Neurological Function & AnatomyNeurological Function & Anatomy

• The nervous system is one of the body’s control systems; by means of electrochemical impulses we are able to detect changes and feel sensations, initiate appropriate responses to changes, and organize and store information for future use. Some of this is conscious activity, but much of it is reflexive in nature and happens without our awareness. The nervous system has two divisions. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of cranial nerves and spinal nerves, which include the nerves of the autonomic nervous system (ANS).

NERVE TISSUE

•  Nerve tissue consists of neurons and specialized supporting cells. There are many kinds of neurons (nerve cells or nerve fibers), but they all have the same general structure . The cell body contains the nucleus and is essential for the continued life of the neuron. All neuron cell bodies are found in the brain or spinal cord or within the trunk of the body; in these locations they are protected by bone.

NEURONS

• is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells .

• A typical neuron possesses a cell body (often called the soma), dendrites, and an axon. Dendrites are filaments that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree". An axon is a special cellular filament that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 m in humans or even more in other species. There is usually one long axon per neuron that reaches out to connect with other neurons. This may divide into sub-branches, typically towards the end of the axon, thus allowing it to connect with multiple other neurons in a given vicinity.

Myelin Sheath • Myelin Sheath are usually around only the axon of a neuron. It is

essential for the proper functioning of the nervous system. Myelin is an outgrowth of a glial cell Myelin, sometimes referred to as "white matter" because of its white, fatty appearance, protects and insulates the axons. It consists of a protective sheath of many different molecules that include both lipids (fatty molecules) and proteins.  This protective sheath acts in a manner very similar to that of the protective insulation that surrounds an electric wire; that is, it is necessary for the rapid transmission of electrical signals between neurons. It does this primarily by containing the electrical molecules within the axon so that they are all properly transmitted to the next neuron. With the protective myelin coat, neurons can transmit signals at speeds up to 60 meters per second.

• When the coat is damaged, the maximum speed can decrease by ten-fold or more, since some of the signal is lost during transmission. This decrease in speed of signal transmission leads to significant disruption in the proper functioning of the nervous system. The myelin coating along the axons of neurons allows impulses to "jump" more quickly along the length of the axon. Without this insulating layer of myelin, the transmission of nerve impulses is greatly slowed. The myelinated axons are compared to copper wires coated with plastic insulation and are designed for longer distance signal transmission. In peripheral nerves, the insulation is provided by Schwann cells that make a single wrap around each axon.

Nerve Impulse

•  A nerve impulse, which may also be called an action potential, is an electrical change brought about by the movement of ions across the neuron cell membrane. When a neuron is not carrying an impulse, it is in a state of polarization with a positive charge outside the membrane and a relatively negative charge inside the membrane. Sodium ions are more abundant outside the cell, and potassium and negative ions are more abundant inside the cell.

• A neuron is capable of transmitting hundreds of impulses per second, and at great speed, many meters per second.

Central Nervous System

• It contains the majority of the nervous system and consists of the brain and the spinal cord.

BrainBrain – The brain consists of many parts, which function as an

integrated whole. The major parts are the medulla, pons, and midbrain (the brainstem); the cerebellum; the hypothalamus and thalamus; and the cerebrum. 

• Ventricles– The ventricles are four cavities within the brain: two lateral

ventricles within the cerebral hemispheres, the third ventricle within the thalamus and hypothalamus, and the fourth ventricle between the medulla and cerebellum. Each ventricle contains a capillary network called a choroid plexus, which forms cerebrospinal fluid (the tissue fluid of the CNS) from blood plasma.

 • MedullaMedulla• The medullaThe medulla is anterior to the cerebellum, extends from the

spinal cord to the pons, and regulates our most vital functions. Within the medulla are cardiac centers that regulate heart rate, respiratory centers that regulate breathing, and vasomotor centers that regulate the diameter of blood vessels

• and therefore blood pressure. Also in the medulla are reflex centers for coughing, sneezing, swallowing, and vomiting.

 

• PonsPons• The ponsThe pons is anterior to the upper portion of the medulla. Within

the pons are two respiratory centers that work with those in the medulla to produce a normal breathing rhythm.

