3 Major Somatic Sensory Pathways

1. The posterior column pathway

2. The spinothalamic (anterolateral) pathway

3. The spinocerebellar pathway

Posterior Column Pathway

• Carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception

Figure 15–5a

Crosses in medulla oblongata

Figure 15–5a, b

Sensory Homunculus• Functional map of the primary sensory cortex

– Where secondary neuron in thalamus projects to in the primary sensory cortex depends on where the corresponding sensory neuron innervates the body

Size of particular cortical area depends on number of sensory receptors in corresponding region of the body

The Spinothalamic Pathway

• Carry conscious sensations of poorly localized (“crude”) touch, pressure, pain, and temperature

• Crude touch and pressure ascend within the

anterior spinothalamic tracts

Figure 15–5c

• Pain and temperature sensations ascend within the

lateral spinothalamic tracts

The Spinothalamic Pathway

Referred pain • Visceral sensations arriving at a segment of

spinal cord can stimulate interneurons that are part of spinothalamic pathway

• Interneurons stimulate primary sensory cortex- feel pain in specific part of body surface.

The Spinocerebellar Pathway

• Proprioceptive information about position of skeletal muscles, tendons, and joints sent to cerebellum

Figure 15–7

posterior spinocerebellar tracts do not cross

Anterior spinocerebellar tracts do cross

Visceral Sensory Information

• interoceptors monitor visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities

• Not as numerous as in somatic tissues, include:– nociceptors– thermoreceptors– tactile receptors– baroreceptors– chemoreceptors

• Visceral sensory information carried through cranial nerves or spinothalamic tract to:

Solitary Nucleus • Large nucleus in the medulla oblongata

• Extensive connections with cardiovascular and respiratory centers and reticular formation

• Not connected to primary sensory cortex

Somatic Motor Pathways

• Always involve at least 2 motor neurons:– upper motor neuron:

• cell body lies in a CNS processing center• Innervates a single motor unit in a skeletal muscle:• activity in upper motor neuron may facilitate or inhibit

lower motor neuron

– lower motor neuron:• cell body lies in a nucleus of the brain stem or spinal

cord• Innervates muscle

3 Conscious and Subconscious Motor Pathways:

1) corticospinal pathway

2) medial pathway

3) lateral pathway

1

3

21

Corticospinal Pathway

Figure 15–9

• Also called the pyramidal system

• Voluntary (conscious) control of skeletal muscles:– begins at upper motor

neurons of primary motor cortex

– axons descend into brain stem and spinal cord to synapse on lower motor neurons

• 3 pairs of descending tracts:– coricobulbar tracts

– lateral corticospinal tracts

– anterior corticospinal tracts

Corticobulbar Tracts

• Conscious control over skeletal muscles that move eye, jaw, face, and some muscles of neck and pharynx

• Also innervate motor centers of medial and lateral motor pathways (described later)

The Pyramids and Crossing Over

• Corticospinal tracts visible along ventral surface of medulla oblongata as pair of thick bands, the pyramids

• lateral corticospinal tracts; cross at the pyramidal decussation

• anterior corticospinal tracts; cross to opposite side of spinal cord in anterior white commissure

Rest of somatic muscles controlled by:

Motor Homunculus

Figure 15–9

Similar to sensory homunculus, but in primary motor cortex (pre-central gyrus)

medial pathway:• control muscle tone and gross movements of trunk and

proximal limb muscles

• Upper motor neurons in;– vestibular nuclei– superior and inferior colliculi– reticular formation

lateral pathway:• control muscle tone and distal limb muscles that perform

more precise movements

• upper motor neurons in red nuclei

Subconscious somatic motor commands through:

Vestibular Nuclei• Info from vestibulococlear nerve (VIII); receptors in

inner ear• Monitor position and movement of the head to

maintain posture and balance• Axons desend in vestibulospinal tracts

Superior and Inferior Colliculi

• Receive visual (superior) and auditory (inferior) info• Reflexes in head, neck, upper limbs to visual and

auditory stimuli• Axons descend in tectospinal tracts

Reticular Formation

• Input from all pathways• Controls many reflexes• Axons descend in reticulospinal tracts

Lateral Pathway• Upper motor neurons in red nuclei in mesencephalon• Not as significant as lateral corticospinal tracts• axons descend in rubrospinal tracts

Cerebellum• As movements are under way, monitors:

– proprioceptive info– visual info from eyes– vestibular (balance) info from inner ear

Feedback

Basal Nuclei• Provide background patterns of movement

involved in voluntary motor activities

Coordinate reflexes with voluntary control

Integrative Centers in the Brain

• As we move from medulla oblongata to primary motor cortex, motor patterns become increasingly complex and variable