| The Nervous System The nervous system is the most complex system of the body, yet it is very conservative in terms of change Show Neurons The nerve impulse is an electrical phenomenon that passes as a wave along the surface membrane of a nerve fiber Normally the nerve cell is at resting potential, based on the concentration of sodium and potassium ions inside and outside of the cell During an action potential, the neuron goes through several steps: 1. High concentration of sodium ions outside; negative charge inside cell compared to outside 2. Axon stimulated, ion channels open causing action potential; sodium ions rush into cell, membrane potential reversed and membrane is depolarized 3. Nerve impulse travels down the axon as a wave of depolarization 4. Sodium pumped outside of cell and resting potential restored Between neurons, nerve impulse must travel across a space or synapse - the telodendria contains synaptic vesicles that contain neurotransmitters (e.g., acetylcholine, noradrenaline, serotonin, dopamine) Neurotransmitters are released when the nerve impulse reaches the telodendria, and then cross the synaptic cleft to reach the dendrites of the next neuron in line causing the impulse to be sent by the postsynaptic nerve cell Other cells of the nervous system:
The general parts of the neuronal circuitry include three basic types of neurons: Primary sensory neurons, or afferent neurons: carry impulses from free nerve endings or receptor cells into the central nervous system Motor, or efferent neurons: carry impulses from the central nervous system to effectors, such as muscles or glands Interneurons: receive signals from sensory neurons, integrate information and send signals to motor neurons Somatic fibers relate to the skin and its derivatives, and to voluntary muscles Visceral fibers related to involuntary muscles and glands of the organ systems Spinal cord and spinal nerves In the simplest reflex arc, messages from receptor organs are transferred within the spinal cord directly from afferent fibers to efferent fibers, which then send appropriate messages to effector organs The function of the spinal cord is to receive incoming impulses, integrate and coordinate them, transmit them to wherever they should go within the central nervous system, and send responses to the peripheral nervous system as appropriate The general structure of the spinal cord is best exemplified by a cross-section of the spinal cord of an amniote (Fig. 16.7, p. 593): � the grey matter lies on the interior of the cord while white matter lies on the exterior � the grey matter resembles the letter H, with the upper arms called the dorsal columns or horns, and the lower arms called the ventral columns or horns � the grey commissure makes up the cross arm of the H and transmits fibers from one side of the cord to the other � the external white matter is divided into right and left sides by the dorsomedian sulcus and the ventromedian fissure � the dorsal horn of the cord receives terminations of primary sensory neurons � the ventral horn contains the dendrites and cell bodies of motor neurons Three types of neuronal pathways are common to all vertebrates:ReflexesInvolve only sensory, motor, and interneurons of the spinal cord which constitutes a three-neuron reflex arc - helps the body to perform a rapid integrated movements that requires that certain muscles contact with the appropriate force at the appropriate time Other reflexes include intersegmental reflexes that involve neurons that decussate on the other side of the body - decussation is the crossing of neuronal tracts in the midline of the central nervous system - intersegmental reflexes are responsible for maintaining coordinated movements such as swimming or walking. Conditioned reflexes are innate reflexes that become elaborated as a result of an animal�s repetitive experiences Pathways from lower to higher brain centersUtilize ascending tracts in the spinal cord - most ascending impulses decussate along the way Pathways from the brain to lower centersImpulses utilize descending tracts in the spinal cord, which may decussate in the brain prior to traveling to the appropriate muscle The central pattern generators, groups of neurons in the spinal cord and in the brain, whose activity is responsible for innate cyclical movements of body parts - central pattern generators do not require continued sensory input in order to cause a response Spinal nerves usually attach by roots to the spinal cord In more primitive species, the dorsal and ventral roots form separate dorsal and ventral spinal nerves In all other vertebrates, dorsal and ventral nerves unite to form single spinal nerve, with sensory fibers entering through the dorsal root (ramus) and motor fibers leaving through the ventral root (ramus) Spinal nerves are defined by their location and include cervical, thoracic, lumbar, sacral and caudal nerves - the more caudal spinal nerves form a bundle known as the cauda equina Plexuses are networks of nerves or blood vessels formed before nerves are distributed to the muscles� the cervical plexus supplies the ventral neck muscles Autonomic nervous system Distinctive features of the autonomic nervous system are: 1. Every pathway includes a neuron having its cell body inside the CNS and a neuron cell body outside CNS.In mammals, postganglionic parasympathetic fibers secrete acetylcholine and are called cholinergic fibers. Postganglionic sympathetic fibers (which elicit the "fight or flight" response) secrete noradrenaline (norepinephrine) and are called adrenergic fibers. The effects of stimulation of the two systems are shown in Table 12-3, p. 481 in the text, and in the figure of the mammalian autonomic nervous system in the handout.Fibers between ganglia and CNS are preganglionic and myelinated. Fibers between ganglia and end organs are postganglionic and unmyelinated2. Divisible into several sets of fibersSympathetic - part of autonomic nervous system that leaves CNS from parts of spinal cord. Activity of the sympathetic nervous system helps an animal adjust to stress by promoting physiological processes that increase energy available to body tissues. Also called thoracolumbar outflow. Brain
Development prosencephalon - forebrainThese three regions then separate into several additional regions The anterior part of the prosencephalon develops into
The telencephalon will also differentiate anteriorly to form the olfactory bulbs The rhombencephalon forms an anterior metencephalon (which will form the adult cerebellum) and a posterior myelencephalon Within the division of the brain are cavities called vesicles, which later grow to form expansions or ventricles.