• Midbrain• The midbrain extends from the pons to the hypothalamus and

encloses the cerebral aqueduct, a tunnel that connects the third and fourth ventricles. Primarily a reflex center, the midbrain regulates visual reflexes (coordinated movement of the eyes), auditory reflexes (turning the ear to a sound and righting reflexes that keep the head upright and contribute to balance. 

• Cerebellum• The cerebellum is posterior to the medulla and pons, separated

from them by the fourth ventricle; it is overlapped by the occipital lobes of the cerebrum. The functions of the cerebellum are concerned with the involuntary aspects of voluntary movement: coordination, regulation of muscle tone, the appropriate trajectory and endpoint of movements, and the maintenance of posture and balance or equilibrium. For the maintenance of balance, the cerebellum (and midbrain) uses sensory information provided by the receptors

• in the inner ear that detect movement and changes in position of the head

Hypothalamus• The hypothalamus is located above the pituitary gland and

below the thalamus. It has many diverse functions: • 1. Production of antidiuretic hormone (ADH) and oxytocin;

these hormones are then stored in the posterior pituitary gland. ADH increases the reabsorption of water by the kidneys and thus helps maintain blood volume. Oxytocin causes contractions of the myometrium of the uterus to bring about labor and delivery.

• 2. Production of releasing hormones that stimulate secretion of the hormones of the anterior pituitary gland. An example is growth hormone–releasing hormone (GHRH), which stimulates the anterior pituitary to secrete growth hormone.

• 3. Regulation of body temperature by promoting responses such as shivering in a cold environment or sweating in a warm environment.

• 4. Regulation of food intake; the hypothalamus is believed to respond to changes in blood nutrient levels or chemicals secreted by adipose tissue and bring about feelings of hunger or fullness.

• 5. Integration of the functioning of the autonomic nervous system, which is covered in a later section.

• 6. Stimulation of visceral responses in emotional situations, such as increased heart rate when angry or afraid. The neurological basis of emotions is not well understood, but the hypothalamus brings about bodily changes by way of the autonomic nervous system.

Thalamus The thalamus is above the hypothalamus and below

the cerebrum; its functions are concerned with sensation. Sensory pathways (except olfactory ones) to the brain converge in the thalamus, which begins to integrate sensations, which in turn permits more rapid interpretation by the cerebrum. The thalamus is also capable of suppressing unimportant sensations, permitting the cerebrum to concentrate without the distraction of minor sensations

CEREBRUM

• The two cerebral hemispheres form the largest part of the human brain. The right and left hemispheres are connected by the corpus callosum, a band of about 200 million nerve fibers that allows each hemisphere to know what is going on in the other. The cerebral cortex is the surface of the cerebrum; it is gray matter that consists of the cell bodies of neurons. The cerebral cortex is folded extensively into convolutions (or gyri) that permit more space for neurons. The grooves between the folds are called fissures or sulci. Interior to the gray matter is white matter.

Spinal Cord » The spinal cord transmits impulses to

and from the brain and is the integrating center for the spinal cord reflexes. The spinal cord is within the vertebral canal formed by the vertebrae and extends from the foramen magnum of the occipital bone to the disk between the first and second lumbar vertebrae.

PERIPHERAL NERVOUS SYSTEM

Cranial Nerves

• Cranial nerve I: Olfactory nerve • The olfactory nerve is composed of axons from the olfactory

receptors in the nasal sensory epithelium. It carries olfactory information (sense of smell) to the olfactory bulb of the brain. This is a pure sensory nerve fiber.

• Cranial nerve II: Optic nerve • The optic nerve is composed of axons of the ganglion cells in

the eye. It carries visual information to the brain. • Cranial nerve III: Oculomotor nerve • The oculomotor nerve is composed of motor axons coming

from the oculomotor nucleus and the edinger-westphal nucleus in the rostral midbrain located at the superior colliculus level. This is a pure motor nerve. It provides somatic motor innervation to four of the extrinsic eye muscles:

• Cranial nerve IV: Trochlear nerve • The trochlear nerve provides somatic motor innervation to the

superior oblique eye muscle.• Cranial nerve V: Trigeminal nerve • The trigeminal is the largest cranial nerve . It provides sensory

information from the face, forehead, nasal cavity, tongue, gums and teeth (touch, and temperature) and provides somatic motor innervation to the muscles of mastication or “chewing”

• Cranial nerve VI: Abducens nerve • The abducens nerve carries somatic motor innervation to one

of the extrinsic eye muscles, the lateral rectus muscle • Cranial nerve VII: Facial nerve • The facial nerve carries somatic motor innervation to the many

muscles for facial expression

• Cranial nerve VIII: Vestibulocochlear nerve • The vestibulocochlear nerve innervates the hair cell receptors

of the inner ear. It carries vestibular information to the brain from the semicircular canals, utricle, and saccule providing the sense of balance.

