone for protection.

• Taste and smell receptors can regenerate, but others in the central nervous system cannot.

• Damage to vision receptors can lead to blindness, as these neurons don't regenerate.

• General senses are part of the peripheral nervous system and can repair.

General Senses

• Receptors located all over the body.

• Include:

  • Proprioception (body position)

  • Touch

  • Temperature

  • Pain

  • Pressure

  • Vibration

• Internal receptors provide information to the central nervous system about internal conditions.

Receptor Types and Stimuli

• Receptor structure dictates function.

• Receptor types:

  • Mechanoreceptors: Respond to physical deflection or movement of the cell membrane of the receptor.

    • $action potential \propto movement$

  • Proprioceptors: Body position.

  • Chemoreceptors: Chemicals binding to receptors which open the sodium gates and trigger an action potential.

  • Nociceptors: Pain.

  • Baroreceptors: Pressure. Function like a barometer.

• Stimuli are translated into action potentials for the central nervous system.

• Sensations are felt in the primary sensory cortex, not at the point of stimulus.

• Example: Referred pain occurs because internal organ pain receptors have large receptive fields.

Receptive Fields

• Each part of the skin/tissue is monitored by a receptor.

• Receptive field: area monitored by a single receptor cell.

• Multiple touches within the same receptive field are felt as one touch.

• Smaller receptive fields allow for more precise localization of stimuli.

• Larger receptive fields make it harder to pinpoint sensation location.

• Tongue and fingertips have small receptive fields, enabling Braille reading.

Adaptation

• Reduction in sensitivity in the presence of a constant stimulus.

• Peripheral nervous system: Receptor activity changes with fast adapting receptors (e.g., temperature receptors).

• Central adaptation: Inhibition along the sensory nervous system (e.g., smell).

• Peripheral adaptation occurs at the receptor level.

• Central adaptation involves action potential blockage along the pathway to the brain.

General Sensory Receptors

• Scattered throughout the body and relatively simple in structure.

• Named for the type of stimulus that generates the action potential:

  • Nociceptors: Respond to pain.

  • Thermoreceptors: Respond to temperature.

  • Mechanoreceptors: Respond to mechanical force/movement of the cell membrane.

  • Chemoreceptors: Respond to chemicals.

  • Baroreceptors: Respond to pressure changes.

  • Proprioceptors: Respond to body position.

• Skin receptors are mostly mechanoreceptors that send different signals.

  • Free nerve endings are very sensitive.

  • Root hair sensory neurons respond to hair movement.

Baroreceptors and Chemoreceptors

• Baroreceptors:

  • Respond to pressure.

  • Provide information about blood pressure, lung expansion, digestive tract pressure, and bladder expansion.

• Chemoreceptors:

  • Respond to chemicals.

  • Internal organs: Monitor carbon dioxide, pH, and oxygen levels in the brainstem, heart, and aorta.

  • Important for blood gas monitoring and maintaining blood pH.

Sensory Pathway Organization

• Sensory signals travel through a series of neurons.

• Terminology varies depending on the specific nervous system branch.

• General Senses Pathway:

  • First-order neuron: From peripheral tissue to spinal cord.

  • Second-order neuron: Interneuron within the central nervous system.

  • Third-order neuron: From thalamus to the primary sensory cortex of the parietal lobe in the cerebrum.

• Action potential starts in the sensory neuron and heads to the central nervous system.

Neural Pathways and Decussation

• Action potential goes to central nervous system via 1st order neuron. May synapse on motor neuron for reflexes.

• For awareness (e.g., mosquito on hand):

  • Action potential goes to the brain.

  • Synapses on a second-order neuron (interneuron).

  • Second order neuron synapses on third order neuron. Third order neuron goes to the primary sensory cortex of the parietal lobe of cerebral.

• Thalmus relays with 3rd order neuron to the Cerebral in very specific locations for awareness.

• One of the neurons will cross over to opposite side.

  • This is called decussation.

  • Left side of the brain controls the right side of the body vice versa.

  • Losing neurons on one side causes loss of function on the opposite side.

Sensory Pathway Details

• Action potential starts in the first-order neuron in the spinal nerve.

• Sensory neuron splits away and goes to the dorsal (posterior) root of the spinal cord.

• First order neuron synapses (cell body) on a second-order neuron in the gray horn then the second order neuron crosses to opposite of the spinal cord via decussation.

• Action potential then travels up to the brainstem. First the Medulla then midbrain before going to the Thalamus.

• Third order neuron ends at the Primary Sensory Cortex.

Sensory Pathways and Homunculus

• Specific pathways for different general senses exist.

• Homunculus: Representation of the body in the primary sensory and motor cortex based on neuron density.

  • Body parts are sized according to the amount of cortical space devoted to them.

  • Fingertips have small receptive fields and a large representation in the cortex.

  • Also known as the "little man"

• Frontal and Parietal Lobes are divided by the Central Sulcus.

Somatic Sensory Pathways

• Sensory information from skin, skeletal muscles, and proprioception.

• Include the spinothalamic, posterior column, and spinal cerebellar pathways.

• If a sensation reaches awareness, it involves a third-order neuron to the primary sensory cortex.

• Many general senses do not reach the cerebrum; some end in the brainstem or hypothalamus.

• Cerebellum monitors body position, so sensory signals for the same are getting conducted there where the cerebellum is processing them (ie keeping consistent posture).

  • These actions occur without 3rd order neuron.

Efferent Division of Somatic Nervous System

• Efferent: Motor pathways from the central nervous system.

• Afferent: Sensory pathways to the central nervous system.

• The efferent division controls skeletal muscle contractions.

• Motor pathways from the central nervous system.

• Upper motor neuron, lower motor neuron.

  • Upper motor neurons start in the primary motor cortex (frontal lobe) and synapse on the lower motor neuron, goes out the ventral branch, the anterior gray horn and the ventral branch of the spinal nerve.

  • Lower motor neurons go out to their muscles.

• Upper motor can block activation of the lower motor neuron.

• Once Cocholine is released the action potential will be started, muscle contraction will occur.

• Cerebellum monitors movements and adjust activities of the upper motor if necessary. creates neural loops to get more proficient with practice.