Biology 42 – Human Biology (Spring 2007)
[ Students and professors, please read. ]
Assignment: Write a description of the structure and function of your assigned organ system.
The Nervous System
The nervous system includes the brain, spinal cord, and nerves. Nerve impulses travel via nerves from sensory receptors to the brain and spinal cord for input integration. Sensation, interestingly, occurs only in the brain. A backward path of nerve impulse – from brain and spinal cord to muscles and glands – allows coordinated response to external (motor output) and internal (secretion) stimuli.
Nervous tissue contains nerve cells called neurons, present in the brain and spinal cord. A neuron has three parts: 1) The dendrites are extensions that receive signals from sensory receptors and other neurons, 2) The cell body is what the neuron has in common with other living cells, studied last chapter, 3) The axons (also called fibers) conduct the nerve impulses. Long axons are covered by myelin. Outside the brain and spinal cord, these (axons/fibers, with or without myelin) are called nerves and are bound by connective tissue.
The fingertips contain the most touch receptors, which are a type of the sensory receptors or specialized nerve endings in the dermis (see Integumentary System) that respond to external stimuli. Touch receptors add to our ability to use our fingers for delicate tasks.
Nervous tissue also contains neuroglia, or glial cells, which primarily support and nourish neurons, but may contribute to brain function as well (the focus of current research). Neuroglia outnumber neurons 9-to-1, taking up more than half the volume of the brain. Microglia, astrocytes, oligodendrocytes and Schwann cells are types of neuroglia. Microglia support neurons and engulf bacterial and cellular debris. Astrocytes feed neurons with nutrients and the hormone ‘glia-derived growth factor’. Oligodendrocytes form myelin around the axon in the central nervous system, while Schwann cells form myelin around axons in the peripheral nervous system.
The nervous system contributes to homeostasis among the other systems of the body, by regulating and coordinating their activities and responding quickly (more quickly than the slow, but longer-lasting endocrine system) to stimuli coming in from them, bringing about rapid response to any changes in the internal environment by issuing commands by electro-chemical signals that are rapidly transmitted to effector organs, like muscles and glands. The hypothalamus is the sensor and control center in the brain for body temperature, responding with negative feedback, causing the blood vessels of the skin to dilate (release heat in hot weather) or constrict (conserve heat in cold weather).
The nervous system can also counteract homeostasis, as in the case of positive feedback. For example, when nerve impulses from the sensory receptors of the cervix reach the brain during child birth, because the baby is pushing down on the cervix, then the brain responds with positive feedback, causing the pituitary gland to secrete oxytocin, which increases contractions – rather than stopping them (which would return the body to homeostasis).