• Cranial nerve IX: Glossopharyngeal nerve • The glossopharyngeal nerve innervates the pharynx (upper

part of the throat), the soft palate and the posterior one-third of the tongue. It carries sensory information (touch, temperature, and pressure) from the pharynx and soft palate.

• Cranial nerve X: Vagus nerve • The vagus nerve consists of many rootlets that come off of the

brainst just behind the glossopharyngeal nerve. The branchial motor component originates from the nucleus ambiguous in the reticular formation of the medulla. Other parasympathetic ganglia include CN III , CN VII and CN IX .

• Cranial nerve XI: Spinal Accessory nerve • The spinal accessory nerve has two branches. The cranial

branch provides somatic motor innervation to some of the muscles in the throat involved in swallowing

• Cranial nerve XII: Hypoglossal nerve • The hypoglossal nerve provides somatic motor innervation to

the muscles of the tongue. This pure motor nerve originates from the hypoglossal nucleus located in the tegmentum of the medulla.

ADRENAL GLANDS

• Adrenal Medulla• The cells of the adrenal medulla are called chromaffin cells.

They secrete epinephrine and norepinephrine, which are collectively called catecholamines and are sympathomimetic (mimicking the sympathetic nervous system). Secretion of both hormones is stimulated by sympathetic impulses from the hypothalamus in stressful situations. The functions of the catecholamines mimic and prolong those of the sympathetic nervous system, which enable the individual to respond physiologically to stress situations. Of the two hormones, epinephrine is secreted in larger amounts (approximately four times that of norepinephrine) and has many effects. It increases the heart rate and force of contraction, stimulates vasoconstriction in skin and viscera and vasodilation in skeletal muscles, dilates the bronchioles, decreases peristalsis, stimulates the liver to convert glycogen to glucose, increases the use of fats for energy, and increases the rate of cell respiration. The most significant function of norepinephrine is to cause vasoconstriction in the skin, viscera, and skeletal muscles, thereby raising blood pressure.

• Adrenal Cortex• The adrenal cortex secretes three

types of steroid hormones: sex hormones, mineralocorticoids, and glucocorticoids. The sex hormones are small amounts of male androgens and even smaller amounts of female estrogens. Their function is not known with certainty, though they may contribute to the growth spurt that often occurs just before puberty and to the libido (sex drive) in adult women.

Hormones of the Adrenal Glands

• Aldosterone is the most abundant of the mineralocorticoids, and its target organs are the kidneys. Aldosterone increases the reabsorption of sodium ions and the excretion of potassium ions by the kidney tubules.

• Cortisol is the most abundant of the glucocorticoids and has many target tissues. Cortisol stimulates the liver to change glucose to glycogen (glycogenesis) for storage. It increases the conversion of excess amino acids to carbohydrates (gluconeogenesis) for energy production and increases the use of fats for energy. By providing these

• secondary energy sources to most cells, cortisol ensures that whatever glucose is present will be available for the brain (the glucose-sparing effect). Cortisol also has an anti-inflammatory effect because it blocks the effects of histamine and stabilizes the lysosomes in cells. Normal cortisol secretion seems to limit the inflammation process to what is useful for tissue repair and to prevent excessive tissue destruction. Excess cortisol has damaging effects, however: It raises blood glucose levels, decreases the immune response, and delays healing of damaged

• tissue. The direct stimulus for cortisol secretion is ACTH from the anterior pituitary gland. Cortisol is also a “stress” hormone, and any type of physiological stress (injury, disease, malnutrition) stimulates the hypothalamus to secrete CRH. CRH increases the secretion of ACTH by the anterior pituitary, which increases cortisol secretion by the adrenal cortex.