Question

Physiology question

Spinal cord reflexes are important physiological mechanisms for maintaining body homeostasis under certain situations. Describe what a spinal cord reflex is and how it arises. As an example, explain how reflexes are involved in the central pattern generation of locomotion.

Answer 1

The spinal cord, along with the brain, makes up the central nervous system. It resembles a thick, cream-colored rope and is made up of nerves that relay messages between the brain and the rest of the body. It stretches from the medulla oblongata, at the base of the brain, to the lower back, and is housed in a tunnel made by the vertebrae, or bones of the spinal column. All vertebrate animals have spinal cords, from simple jawless fish to complex birds and mammals.

Functions

The spinal cord works a bit like a telephone switchboard operator, helping the brain communicate with different parts of the body, and vice versa. Its three major roles are:

• To relay messages from the brain to different parts of the body (usually a muscle) in order to perform an action
• To pass along messages from sensory receptors (found all over the body) to the brain
• To coordinate reflexes (quick responses to outside stimuli) that don't go through the brain and are managed by the spinal cord alone

General Characteristics

In an adult human, the spinal cord measures about 44 cm, or 17-1/4 inches, in length and is as wide as a thumb at the top and as thin as a drinking straw at the bottom. The thick bundle of nerves are protected by three layers of membrane called meninges, like those surrounding the brain. The bundle resembles a rough jute rope with a thick hot dog casing around it. Between the nerve bundle core and the meninges, there is also cerebrospinal fluid for added cushioning. The spinal cord ends in a cascade of nerves resembling a horse's tail, which is why this part is called the cauda equina.

Anatomy

The cord is organized into five major regions consisting of a total of 33 segments (two of these segments are fused, so it is usually described as having 31 segments). Each segment contains nerves connected to different parts of the body.

• The cervical region is connected to the head, neck, upper body, arms, and hands
• The thoracic region is connected to the hands, fingers, chest, and abdominal muscles
• The lumbar region is connected to the hips, knee, ankles, and toe muscles
• The sacral region is connected to the legs, toes, bladder, and anal muscles
• The coccygeal region is connected to the skin around the coccyx

The cross section of the spinal cord looks like a taffy candy with a butterfly in the middle. The central core contains grey matter ,the bodies and dendrites of the neurons in the bundle, and is surrounded by white matter, the neuron axons. Each segment has a pair of spinal nerves coming out of it, and each of these nerves has two roots. The dorsal root, in the back of the spinal cord, carries sensory messages from the body to the brain so you can detect things like touch, smells, pain, or temperature The ventral root, in the front of the spinal cord, carries motor messages from the brain to the body, thus controlling the different muscles of the body.

Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information. General principles of the organization of these circuits and their control by higher brain centers have come from the study of smaller circuits found in invertebrates. Recent work on vertebrates highlights the importance of neuro-modulatory control pathways in enabling spinal cord and brain stem circuits to generate meaningful motor patterns. Because rhythmic motor patterns are easily quantified and studied, central pattern generators will provide important testing grounds for understanding the effects of numerous genetic mutations on behavior. Moreover, further understanding of the modulation of spinal cord circuitry used in rhythmic behaviors should facilitate the development of new treatments to enhance recovery after spinal cord damage.

Central pattern generators are complex structures for which many of the cellular elements have not yet been unraveled. Nonetheless, compelling evidence supports key roles in controlling biological rhythms such as locomotion in most if not all vertebrate species. Several neurological conditions and disorders displaying abnormal rhythmic and locomotor-like movements have been presumably associated with dysfunctional CPG activity. Although the causes are often complex and still incompletely understood in most cases, several research avenues have already been pursued (e.g., RLS, myoclonus) and could still be explored (UTS) to restore normal CPG activity and corresponding bipedal walking capabilities. The CPG for locomotion has clearly been shown to be both flexible and adaptable providing hope for the development in a near future of safe therapeutic approaches capable of re-establishing normal CPG activity.

The existence of spinal locomotor CPGs in animals has been established beyond reasonable doubt, but the relative importance of CPG activity in the control of human locomotion remains to be elucidated. Accumulating physiological and behavioral evidence that adaptive processes can occur within the spinal cord has challenged the dogma that the spinal cord is a relatively nonplastic, hardwired conduit for relaying supraspinal commands. It has become clear, however, that in the intact nervous system, CPGs do not operate in a vacuum but depend on the interplay of information between the brain and spinal cord, with the final motor output shaped by sensory feedback from peripheral receptors and reconfigured by neuromodulators. Further research at each level of interaction, from molecular, cellular, and intercellular to behavioral, will inform the other levels, and, one by one, the mysteries of animal and human locomotion will be solved.