Skeletal muscle contraction:
Electron microscopy and biochemical analysis show that in muscle fibres cell bends are due to the placement of muscle protein action and myosin with myofibrils. Myosin occurs as thick filaments and actin as thin filaments. The lightest region of a myofibril contains only actin whereas the darkest region contains both.
The functional (contractile) unit of a myofibril is sarcomere each of which extends from one Z line to another z line. The actin filaments attach to the Z lines whereas myosin filaments do not.
When a sarcomere contracts the actin filament slide pas the myosin filaments as they approach one another. This process shortens the sarcomere. The combined decreases in length of the individual sarcomeres account for contraction of the whole muscle fibres and in turn, the whole muscle.
A ratchet mechanism between two filament types produces the actual contraction. Myosin contains globular projections that attach to actin as specific active binding sites, forming attachments called cross bridges. Once cross bridges form, they exert a force on thin actin filament and cause it to move.
Neuron and kinds of Neuron:
Neuron is functional unit of nervous system. Neurons are specialized to produce signals that can be communicated over short to long distances from one part of animals’ body to another. There are three neurons.
(1) Sensory neurons (receptor or afferent): Either act as receptors of stimuli themselves or are activated by receptors. Changes in the internal or external environments stimulate sensory nervous which respond by sending signals to the major integrating centers where information is processed.
(2) Inter neurons: It comprise the integrating centres and receive signals from sensory neurons and transmit them to motor neurons.
(3) Motor neurons (effector or efferent): It sends the processed information via a signal to the body’s effector muscles, causing them to contract or to glands causing them to secret.
Importance of Spinal Cord:
Spinal cord is the part of central nervous system that extends from the brain to near of into the tail. A cross section shows a neural canal that contains cerebro spinal fluid. The grey matter consists of cell bodies and dendrites and is concerned with reflex connections at various levels of spinal cord. Extending from spinal cord are ventral and dorsal roots of spinal nerves. These roots contain main motor and sensory fibres (axon or dendrites) that contribute to the major spinal nerves. White matter of spinal cord gets its name from the whitish myelin that covers it.
Function: Spinal cord is link between brain and most of body parts and is involved in reflex actions.
A reflex is predictable involuntary response to a stimulus. Thus both voluntary and involuntary limb movements, as well as certain organ functions depend on this link.
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