Brief Anatomy and Physiology of the Nervous System

Brief Anatomy and Physiology of the Nervous System

Neuron: Structures and Functions

The nervous system is an organ system containing network of specialized cells called neurons; it is the structural and functional unit of the nervous system. The human nervous system consists of more than 100 billions of nerve cells or neurons. It is specialized to be irritable and transmit signals, or impulses. The neurons are held together and supported by another nervous tissue known as neuroglia, or simply glia. The word glia, which means "glue," implies one of their roles; they bind neurons together and provide a supportive framework for the nervous tissue. An excitable cell reacts to stimuli by altering its membrane characteristics. There are two types of excitable cells: nerve cells, which transmit and modify impulses within the nervous system, and muscle cells, which contract either in response to nerve stimuli or autonomously.

The communicative role of the nervous system is carried out by nerve cells, or neurons. These cells have three fundamental physiological properties that are necessary to this function:

• Excitability (irritability): All cells possess excitability, the ability to respond to environmental changes called stimuli. Neurons have developed this property to the highest degree.
• Conductivity: Neurons respond to stimuli by producing traveling electrical signals that quickly reach other cells at distant locations.
• Secretion: When the electrical signal reaches the end of a nerve fiber, the neuron secretes a chemical neurotransmitter that "jumps the gap" and stimulates the next cell.

Functional Classes of Neurons

There are three general classes of neurons corresponding to the three major aspects of nervous system function listed earlier:

Sensory (afferent) neurons are specialized to detect stimuli such as light, heat, pressure, and chemicals, and transmit information about them to the CNS. These neurons can begin in almost any organ of the body and end in the CNS; the word afferent refers to signal conduction toward the CNS. Some sensory receptors, such as pain and smell receptors are themselves neurons. In other cases, such as taste and hearing, the receptor is a separate cell that communicates directly with a sensory neuron.

Inter neurons (association neurons) lie entirely within the CNS. They receive signals from many other neurons and carry out the integrative function of the nervous system that is, they process, store, and retrieve information and "make decisions" that determine how the body responds to stimuli. About 90% of our neurons are inter neurons. The word interneuron refers to the fact that they lie between, and interconnect, the incoming sensory pathways and the outgoing motor pathways of the CNS.

Motor (efferent) neurons send signals predominantly to muscle and gland cells, the effectors that carry out the body's responses to stimuli. These neurons are called motor neurons because most of them lead to muscle cells, and efferent neurons to signify the signal conduction away from the CNS.

The nervous system has three major subdivisions:

1. The central nervous system (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column.
2. The peripheral nervous system (PNS).
3. The autonomic nervous system (ANS).

A nerve is a bundle of nerve fibers wrapped in fibrous connective tissue. Nerves emerge from the CNS through foramina of the skull and vertebral column and carry signals to and from other organs of the body. Ganglion/ganglia) is a knot like swelling in a nerve where the cell bodies of neurons are concentrated.

The Central Nervous System (CNS)

Is central in both location and function. A fiber tract is a collection of neuron processes, together and inside the CNS

Major levels of central nervous system function: The three major levels of the central nervous system have specific functional characteristics:

The spinal cord level: In fact, the upper levels of the nervous system often operate not by sending signals directly to the periphery of the body but by sending signals to the control centers of the cord, simply "commanding" the cord centers to perform their functions. For instance, neuronal circuits in the cord can cause:

• Walking movements
• Reflexes that withdraw portions of the body from painful objects
• Reflexes that stiffen the legs to support the body against gravity
• Reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion.

Peripheral Nervous System (PNS)

Peripheral nerves are nerves which pass from the CNS to the periphery of the body carry commands for skeletal muscles and other muscles made up of striated muscle tissue, as well as sensory information from the periphery of the body is carried to the CNS where it is processed. The PNS is made up of a large number of individual nerves. A nerve is a collection of neuron processes, together and outside the CNS. These nerves are arranged in pairs. Each pair includes one nerve on the left side of the brainstem or spinal cord and one nerve on the right side. PNS nerves include 12 pairs of cranial nerves and 31 pairs of spinal nerves.

