Nervous System PDF

Nervous system Nervous system To carry out its functions, the nervous system is organized anatomically in: • Central Nervous System (CNS): formed by the encephalon and spinal cord. • Peripheral Nervous System (PNS): located outside the CNS and formed by nerves and ganglia. The PNS can functiona...

Nervous system Nervous system To carry out its functions, the nervous system is organized anatomically in: • Central Nervous System (CNS): formed by the encephalon and spinal cord. • Peripheral Nervous System (PNS): located outside the CNS and formed by nerves and ganglia. The PNS can functionally be divided into: o Sensitive (afferent): receives signals from the internal or external environment and sends them to the CNS. o Motor (efferent): originating in the CNS and sending impulses to effector organs. This motor component can also be divided into: ➢ Somatic system: is responsible for all voluntary functions. A single neuron transmits impulses from the CNS to a skeletal muscle. ➢ Autonomous system: is responsible for all involuntary functions. The impulses that leave the CNS go to a ganglion through a neuron and then a second neuron will take them to a smooth muscle, a gland or the heart. Central nervous system Composed by the brain and the spinal cord. Here, the neurons are organized giving rise to two parts: - White matter: formed mostly by nerve fibers and neuroglial cells. Its white color is due to the abundance of myelin. - Gray matter: formed by aggregates of neuronal bodies, dendrites and unmyelinated portions of axons, as well as neuroglials cells. Spinal cord The gray matter is in the center of the spinal cord, with the shape of an H and around it is the white matter. In the center of the spinal cord is the ependymal channel, lined by ependymocytes. The posterior horns of the H receive prolongations of sensory neurons, whose bodies are in the ganglia of the posterior root. These extensions reach interneurons that are also found in these posterior horns. In the anterior horns there are bodies of motor neurons, whose axons will come out using the anterior roots. The disposition of the neurons in the spinal cord allow it to communicate with the brain and transmit the impulses received. The brain will elaborate a response that will also travel through the cord. However, the spinal cord can sometimes also develop simple and quick responses called reflex arcs. Reflex arc Brain In the brain, the gray matter is found in the cortex and the white matter in the interior. The majority of interneurons are found here, where they form communication networks for the integration of sensory and motor neurons. Cerebellum In the cerebellum there are the smallest neurons of the body and are distributed in two layers of gray matter: the outermost called molecular layer, with fewer neuronal bodies, and the innermost called granulosa, with a large number of neuronal bodies. Between these two layers, there is a row of large interneurons called Purkinje cells. Inside this gray matter we will find the white matter. Meninges They are three layers of connective tissue that surrounds the CNS to give it protection. The outermost, in contact with the bones of the skull and spine, is called dura mater. It has a periosteal layer formed by osteoprogenitor cells, fibroblasts and collagen fibers that bind to bone, and another meningeal layer formed by dense conjunctiva. The second meninge is the arachnoid, which is also formed by dense conjunctiva and has a flat layer adhered to the dura and another formed by a network of trabeculae that contact the pia mater. In the spaces between these trabeculae there is cerebrospinal fluid and blood vessels. The pia mater is the innermost of the meninges, formed by a thin layer of connective tissue and associated with nervous tissue. Peripheral nervous system • Nerves • Ganglia Nerves They are bundles of nerve fibers located outside the CNS surrounded by sheaths of connective tissue. Each fiber is surrounded by loose connective tissue called endoneuro. Several fibers are grouped together forming a fascicle, which is surrounded by dense connective tissue called perineuro; several fascicles form a nerve surrounded by more dense connective tissue called epineuro. There may be sensory nerves or motor nerves, but most are mixed. Autonomous nervous system It is a motor and involuntary system that helps to maintain the homeostasis of the organism since it controls the viscera (smooth muscle, cardiac muscle) and the secretion of the glands. Their neurons originate in the CNS and are directed to an autonomous ganglion, where they will synapse with a second neuron that will reach the effector organ. It is subdivided into two systems, sympathetic and parasympathetic. The sympathetic nervous system, generally prepares the body for action, acts in situations of stress. Increases breathing, blood pressure, heart rate, blood flow to skeletal muscles, dilates pupils and slows visceral functions. Their neurons originate in the dorsal and lumbar region of the cord and some synapses in a ganglion of the paravertebral chain and others cross this chain to enter the abdominal cavity and synapse in the collateral ganglia along the abdominal aorta. The parasympathetic nervous system tends to be sympathetic antagonist, produces the opposite reactions. It helps the maintenance of homeostasis. Its neurons originate in the brain or in the sacral region of the spinal cord and synapses in ganglia located in the vicinity of the effector organ or even in the organ itself (terminal ganglia). Nervous ganglia They are aggregates of neuronal bodies outside the CNS. They can be of two types: - Sensitive: they lodge bodies of sensitive neurons and are associated to some cranial pairs and to all the spinal nerves. They are surrounded by a dense connective capsule and inside of it there are abundant neurons all surrounded by satellite cells. - Autonomous: they lodge the neuronal bodies of the postsynaptic motor neurons of the ANS. They are the ganglia of the paravertebral sympathetic chain, the collateral ganglia and the terminal ganglia. They do not have their own capsule; they are usually inside the same nerve sheath and have few neurons and satellite cells.