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neurons and why they matter

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Basic principles

Nervous tissue is an excitable tissue that is capable of receiving and tra

nsmitting electrical impulses. The central cell type is called a neuron. Neurons usually have a cell body, dendrites that receive inputs, and an axon that transmits electrical potentials.

Neurons may be classified as sensory, motor, secretory or association neurons. They are often classified by conduction speed, diameter and the presence or absence of specialized lipoprotein insulation called myelin. Type A fibers are myelinated and can conduct imp

ulses at 12 -120 m/sec. Type B are also myelinated fibers but they only transmit impulses at 3-5 m/sec. Type C fibers are unmyelinated, small in diameter and very slow (2.5 m/sec). An example of a Type A fiber is a motor neuron innervating the gastrocnemius. An autonomic preganglionic efferent neuron is an example of a Type B fiber and a sensory neuron carrying information about diffuse pain is an example of a slow Type C fiber.

Sensory neurons are adapted to detect certain types of information from the environment. These include mechanoreceptors sensing things like pressure or stretch, thermoreceptors, photoreceptors in the retina, and chemoreceptors such as the taste bud or those for olfaction. Association neurons, or interneurons are usually found in the spinal cord and brain where they connect sensory afferent neurons to efferent motor or secretory neurons.

Neurons communicate with one another via a structure called the synapse. An axon ends in one or more terminal buttons that contain numerous small vesicles. These small vesicles are filled with chemical substances called neurotransmitters. Acetylcholine is most often the neurotransmitter at the synapse although other chemicals like norepinephrine, serotonin and GABA may be used dependent on the neuron. When an impulse travels down the axon and reaches the terminal buttons the vesicles fuse with the neuronal membrane and the neurotransmitter is released. The chemical then diffuses across the narrow synaptic cleft to specific receptors for the chemical on the postsynaptic membrane of the receiving neuron.

The interaction of the neurotransmitter with the receptor causes a change in the membrane potential that may induce a new impulse postsynaptic neuron. The enzyme acetylcholinesterase is present in synapse to breakdown acetycholine and terminate the stimulus. Other neurotransmitters are either broken down or taken back up into the presynaptic neuron to terminate the stimulus.

In the central nervous system many neurons may converge on a single neuron. When each of the presynaptic neurons releases neurotransmitter into its synapse with the postsynaptic neuron, local membrane potentials occur that are integrated and summed. These incoming signals may be inhibitory or stimulatory. If the resulting summed membrane potential reaches the minimum threshold for that neuron, then an action potential will be initiated.

Action potentials travel in one direction away from the cell body by saltatory conduction. The fastest neurons are covered in myelin sheaths arranged in discreet segments separated by nodes of naked neuronal membrane called nodes of Ranvier. In saltatory conduction, the electrical potential jumps from node to node, thereby reducing the membrane area involved in conduction of the action potential and speeding up conduction.

Non-neural cells found in the nervous system are called glial cells. Astrocytes are the most numerous and provide support and nourishment of neurons. Microglia are small phagocytic cells specific to neural tissue. Cells that line the ventricular system and central canal of the spinal cord and make cerebrospinal fluid are called ependymal cells. In the central nervous system, an oligodendrocyte forms segments of the myelin sheaths of multiple neurons. In the peripheral nervous system, each segment of the myelin sheath is made by a single Schwann cell.

Central nervous system

The central nervous system (CNS) consists of the brain and spinal cord. The meninges (dura mater, arachnoid and pia mater) protect and nourish the CNS in addition to the protection afforded by the bony skull and vertebrae. Cerebrospinal fluid is found in the the subarachnoid space, central canal of the spinal column and the ventricles of the brain. The pia mater is the innermost layer and is adherant to the nervous tissue. Between the pia mater and the dura mater lies the arachnoid layer. The tough fibrous dura mater lies just beneath the skull.