The Autonomic Nervous System

The autonomic nervous system (ANS) is that portion of the nervous system concerned with commands for smooth muscle tissue, cardiac muscle tissue, and glands responsible for those functions in the body which are carried out automatically (we do not think about them consciously). The visceral organs are innervated by ANS; the term visceral organs is used to include the various hollow organs of the body whose walls have smooth muscle tissue in them. Examples are the blood vessels and the gut. Its effects include control of heart rate and force of contraction, constriction and dilatation of blood vessels, contraction and relaxation of smooth muscle in various organs, visual accommodation, pupillary size and secretions from exocrine and endocrine glands.

The autonomic nervous system is divided into two separate parts sympathetic & parasympathetic, on the basis of their anatomical origins and physiological actions. Both of these systems consist of myelinated preganglionic fibers which make synaptic connections with unmyelinated postganglionic fibers, and it is these which then innervate the effector organ. These synapses usually occur in clusters called ganglia. Most organs are innervated by fibers from both divisions of the ANS, and the influence is usually opposing (e.g. the vagus slows the heart, whilst the sympathetic nerves increase its rate and contractility). The responses of major effector organs to autonomic nerve impulses are summarized.

The Parasympathetic Nervous System

The preganglionic outflow of the parasympathetic nervous system arises from the cell bodies of the motor nuclei of the cranial nerves III, VII, IX and X in the brain stem and from the second, third and fourth sacral segments of the spinal cord. It is therefore also known as the cranio-sacral outflow. Preganglionic fibers run almost to the organ which is innervated, and synapse in ganglia close to or within that organ, giving rise to postganglionic fibers which then innervate the relevant tissue. The cranial nerves III, VII and IX affect the pupil and salivary gland secretion, whilst the vagus nerve (X) carries fibers to the heart, lungs, stomach, upper intestine and ureter. The sacral fibers form pelvic plexuses which innervate the distal colon, rectum, bladder and reproductive organs.

In physiological terms, the parasympathetic system is concerned with conservation and restoration of energy, as it causes a reduction in heart rate and blood pressure, and facilitates digestion and absorption of nutrients, and consequently the excretion of waste products.

The chemical transmitter at both pre and postganglionic synapses in the parasympathetic system is Acetylcholine (Ach). Ach is also the neurotransmitter at sympathetic preganglionic synapses, some sympathetic postganglionic synapses, the neuromuscular junction (somatic nervous system), and at some sites in the CNS. Nerve fibers that release Ach from their endings are described as cholinergic fibers.

Sympathetic Nervous System

The cell bodies of the sympathetic preganglionic fibres are in the lateral horns of the spinal segments T1-L2, the so called thoraco-lumbar outflow. The preganglionic fibres travel a short distance in the mixed spinal nerve, and then branch off as white rami (myelinated) to enter the sympathetic ganglia. These are mainly arranged in two paravertebral chains which lie anterolateral to the vertebral bodies and extend from the cervical to the sacral region. They are called the sympathetic ganglionic chains. The short preganglionic fibres which enter the chain make a synapse with a postsynaptic fibre either at the same dermatomal level, or at a higher or lower level, and then the longer postganglionic fibres usually return to the adjacent spinal nerve via grey rami (unmyelinated) and are conveyed to the effector organ.

Some preganglionic fibres do not synapse in the sympathetic chains travel in the greater splanchnic nerve and directly synapse with chromaffin cells in the adrenal medulla. Ach is the neurotransmitter via a nicotinic receptor at the preganglionic synapse. The adrenal medulla is innervated by preganglionic fibres and therefore adrenaline is released from the gland by stimulation of nicotinic Ach receptors. At most postganglionic sympathetic endings, the chemical transmitter is noradrenaline, which is present in the presynaptic terminal as well as in the adrenal medulla. In sweat glands, however, postganglionic sympathetic fibres release Ach and this transmission is nicotinic. In contrast to the parasympathetic system, the sympathetic system enables the body to be prepared for fear, flight or fight. Sympathetic responses include an increase in heart rate, blood pressure and cardiac output, a diversion of blood flow from the skin and splanchnic vessels to those supplying skeletal muscle, increased pupil size, bronchiolar dilation, contraction of sphincters and metabolic changes such as the mobilization of fat and glycogen.

