Spinal Cord | Reflex Action | | Human Anatomy And Physiology - 2 | B Pharma 2nd Semester
This lecture covers the spinal cord as part of the Central Nervous System, including its structure, vertebral column anatomy, internal anatomy (grey and white matter), and reflex actions. The instructor explains how the spinal cord extends from the medulla oblongata to the L2 vertebra and plays a critical role in reflex activities. The session concludes Unit 1 of Human Anatomy and Physiology for B.Pharma 2nd semester students.
Summary
The lecture begins by placing the spinal cord within the broader nervous system, identifying it as the second major organ of the Central Nervous System (CNS) alongside the brain. The spinal cord is described as a long, tube-like structure extending from the medulla oblongata of the brain down to the L1-L2 lumbar vertebrae. Its length is approximately 45 cm in males and 43 cm in females, with a weight of about 35 grams.
The vertebral column (backbone) is discussed in detail as the protective housing for the spinal cord. It consists of 33 bones before age 20, divided into cervical (7), thoracic (12), lumbar (5), sacral (5), and coccyx (4) vertebrae. In adults, the sacral and coccyx bones fuse, reducing the total count to 26 vertebrae. The spinal cord itself terminates at L2, even though the vertebral column continues further.
The protective coverings of the spinal cord are explained through the concept of meninges — three tissue layers: the innermost Pia Mater, the middle Arachnoid Mater, and the outermost Dura Mater. These meninges, along with the bones of the vertebral column, provide dual-layer protection to this sensitive structure.
The spinal cord is divided into 31 segments, from which 31 pairs of spinal nerves arise. A notable distinction is made for the cervical region: while there are only 7 cervical vertebrae, there are 8 cervical nerves. The nerves from sacral and coccygeal segments extend beyond the termination point of the spinal cord as a bundle called the cauda equina.
Two enlargements of the spinal cord are identified: the superior (cervical) enlargement from C4 to T1, and the inferior (lumbar) enlargement from T9 to T12, where the cord appears slightly wider due to increased nerve density.
The internal anatomy section explains the cross-sectional structure of the spinal cord. The center contains grey matter (composed of neuron cell bodies) shaped like horns — dorsal horns and ventral horns. The surrounding white matter is composed of axons covered in myelin sheaths, which are white due to the lipid nature of myelin. A central canal runs through the middle of the grey matter.
Signal flow is explained: sensory information enters through the dorsal horns, travels to the brain, and motor commands exit through the ventral horns back to effector organs via motor neurons.
The final section covers reflex activity, defined as an involuntary, immediate response of voluntary muscles to a specific stimulus. The instructor uses relatable examples such as pulling one's hand away from steam or a hot surface. The reflex arc involves five components: receptor, sensory neuron, control center (spinal cord), motor neuron, and effector (muscle). Crucially, in spinal reflexes, the signal does not travel to the brain — the spinal cord processes and responds independently, enabling extremely fast reactions. If the brain controls the reflex, it is called a cranial reflex; if the spinal cord controls it, it is a spinal reflex.
Key Insights
- The instructor explains that the spinal cord terminates at the L2 lumbar vertebra, even though the vertebral column continues further, and the nerves from sacral and coccygeal segments extend beyond this point as a tail-like bundle from the cord's terminal end.
- The instructor points out a key distinction: while there are only 7 cervical vertebrae, there are 8 cervical spinal nerves, because the first nerve exits above the first vertebra, creating an extra nerve-bone pairing unique to the cervical region.
- The instructor argues that in reflex activity, the spinal cord bypasses the brain entirely and generates a response on its own — this is because sending signals all the way to the brain and back would introduce delay, and reflex actions must occur almost instantaneously.
- The instructor explains that white matter appears white because the axons are covered in myelin sheaths, which are lipid in nature, and lipids are generally white in color — making the axonal regions of the spinal cord visibly distinct from the grey neuron cell body regions.
- The instructor clarifies that reflex activity is defined as an involuntary response of voluntary muscles — meaning the skeletal muscles, which are normally under conscious control, act automatically without deliberate thought in response to specific stimuli.
Topics
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