The brain can be divided into 3 basic areas of the forebrain, midbrain, and brain stem. The forebrain includes the thalamus, hypothalamus, basal ganglia, and cerebrum. The cerebrum is responsible for conscious thought, interpretation of sensations, all voluntary movements, mental faculties, and the emotions.

Cerebral tissue can be divided into structural and functional areas. The surface of the cerebrum is convoluted into gyri (ridges) and sulci (grooves). The cortical sensory and motor areas can be mapped to the post central gyrus and central sulcus, respectively. The sensory area receives sensory info from the opposite side of the body that is projected after thalamic processing. Those parts of the body with more sensory nerve endings are represented by more cortical sensory area. The motor area controls voluntary muscle movements of the contralateral body parts but the association areas are important for the initiation of movement.

The cerebrum is the largest part of the brain and is divided into two hemispheres, right and left, having several lobes. The frontal lobe contains the motor area, Broca’s speech area, association areas, and functions in intelligence and behavior. The parietal lobe contains sensory areas and function in feeling and hearing. Primary visual association areas are located in the occipital lobe and the temporal lobe contains areas for auditory association, smell and memory storage.

The thalamus is located between the cerebral cortex and brainstem. All sensory input except the sense of smell is processed here before being projected to other areas of the brain. The hypothalamus is located beneath the thalamus and is responsible for processing internal stimuli and the maintenance of the internal environment. Moment by moment unconscious control of blood pressure, temperature, heart rate, respiration, water metabolism, osmolality, hunger, and neuroendocrine activities are handled here. Nuclei of the neuroendocrine cells that release oxytocin and ADH from the posterior pituitary are located in the hypothalamus.

The basal ganglia (caudate nucleus, globus palladus, substantia nigra, subthalamic nucleus, red nucleus) are groups of neurons embedded within each hemisphere of the cerebrum. They are involved in the control of complex motor control, information processing and unconscious gross intentional movements.

The brainstem includes the medulla oblongata and pons. The medulla oblongata contains important functional areas and relay centers for the control of respiration, cardiac and vasomotor reflexes. The pons contains the pneumotaxic center which is involved in the regulation of respiration.

The cerebellum lies above the brainstem and uses sensory information processed elsewhere about the position of the body, movement, posture and equilibrium. Movements are not initiated in the cerebellum but it is necessary for coordinated movement.

Peripheral nervous system

The peripheral nervous system includes nerves, ganglia, spinal and cranial nerves located outside the brain and spinal cord. The twelve cranial nerves arise from nuclei located in the brainstem and travel to specific locations carrying impulses to control various autonomic functions like smell, vision, salivation, heart rate and cutaneous sensation. Cranial nerves are often mixed in that they carry sensory and motor components but they may have only motor or sensory fibers. The following table lists the cranial nerves and their functions.

Cranial Nerves

Number Name Function
I Olfactory Sense of smell
II Optic Vision
III Oculomotor Motor control of some eye muscles and eyelid
IV Trochlear Motor control of some eye muscles
V Trigeminal Chewing muscles and some facial sensation
VI Abducent Motor control of some eye muscles
VII Facial Motor control of facial muscles, salivation. Taste and cutaneous sensations.
VIII Acoustic Equilibration, static sense and hearing
IX Glossopharyngeal Salivation, sensations of skin, taste and viscera
X Vagus Motor control of the heart and viscera, sensation from the thorax, pharynx and abdominal viscera
XI Accessory Motor impulses to the pharynx and shoulder
XII Hypoglossal Motor control of the tongue, some skeletal muscles, some viscera, sensation from skin and viscera

The sensory division of the peripheral nervous system takes input from various types of receptors, processes it and sends to the central nervous system. Sensory input can come from internal sources as in proprioception (sense of position of the joints and muscles) or external sources as in the sensation of pressure or heat on the skin. Areas of the skin innervated by specific spinal nerves are called dermatomes. Afferent fibers collect sensory input and travel up the spinal cord, converge in the thalamus, and end finally on the sensory cortex of the cerebrum. Those areas with more sensory receptors, i.e. the fingertips or lips, correspond to a larger area on the sensory cortex of the brain. Fibers carrying proprioceptive information are dispersed to the cerebellum as well. Almost all sensory systems transmit impulses to parts of the thalamus. The cerebral cortex is involved in conscious perception and interpretation of sensory stimuli.

