Chapter 15: Neuronal Integration I: Sensory Pathways and the Somatic Nervous System

general senses

used to describe sensitivity to: temperature, pain, touch, pressure, vibration and proprioception

four basic types of receptors

1. nociceptors

2. thermoreceptors

3. mechanoreceptors

4. chemoreceptors

nociceptors

pain receptors = found everywhere

• free nerve endings with large receptive fields

thermoreceptors

temperature receptors

• free nerve endings found in dermis, skeletal muscles, liver and hypothalamus

mechanoreceptors and types

stimulus occurs when plasma membrane is distorted

1. tactile receptors

2. baroreceptors

3. proprioceptors

tactile receptors

touch, pressure, and vibration

• found in skin

• very sensitive and have narrow receptive fields

baroreceptors

pressure changes

• found in blood vessels and lungs, digestive, reproductive and urinary tracts

proprioceptors

position of joints - state of muscle contraction

• tension in tendons and ligaments

• state of muscular contraction

chemoreceptors

detect changes in concentration of particular chemicals or compounds

decussation

most pathways cross from one side of the CNS to the other at same point

relay

most pathways have two or more neurons relaying the signal from the PNS to CNS

symmetry

all pathways are paired symmetrically. one member of each pair on separate sides of the spinal cord

sensory pathway - basic plan

• stimulus applied

• detected by receptor - monitors specific conditions in body or external environment

• first order neuron - cell body of this sensory neuron is in the dorsal root ganglion

• second order neuron - interneuron in the spinal cord or brain

  • crosses - this must occur if sensation is to reach our awareness, occurs somewhere along its length

• third order neuron - thalamus

• neuron in primary sensory area - postcentral gyrus and cerebral cortex

• conscious perception of stimulus

posterior column pathway

carries sensations of highly localized touch, pressure, vibration, and proprioception

• 1st order neuron - ascends through posterior column and synapses in medulla oblongata

• 2nd order neuron - ascends through medulla oblongata, crosses over, and synapses on thalamus

• 3rd order neuron - travels to post-central gyrus

anterior spinothalamic pathway

carries sensations of poorly localized touch and pressure

• 1st order neuron - synapses within posterior gray horn

• 2nd order neuron - crosses over and ascends through anterior column to synapse in thalamus

• 3rd order neuron - travels to post-central gyrus

lateral spinothalamic pathway

carries poorly localized sensations of pain and temperature

• 1st order neuron - synapses within posterior gray horn

• 2nd order neuron - crosses over and ascends through lateral column to synapse in thalamus

• 3rd order neuron - travels to post-central gyrus

types of pain

somatic pain and visceral pain

somatic pain

pain associated with muscle and skin

visceral pain

pain associated with organs

• often poorly localized - may be felt in areas not commonly associated with that organ - referred pain

causes

multiple sensory neurons converge on a single ascending tract

• brain is unable to distinguish visceral signal from somatic signal

locations

• liver and gallbladder - shoulder

• heart - arm

• stomach - upper abdomen

• small intestine - middle abdomen

• appendix - upper right lower abdomen

• colon - lower abdomen

• ureters - groin

somatic

voluntary control over skeletal muscles

descending tracts

only two are involved

• upper motor neuron

• lower motor neuron

upper motor neuron

cell body lie in pyramidal cells of the primary cortex

lower motor neuron

cell bodies are located in motor nucleus of brain stem or spinal cord = trigger muscle contraction

motor pathway

upper motor neuron always goes to lower motor neuron

lower motor neuron always goes to effector

crossing over occurs in medulla oblongata

1st order neuron - upper MN

2nd order neuron - lower MN

pyramidal cells of the primary motor cortex