Lecture 18 - Sensory Pathways and the Somatic Nervous System

What are the two types of sensory receptors?

1. Exteroceptors

2. Proprioceptors

Exteroceptors

Provides information about the EXTERNAL environment

Proprioceptors

A type of exteroceptor that reports POSITION of the skeletal muscles and joints

Somatic sensory receptors

Monitors the skeletal muscles, joints, and skin surface. They provide info about the general sensations and position

What are the general sensations and the receptors attached to them?

1. Position → Proprioceptors

2. Pain → Nociceptors

3. Temperature → Thermoreceptors

4. Touch, pressure, vibration → Tactile receptors and mechanoreceptors

Sensory receptors

Specialized cells that monitor specific conditions in the body or external environment

• Passes info to the CNS when stimulated in the form of action potentials along the axon of a neuron

Sensory pathways

Delivers somatic and visceral sensory information to their final destinations inside the cell using nerves, nuclei, and tracts

Nerves

An axon

Nuclei

A group of cell bodies

Tracts

Groups of axons

Describe the sensory pathway

1. Depolarization of receptors

2. Action potential is generated

3. Propagation

4. CNS processing

   1. Immediate involuntary response

   2. Perception

      1. Voluntary response

Sensation

Arriving information from the senses

Perception

Conscious awareness of a sensation

Depolarization of the receptor

A stimulus produces a graded change in the membrane potential of a receptor cell

Action potential generation

IF the stimulus depolarizes the receptor cell to threshold, then an action potential develops in the initial segment

Propagation

Axons of sensory neurons carry info about the type of stimulus as action potentials to the CNS

CNS processing

Information processing occurs at every relay synapse. Sensory info may be distributed to MULTIPLE nuclei and centers in the spinal cord and brain

Immediate involuntary response

CNS may direct an immediate reflex response before sensations reach cerebral cortex

Perception

Only about 1% of arriving sensations are relayed to the primary sensory cortex

Voluntary response

A response that can moderate, enhance or supplement the relatively simple involuntary reflexive response

First order neurons

Sensory neurons that deliver sensations to the CNS. Located in the dorsal root ganglion or cranial nerve ganglion

Second order neuron

Axon of the sensory neuron synapses of an interneuron in the CNS. Located in the spinal cord or the brain stem

Third order neuron

Picks up neural impulse and carries them to cerebral cortex

What are the three major somatic sensory pathways?

1. Posterior column pathway

2. Spinocerebellar pathway

3. Spinothalamic pathway

Spinal tracts of the posterior column pathway

1. Fasciculus gracilis

2. Fasciculus Cuneatus

Spinal tracts of the spinocerebellar pathway

1. Posterior spinocerebellar tract

2. Anterior spinocerebellar tract

Spinal tracts of the spinothalamic pathway

1. Lateral spinothalamic tract

2. Anterior spinothalamic tract

Posterior column pathway

Carries sensations of highly localized “fine” touch, pressure, vibration, and conscious proprioception

What does determined when processing occurs in the thalamus?

1. Determines WHAT the sensation is

2. Determines WHERE on the body the sensation is located

Projection fibers

Fibers in the brain that serves as the destination for conscious sensory pathways

Sensory homunculus

A functional map of primary sensory cortex

Why do distortions occur in the sensory homunculus?

The area of sensory cortex is devoted to particular body region is proportional to the AMOUNT of sensory receptors and NOT the surface area

Spinothalamic pathway

Provides conscious sensations of poorly localized

Anterior tracts

(Of the spinothalamic pathway) Carries crude touch and pressure sensations

Lateral tracts

(Of the spinothalamic pathway) Carries pain and temperature sensations

Spinocerebellar pathway

Unconscious cerebellum receives proprioceptive information about the position of skeletal muscle, tendons, and joints

Posterior Spinocerebellar Tracts

Contains second order axons that DO NOT cross over to the opposite side of the spinal cord

• Axons reach cerebellar cortex via inferior cerebellar peduncles of that side

Anterior Spinocerebellar Tracts

Dominated by second order axons that have crossed over to the opposite side of spinal cord

• Many uncrossed axons are present

• Crossed axons will likely cross again

*Sensations never reach cerebral cortex

Interoceptors

Monitors visceral organs and functions

Mechanoreceptors

With mechanically gated ion channels

Baroceptors

Detects pressure changes

Chemoreceptors

Monitors pH, carbon dioxide, and oxygen levels