Motor inputs to muscles and glands occur via the autonomic and somatic efferent systems. CNS innervation of the joints, tendons and muscles travel via the somatic efferent system. Some muscular responses are handled via spinal reflexes. An example of this is the withdrawal reflex seen when the finger contacts a hot stove. The movement to remove the finger occurs via a simple spinal reflex long before the sensation of pain reaches the brain. Clearly this is protective mechanism to avoid further injury. Motor inputs to glands and smooth muscle usually occur via the autonomic system.

Most organs receive input from both branches of the autonomic nervous system. One branch will generally be excitatory while the other is inhibitory in that organ or tissue. The sympathetic branch of the autonomic system acts to prepare the body for physiologic stress. Stimulation of the sympathetic branch is like stepping on the gas in that the body prepares to run or fight in response. Effects such as an increased heart rate, dilation of airways and mobilization of glucose from glycogen stores are seen. Sympathetic nerves arise from the 1st thoracic to the 4th lumbar vertebra. They have a short preganglionic neuron that ends in one of the chain ganglia that lie along the spinal column. Acetylcholine is the neurotransmitter at the synapse with the long postganglionic neuron which then travels to the target tissue where norepinephrine is released at the majority of sympathetic nerve endings. A few sympathetic post ganglionic neurons, such as those innervating sweat glands or skeletal muscle vasculature, release acetylcholine.

The parasympathetic branch acts to counterbalance the sympathetic branch via neurons that arise from the cranial and sacral regions of the CNS. For instance, parasympathetic stimulation constricts airways and decreases heart rate. It regulates resting activities such as digestion, micturation and erection. Long preganglionic neurons release acetylcholine at synapses close to the end organ. Short postganglionic neurons also release acetylcholine on the effector tissue.

Cerebrospinal fluid and blood supply

The cerebral spinal fluid (CSF) is a clear, watery and colorless secretion of the choroid plexus within the ventricles that acts as a shock absorber for the brain and spinal cord. CSF also acts to support the brain as it becomes buoyant floating in the CSF. It is made, circulates throughout the ventricular spaces and the subarachnoid space and resorbed continuously. The volume of CSF is changed to adjust for shrinking or swelling of the nervous tissue by altering absorption or secretion. CSF is made by filtration, diffusion and active transport of substances from the blood. Glucose levels are usually about 60% of that in blood and CSF normally contains only a few cells, almost all of which are lymphocytes.

Analysis of the CSF can give clues to the nature of CNS disease. Total protein is increased in infections and viral diseases and cell counts in CSF increase in cases of meningitis or encephalitis.

Autoregulatory mechanisms in the healthy brain allow for constant blood flow even when physiologic conditions change radically. The concentrations or tension of oxygen and carbon dioxide within the tissues change the local blood flow by causing vasodilation or vasoconstriction. Wide fluctuations in arterial blood pressure have virtually no effect on cerebral blood flow because of this intrinsic control. However, decreased cardiac output due to heart failure or decreased blood volume compromises cerebral blood flow.

The brain is supplied by the internal carotid arteries which branch and become the anterior and middle cerebral arteries and posteriorly via the vertebral arteries that become the basilar artery. These anterior and posterior arteries are joined together by smaller arterial connections to form the Circle of Willis. The small diameter arteries of the Circle of Willis are a frequent site of infarction and stroke. Venous drainage occurs via the deep veins and dural sinuses that finally drain into the internal jugular veins.

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