The brainstem is the first region to form in development, is the least variable, and receives all the cranial nerves (except for the terminal and olfactory nerves). Part of the adult metencephalon and all of the diencephalon , mesencephalon and myelencephalon are included in the brainstem. The brainstem controls most of the vegetative functions of the body, and is thus vital for life. The cerebellum and pons (the ventral part of the metencephalon of birds and mammals that has a band of transverse fibers) are the principal adult derivatives of the metencephalon. The cerebellum and pons contribute to coordination of motor function. The cerebrum is the adult derivative of the telencephalon, and dominates the brain in both size and control. Surrounding the adult brain are layers of mesodermally-derived connective tissue called meninges (singular: meninx). Whereas cyclostomes and fishes only have a single envelope called the primitive meninx, amphibians have two layers, consisting of an outer dura mater which is extremely dense and protective, and a pia-arachnoid or secondary meninx which is more delicate and vascular. Mammals have three meninges: pia mater (which follows all the convolutions of the brain and is the most interior), the arachnoid layer (which is delicate and sends strands to the pia mater), and the dura mater (the outer, more protective meninx). The area between the dura mater and the arachnoid layer is called the subdural space; the area between the arachnoid layer is called the subarachnoid space. An additional layer of tissue lies between the two hemispheres of the cerebrum and is called the falx cerebri. Posterior brainstem: medulla through midbrain The reticular formation is found in all vertebrates, and is a network of short interneurons in the brainstem that forms a primitive integrating system. It projects into the cerebrum, cerebellum, cranial nuclei, and the spinal cord, and is essential for consciousness as well as control of the cardiovascular and respiratory systems. The ruber nucleus and substantia nigra are two other important parts of the brain that are located in the posterior brainstem. The ruber nucleus plays a role in the coordination of motor functions. The substantia nigra is involved in the memory of learned tasks, and death of its cells is associated with Parkinson�s disease. The roof of the midbrain is called the tectum. The tectum of non-mammalian vertebrates is the site for the optic lobes, which are the primary center for the perception of vision. In mammals, vision is perceived in the cerebrum. However, while the mammalian cerebrum tells the animal what an object is, the tectum tells the mammal where in space a visual object is. In the tectum the optic lobes are called anterior colliculi. Behind them are the posterior colliculi which may be important in coordination of auditory reflexes. Together the colliculi form four bumps called the corpora quadrigemina. Other features of the posterior brainstem include are the pyramidal tracts, which are tracts of motor fibers that run from the cerebral cortex to the spinal cord without interruption. The cerebra peduncles are also important, as they are the sites where the cerebellum joins the brainstem. Anterior brainstem: diencephalon Cerebellum and pons Ganglion - group of neuron cell bodies that lie peripheral to the central nervous system in vertebrates Neuroglia - cells in the central nervous system that help to support, protect and maintain the neurons Microglia - small neuroglial cells of mesodermal origin, some of which are phagocytic Node of Ranvier - regions of the axon that lie between the Schwann cells, where the plasma membrane of a myelinated axon is close to the extracellular fluid Oligodendrocytes - neuroglial cells of ectodermal origin that myelinate axons in the central nervous system, and forms the white matter of the central nervous system (unmyelinated axons are grey matter) Plexus - networks of nerves or blood vessels formed before nerves are distributed to the muscles Schwann cells - also called neurilemma. Cells of neural crest origin that form a thin sheath that surrounds an unmyelinated axon, or, after having myelinated an axon, lies on the surface of the myelin sheath Telodendria - the terminal branches of an axon Which nerve carries messages from the brain or the spinal cord to the muscles or glands?You have two main types of nerves: Sensory nerves carry signals to your brain to help you touch, taste, smell and see. Motor nerves carry signals to your muscles or glands to help you move and function.
What type of nerve carries messages to the brain and spinal cord?There are basically two major types of nerves: sensory and motor. Sensory nerves send information such as touch, temperature, and pain to the brain and spinal cord. Motor nerves send signals from the brain back into the muscles, causing them to contract either voluntarily or reflexively.
What part of the brain sends messages to muscles and glands?A sensory (or afferent) neuron carries information from the sensory receptors, whereas a motor (or efferent) neuron transmits information to the muscles and glands.
What are the nerves that carry messages from the brain to the body?Motor nerves are the network of neurons that extend between the brain and the muscles or the effector organs.. They carry the messages from the brain to the body parts.. |
Which nerves carry messages from the brain or the spinal cord to the muscles and glands?
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