Chapter 29 By: Christian Borruso

The central nervous system (CNS) consists of the brain and the spinal cord. It is in the CNS that all of the analysis of information takes place.

The peripheral nervous system (PNS), which consists of the neurons and parts of neurons found outside of the CNS, includes sensory neurons and motor neurons. Sensory neurons bring signals into the CNS, and motor neurons carry signals out of the CNS.

The somatic nervous system is responsible for voluntary movements. It conducts impulses from the CNS to skeletal muscles. Sensory neurons relay messages fomr the eys, ears, nose, tongue, and skin to the CNS. Motor neurons carry impulses from the CNS to skeletal muscles.

The autonomous nervous system control involuntary functions. It consists of a network of nerves that is divided into two smaller networks, the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system contains chiefly adrenergic fibers. This system kicks in when you are startled, causing a heart rate increase and the "fight or flight" response. This is a reflex, and this system controls many reflexes.

The parasympathetic nervous system opposes the actions of the sympathetic nervous system by slowing down body functions. In rest periods, it slows heartbeat, relaxes blood vessels, and lowers blood pressure to conserve energy. Also stimulates saliva and stomach secretion production.

Sensory neurons get information about what's going on inside and outside of the body and bring that information into the CNS so it can be processed. For instance, if you picked up a hot coal, sensory neurons with endings in your fingertips would convey the information to your CNS that it was really hot.

Interneurons connect neurons. They are only found in the CNS. They receive and transmit information from neurons to other neurons. For instance, if you picked up a hot coal, the signal from the sensory neurons in your fingertips would travel to interneurons in your spinal cord. Some of these interneurons would signal to the motor neurons controlling your finger muscles which makes you let go, while others would transmit the signal up the spinal cord to neurons in the brain, where it would be perceived as pain. Interneurons are the most numerous class of neurons and are involved in processing information, both in simple reflex circuits and in more complex circuits in the brain. It would be combinations of interneurons in your brain that would allow you to draw the conclusion that things that looked like hot coals weren't good to pick up, and, hopefully, retain that information for future reference.

Motor neurons get information from other neurons and convey commands to your muscles, organs and glands. For instance, if you picked up a hot coal, the motor neurons innervating the muscles in your fingers would cause your hand to let go.

A neuromuscular junction is a synapse between a motor neuron and a skeletal muscle fiber. Action potentials travel along the neurons axon to the axon terminals. The terminals have vesicles filled with neurotransmitter, which is a chemical signal released by axon terminals and is also a type of signaling molecule that relays messages between cells at a synapse. Neurotransmitter is made in the cell body, then moved to axon terminals where it is stored until an action potential arrives. Arrival of an action potential at an axon terminal triggers exocytosis; neurotransmitter-filled vesicles move to the plasma membrane and fuse with it, releasing the neurotransmitter into the synaptic cleft.

A type of neurotransmitter. Motor neurons release acetylcholine (ACh), and skeletal muscle has receptors for this molecule. Binding of ACh to one of these receptors causes a change in the receptor’s shape. The receptor is also a passive transport protein, and when it changes shape, sodium ions can travel through it, from interstitial fluid into the muscle cell. Like a neuron, a muscle fiber can undergo an action potential. The influx of sodium caused by the binding of ACh drives the muscle fiber’s membrane toward threshold potential. Once this threshold is reached, action potentials stimulate muscle contraction.

Norepinephrine and epinephrine (commonly known as adrenaline) are neurotransmitters that prepare the body to respond to stress or excitement. Dopamine influences reward-based learning and acts in fine motor control Serotonin influences mood and memory. Glutamate is the main excitatory signal in the central nervous system. Endorphins are the body’s natural pain relievers.

Neuron-to-neuron connections are made onto the dendrites and cell bodies of other neurons. These connections are the sites at which information is carried from the first neuron, the presynaptic neuron, to the target neuron. The connections between neurons and smooth muscle cells or glands are known as neuroeffector junctions. At most synapses and junctions, information is transmitted in the form of chemical messengers called neurotransmitters. When an action potential travels down an axon and reaches the axon terminal, it triggers the release of neurotransmitter from the presynaptic cell. Neurotransmitter molecules cross the synapse and bind to membrane receptors on the postsynaptic cell, conveying an excitatory or inhibitory signal. Thus, the third basic neuronal function – communicating information to target cells – is carried out by the axon and the axon terminals. Just as a single neuron may receive inputs from many presynaptic neurons, it may also make synaptic connections on numerous postsynaptic neurons via different axon terminals.

White matter (Central nervous system tissue consisting mainly of myelinated axons.) consists mainly of myelin-sheathed axons. In the central nervous system, a bundle of such axons is called a tract (Bundle of axons in the central nervous system.) , rather than a nerve.

Gray matter (Central nervous system tissue that consists of neuron axon terminals, cell bodies, and dendrites, along with neuroglial cells.) contains cell bodies, dendrites, and unmyelinated axon terminals. Thus, synapses of the central nervous system are located within the gray matter.

The human cerebral cortex (Outer gray matter layer of the cerebrum; region responsible for most complex behavior.) , the outermost portion of the cerebrum, is a 2-millimeter-thick layer of gray matter. Over the course of human evolution, this layer has become increasingly folded. Folds allowed humans to add additional gray matter while minimizing the increase in brain volume; a larger brain requires a larger skull, which can pose problems during childbirth. Prominent folds in the cortex are used as landmarks to define the lobes of each cerebral hemisphere

Frontal Lobe- associated with reasoning, planning, parts of speech, movement, emotions, and problem solving

Parietal Lobe- associated with movement, orientation, recognition, perception of stimuli

Occipital Lobe- associated with visual processing

Temporal Lobe- associated with perception and recognition of auditory stimuli, memory, and speech

Emotions such as sadness, fear, and fury arise in part from the limbic system, a group of structures deep in the brain that function in expression of emotion and a set of structurally and functionally related structures deep within the brain. Exactly how these structures give rise to different emotions is poorly understood, but we do know a bit about some of their functions. For example, the hypothalamus summons up the physiological changes that accompany emotions. Signals from the hypothalamus make our heart pound and our palms sweat when we are fearful. The cingulate gyrus helps us control our emotions. The adjacent, almond-shaped amygdala becomes active when we are fearful, as well as when we perceive fear in others. The amygdala is often overactive in people with panic disorders.

The thalamus (Forebrain region that relays signals to the cerebrum; affects sleep-wake cycles.) is a two-lobed structure that sorts sensory signals and sends them to the proper region of the cerebral cortex. It also influences sleep and wakefulness. Damage to the thalamus can cause a person to go into a permanent sleeplike state called a coma. In the rare genetic disorder fatal familial insomnia, thalamus deterioration causes an inability to sleep, followed by a coma, then death.

The hypothalamus (Forebrain control center for homeostasis-related and endocrine functions.) (“under the thalamus”) is the center for homeostatic control of the internal environment. It receives information about the state of the body and regulates thirst, appetite, sex drive, and body temperature. It also interacts with the adjacent pituitary gland as a central control center for the endocrine system.

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