Functions and ascending tract of spinal cord
- 1. FUNCTION OF THE SPINAL CORD AND ASCENDING TRACT FATIMA WAHID MANGRIO
- 2. FUNCTIONS OF THE SPINAL CORD AND SPINAL NERVES 1. The white matter of the spinal cord contains sensory and motor tracts, the “highways” for conduction of sensory nerve impulses toward the brain and motor nerve impulses from the brain toward effector tissues. 2. The spinal cord gray matter is a site for integration (summing) of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). 3. Spinal nerves and the nerves that branch from them connect the CNS to the sensory receptors, muscles, and glands in all parts of the body.
- 5. 1 Sensory receptors detect a sensory stimulus. 2 Sensory neurons convey this sensory input from sensory receptors into the spinal nerve and then into the posterior root. From the posterior root, axons of sensory neurons may proceed along three possible paths . MECHANISM OF SENSORY INPUT AND MOTOR OUTPUT BY THE SPINAL CORD
- 6. 3 Axons of sensory neurons may extend into the white matter of the spinal cord and ascend to the brain as part of a sensory tract. 4 Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons whose axons extend into the white matter of the spinal cord and then ascend to the brain as part of a sensory tract. 5 Axons of sensory neurons may enter the posterior gray horn and synapse with interneurons that in turn synapse with somatic motor neurons that are involved in spinal reflex pathways.
- 7. The pathways to the cerebral cortex consist of thousands of sets of three neurons: ASCENDING TRACTS OF THE CORD
- 8. 1. 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.
- 9. 2. Second-order neurons Conduct impulses from the brain stem and spinal cord to the thalamus. Axons of second- order neurons decussate 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.
- 10. 3. Third-order neurons conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.
- 11. Somatic sensory impulses ascend to the cerebral cortex via three general pathways: (1) the posterior column–medial lemniscus pathway, (2) the anterolateral (spinothalamic) pathway, and (3) the trigeminothalamic pathway. Somatic sensory impulses reach the cerebellum via the spinocerebellar tracts.
- 12. Posterior Column–Medial Lemniscus Pathway to the Cortex Nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head ascend to the cerebral cortex along the posterior column–medial lemniscus pathway. 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.
- 13. First-order neurons in the posterior column–medial lemniscus pathway 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. The cell bodies of these first-order neurons are in the posterior (dorsal) root ganglia of spinal nerves. In the spinal cord, their axons form the posterior (dorsal) columns, which consist of two parts: the gracile fasciculus and the cuneate fasciculus .
- 14. The axons synapse with the dendrites of second-order neurons whose cell bodies are located in the gracile nucleus or cuneate nucleus of the medulla. 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. 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.
- 15. The axons of the second-order neurons cross to the opposite side of the medulla and enter the medial lemniscus, a thin ribbon like projection tract that extends from the medulla to the ventral posterior nucleus of the thalamus. 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.
- 17. Anterolateral Pathway to the Cortex Nerve impulses for pain, temperature, itch, and tickle from the limbs,trunk, neck, and posterior head ascend to the cerebral cortex along the anterolateral (spinothalamic) pathway. Like the posterior column–medial lemniscus pathway, the anterolateral pathway is composed of three- neuron sets.
- 18. The first-order neurons connect a receptor of the limbs, trunk, neck, or posterior head with the spinal cord. The cell bodies of the 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.
- 19. 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 thirdorder neurons project to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.
- 21. Trigeminothalamic Pathway to the Cortex Nerve impulses for most somatic sensations (tactile, thermal, and pain) from the face, nasal cavity, oral cavity, and teeth ascend to the cerebral cortex along the trigeminothalamic pathway. Like the other somatosensory pathways just described, the trigeminothalamic pathway consists of threeneuron sets First-order neurons extend from somatic sensory receptors in the face, nasal cavity, oral cavity, and teeth into the pons through the trigeminal (V) nerves.
- 22. The cell bodies of these first-order neurons are in the trigeminal ganglion. The axon terminals of some first-order neurons synapse with second-order neurons in the pons. The axons of other first order neurons descend into the medulla to synapse with second order neurons.
- 23. The axons of the second-order neurons cross to the opposite side of the pons and medulla and then ascend as the trigeminothalamic tract to the ventral posterior nucleus of the thalamus. In the thalamus, the axon terminals of the second order neurons synapse with third-order neurons, which project their axons to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.