major characteristics of the somatic sensory and motor pathways

posterior column-medial lemniscus pathway

- the pathway for nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head to ascend to the cerebral cortex. (The name of the pathway comes from the names of two white‐matter tracts that convey the impulses: the posterior column of the spinal cord and the medial lemniscus of the brain stem.)

somatic motor pathways

-Neurons in four distinct but highly interactive neural circuits. -participate in control of movement by providing input to lower motor neurons

somatic sensory and motor map

-Precise localization of somatic sensations occurs when nerve impulses arrive at the primary somatosensory area, which occupies the postcentral gyri of the parietal lobes of the cerebral cortex. -Each region in this area receives sensory input from a different part of the body. -The left cerebral hemisphere has a similar primary somatosensory area that receives sensory input from the right side of the body.

Anterolateral pathway ( spinothalamic pathway)

-Sensory pathway that conveys information related to pain, temperature, crude touch, pressure, tickle, and itch. -The first‐order neurons connect a receptor of the limbs, trunk, neck, or posterior head with the spinal cord

cerebral cortex & somatic sensory input and output

-Specific areas of the cerebral cortex receive somatic sensory input from particular parts of the body. - Other areas of the cerebral cortex provide output in the form of instructions for movement of particular parts of the body. -The somatic sensory map and the somatic motor map relate body parts to these cortical areas.

What is the pathway of the first order neurons on the anterolateral pathway?

-The cell bodies of first‐order neurons are in the posterior root ganglion. -The axon terminals of the first‐order neurons synapse with second‐order neurons, whose cell bodies are located in the posterior gray horn of the spinal cord. -The axons of the second‐order neurons cross to the opposite side of the spinal cord. -Then, they pass upward to the brain stem as the spinothalamic tract. -The axons of the second‐order neurons end in the ventral posterior nucleus of the thalamus, where they synapse with the third‐order neurons. -The axons of the third‐order neurons project to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.

medial lemniscus

-a thin ribbonlike projection tract that extends from the medulla to the ventral posterior nucleus of the thalamus. -The axons of the second‐order neurons cross to the opposite side of the medulla and enter the medial lemniscus. -In the thalamus, the axon terminals of second‐order neurons synapse with third‐order neurons, which project their axons to the primary somatosensory area of the cerebral cortex.

Cerebellar neurons

-aid movement by controlling the activity of upper motor neurons. -Neural circuits interconnect the cerebellum with motor areas of the cerebral cortex (via the thalamus) and the brain stem. -cerebrum coordinates body movements and helps maintain normal posture by issuing commands to upper motor neurons to reduce errors in movement.

Basal nuclei neurons

-assist movement by providing input to upper motor neurons. -Neural circuits interconnect the basal nuclei with motor areas of the cerebral cortex (via the thalamus) and the brain stem. -help initiate and terminate movements, suppress unwanted movements, and establish a normal level of muscle tone.

Somatic Sensory pathways: First‐order neurons

-conduct impulses from somatic receptors into the brain stem or spinal cord. -From the face, mouth, teeth, and eyes, somatic sensory impulses propagate along cranial nerves into the brain stem. -From the neck, trunk, limbs, and posterior aspect of the head, somatic sensory impulses propagate along spinal nerves into the spinal cord.

Somatic Sensory pathways: second‐order neurons

-conduct impulses from the brain stem and spinal cord to the thalamus. -Axons of second‐order neurons decussate (cross over to the opposite side) in the brain stem or spinal cord before ascending to the ventral posterior nucleus of the thalamus. -Thus, all somatic sensory information from one side of the body reaches the thalamus on the opposite side.

Somatic Sensory pathways: third-order neurons

-conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.

motor neurons

-extend out of the brain stem and spinal cord to innervate skeletal muscles in the body. -there are called lower motor neurons

Upper Motor Neurons (UMN)

-give out put to local circuit neurons and lower motor neurons. -synapse with local circuit neurons, which in turn synapse with lower motor neurons. (A few upper motor neurons synapse directly with lower motor neurons.) -UMNs from the cerebral cortex are essential for the execution of voluntary movements of the body. -Other UMNs originate in motor centers of the brain stem. -UMNs from the brain stem regulate muscle tone, control postural muscles, and help maintain balance and orientation of the head and body. -Both the basal nuclei and cerebellum exert influence on upper motor neurons.

lower motor neurons (LMNs)

-have their cell bodies in the brain stem and spinal cord. -From the brain stem, axons of LMNs extend through cranial nerves to innervate skeletal muscles of the face and head. -From the spinal cord, axons of LMNs extend through spinal nerves to innervate skeletal muscles of the limbs and trunk. -Only LMNs provide output from the CNS to skeletal muscle fibers. For this reason, they are also called the final common pathway.

Where are the cells bodies of these first order neurons located?

-in the posterior (dorsal) root ganglia of spinal nerves. -their axons form the posterior (dorsal) columns.

Somatic sensory pathways

-relay information from the somatic sensory receptors just described to the primary somatosensory area in the cerebral cortex and to the cerebellum.

Local circuit neurons

-the interneurons where input arrives at lower motor neurons. -located close to the lower motor neuron cell bodies in the brain stem and spinal cord. -receive input from somatic sensory receptors, such as nociceptors and muscle spindles, as well as from higher centers in the brain. -help coordinate rhythmic activity in specific muscle groups. (such as alternating flexion and extension of the lower limbs during walking)

What is the pathway of First‐order neurons in the posterior column-medial lemniscus pathway

-they extend from sensory receptors in the limbs, trunk, neck, and posterior head into the spinal cord and ascend to the medulla oblongata on the same side of the body.

What are the two parts of the posterior (dorsal) columns?

1. the gracile fasciculus: Nerve impulses for touch, pressure, and vibration from the lower limbs and lower trunk propagate along axons in the gracile fasciculus and arrive at the gracile nucleus. 2. the cuneate fasciculus: Nerve impulses for touch, pressure, vibration, and conscious proprioception from the upper limbs, upper trunk, neck, and posterior head propagate along axons in the cuneate fasciculus and arrive at the cuneate nucleus.

Somatic Sensory Pathways to the Cerebellum Two tracts in the spinal cord:

1. the posterior spinocerebellar tract. 2. the anterior spinocerebellar tract. -routes proprioceptive impulses take to reach the cerebellum. -not consciously perceived, these sensory impulses that travel these pathways are critical for posture, balance, and coordination of skilled movements.

review

Because lower motor neurons provide all output to skeletal muscles, they are called the final common pathway.

review

The anterolateral pathway conveys nerve impulses for pain, cold, warmth, itch, and tickle from the limbs, trunk, neck, and posterior head to the cerebral cortex. Kevin Somerville/Imagineering

How do the somatosensory and motor representations compare for the hand, and what does this difference imply?

The hand has a larger representation in the motor area than in the somatosensory area, which implies greater precision in the hand's movement control than fine ability in its sensation.

Which cranial nerve conveys impulses for most somatic sensations from the left side of the face into the pons?

The left trigeminal (V) nerve conveys nerve impulses for most somatic sensations from the left side of the face into the pons.

review

The posterior column-medial lemniscus pathway conveys nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head to the cerebral cortex

What are the two major tracts that form the posterior columns?

The posterior columns consist of the cuneate fasciculus and the gracile fasciculus.

The pathways to the cerebral cortex consist of thousands of sets of three neurons:

a first‐order neuron, a second‐order neuron, and a third‐order neuron.

The trigeminothalamic pathway

conveys nerve impulses for most somatic sensations (tactile, thermal, pain, and proprioceptive) from the face, nasal cavity, oral cavity, and teeth to the cerebral cortex