Adrenaline and noradrenaline are both catecholamines, and are both synthesized from the essential amino acid phenylalanine. On the arrival of a nerve impulse, noradrenaline is released from granules in the presynaptic terminal into the synaptic cleft. The action of noradrenaline is terminated by diffusion from the site of action, re-uptake back into the presynaptic nerve ending where it is inactivated by the enzyme Monoamine Oxidase in mitochondria or metabolism locally by the enzyme Catechol-O- Methyl-Transferase. The synthesis and storage of catecholamines in the adrenal medulla is similar to that of sympathetic postganglionic nerve endings The actions of catecholamines are mediated by specific postsynaptic cell surface receptors. Pharmacological subdivision of these receptors into two groups (α and β), based upon the effects of adrenaline at peripheral sympathetic sites. Thus β1 adrenoceptor mediated effects in the heart (increased force and rate of contraction) have been differentiated from those producing smooth

muscle relaxation in the bronchi and blood vessels (β2 effects). Similarly, α-adrenoceptor mediated effects such as vasoconstriction have been termed, α1 effects, to differentiate them from the feedback inhibition by noradrenaline on its own release from presynaptic terminals, which is mediated by α2adrenoceptors on the presynaptic membrane.

Cerebral Metabolism

The brain is normally responsible for consumption of 20% of total body oxygen. Most of cerebral oxygen consumption (60%) is used in generating adenosine triphosphate (ATP) support neuronal electrical activity. The cerebral metabolic rate (CMR) is usually expressed in terms of oxygen consumption (cmro2), which averages 3-3.8 ml/100 g/min (50 ml/min) in adults. Because of the relatively high oxygen consumption and the absence of significant oxygen reserves, interruption of cerebral perfusion usually results in unconsciousness within 10sec. As oxygen tension rapidly drops below 30 mm hg. If blood flow is not reestablished within minutes (3-8 min under most conditions

Neuronal cells normally utilize glucose as their primary energy source. Brain glucose consumption is approximately 5 mg/100 g/min, of which over 90% is metabolized aerobically. Cerebral function is normally dependent on a continuous supply of glucose. Acute sustained hypoglycemia is equally as devastating as hypoxia. Paradoxically, hyperglycemia can exacerbate global and focal hypoxic brain injury by accelerating cerebral acidosis and cellular injury.

Cerebral Blood Flow

The brain receives about 15 percent of the cardiac output: yet it represents only 2% of total body weight, reflecting its high metabolic rate. Cerebral blood flow (CBF) parallels the paco2 (increasing the paco2 from 40 mmhg to 80 mmhg doubles the CBF whereas, decreasing the paco2 from 40mmhg to20mmhg half the CBF, which has negative effect on the brain perfusion

CBF is maintained at a constant rate (auto regulated) over a mean arterial pressure (map) from about 50-150mmhg, reflecting appropriate adjustments of the cerebral vascular resistance.

When MAP decreases below 50mmhg, the CBF is reduced and mild symptoms of cerebral ischemia may occur at a perfusion pressure of about 40mmhg. When MAP exceeds the auto regulated range; it may cause disruption of the blood- brain- barrier (bbb) and cerebral edema may develop. Auto regulation can be abolished by trauma, hypoxia and certain anesthetic and adjuvant anesthetic drugs. Systemic hypertension that persists for 1-2 months causes a shift of the autoregulation range to higher pressures, such that cerebral ischemia may occur at a MAP >50mmhg.

Cerebral Perfusion Pressure (CPP)

Is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP). MAP -ICP = CPP. Moderate to severe increases in ICP (> 30 mm hg) can significantly compromise CPP

Cerebro-Spinal Fluid (CSF)

CSF is found in the cerebral ventricles and cisterns and in the subarachnoid space surrounding the brain and spinal cord. Its major function is to protect the CNS against trauma. Most of the CSF is formed by the choroid plexuses of the cerebral (mainly lateral) ventricles. All these chambers are connected with one another, and the pressure of the fluid is maintained at a surprisingly constant level

Intracranial Pressure (ICP)

The cranial vault is a rigid structure with a fixed total volume, consisting of brain (80%), blood (12%), and CSF (8%). Any increase in one component must be offset by an equivalent decrease in another to prevent a rise in ICP. ICP measures normally 10mmhg.