The Nervous System Has Peripheral and Central Units

• The central nervous system (CNS) is the brain and spinal column
• The peripheral nervous system (PNS) consists of nerves outside of the CNS
• There are 31 pairs of spinal nerves (mixed motor & sensory)
• There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)

The pattern of innervation plotted on the skin is called a dermatome

The Nervous System Has Peripheral and Central Units

• The central nervous system (CNS) is the brain and spinal column
• The peripheral nervous system (PNS) consists of nerves outside of the CNS
• There are 31 pairs of spinal nerves (mixed motor & sensory)
• There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)

The pattern of innervation plotted on the skin is called a dermatome

The pituitary gland is pea-sized structure located at the base of the brain. In humans, it consists of two lobes:

• the Anterior Lobe and
• the Posterior Lobe

The Anterior Lobe

The anterior lobe contains six types of secretory cells All of them secrete their hormone in response to hormones reaching them from the hypothalamus of the brain.

Thyroid Stimulating Hormone (TSH)

TSH (also known as thyrotropin) is a glycoprotein The secretion of TSH is

• stimulated by the arrival of thyrotropin releasing hormone (TRH) from the hypothalamus.
• inhibited by the arrival of somatostatin from the hypothalamus.

 TSH stimulates the thyroid gland to secrete its hormone thyroxine (T₄).

Some develop antibodies against their own TSH receptors making more T₄ causing hyperthyroidism. The condition is called thyrotoxicosis or Graves' disease.

Hormone deficiencies

A deficiency of TSH causes hypothyroidism: inadequate levels of T₄ (and thus of T₃ )..

Follicle-Stimulating Hormone (FSH)

FSH is a heterodimeric glycoprotein Synthesis and release of FSH is triggered by the arrival from the hypothalamus of gonadotropin-releasing hormone (GnRH).

FSH in females :In sexually-mature females, FSH (assisted by LH) acts on the follicle to stimulate it to release estrogens.

FSH in males :In mature males, FSH acts on spermatogonia stimulating (with the aid of testosterone) the production of sperm.

Luteinizing Hormone (LH)

LH is synthesized within the same pituitary cells as FSH and under the same stimulus (GnRH). It is also a heterodimeric glycoprotein

LH in females

In sexually-mature females, LH

• stimulates the follicle to secrete estrogen in the first half of the menstrual cycle
• a surge of LH triggers the completion of meiosis I of the egg and its release (ovulation) in the middle of the cycle
• stimulates the now-empty follicle to develop into the corpus luteum, which secretes progesterone during the latter half of the menstrual cycle.

LH in males

LH acts on the interstitial cells (also known as Leydig cells) of the testes stimulating them to synthesize and secrete the male sex hormone, testosterone.

LH in males is also known as interstitial cell stimulating hormone (ICSH).

Prolactin (PRL)

Prolactin is a protein of 198 amino acids. During pregnancy it helps in the preparation of the breasts for future milk production. After birth, prolactin promotes the synthesis of milk.

Prolactin secretion is

• stimulated by TRH
• repressed by estrogens and dopamine.

Growth Hormone (GH)

• Human growth hormone (also called somatotropin) is a protein
• The GH-secreting cells are stimulated to synthesize and release GH by the intermittent arrival of growth hormone releasing hormone (GHRH) from the hypothalamus. GH promotes body growth

In Child

• hyposecretion of GH produces dwarfism
• hypersecretion leads to gigantism

In adults, a hypersecretion of GH leads to acromegaly.

ACTH — the adrenocorticotropic hormone

ACTH acts on the cells of the adrenal cortex, stimulating them to produce

• glucocorticoids, like cortisol
• mineralocorticoids, like aldosterone
• androgens (male sex hormones, like testosterone

Hypersecretion of ACTH cause of Cushing's disease.

Oxygen Transport

In adult humans the hemoglobin (Hb) molecule

• consists of four polypeptides:
  • two alpha (α) chains of 141 amino acids and
  • two beta (β) chains of 146 amino acids
• Each of these is attached the prosthetic group heme.
• There is one atom of iron at the center of each heme.
• One molecule of oxygen can bind to each heme.

The reaction is reversible.

• Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
• Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.

The defecation reflex:

As a result of the mass movements, pressure is exerted on the rectum and on the internal anal sphincter, which is smooth muscle, resulting in its involuntary relaxation. Afferent impulses are sent to the brain indicating the need to defecate. The external sphincter is voluntary muscle and is controlled by the voluntary nervous system. This sphincter is relaxed along with contraction of the rectal and abdominal muscles in the defecation reflex

A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.

Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.

Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.

Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.

Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects 

Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.

1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.

2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.

3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.

4) Ventricular repolarization occurs.

Neurons :

Types of neurons based on structure:

a multipolar neuron because it has many poles or processes, the dendrites and the axon. Multipolar neurons are found as motor neurons and interneurons. There are also bipolar neurons with two processes, a dendrite and an axon, and unipolar neurons, which have only one process, classified as an axon.. Unipolar neurons are found as most of the body's sensory neurons. Their dendrites are the exposed branches connected to receptors, the axon carries the action potential in to the central nervous system.

Types of neurons based on function:

• motor neurons - these carry a message to a muscle, gland, or other effector. They are said to be efferent, i.e. they carry the message away from the central nervous system.
• sensory neurons - these carry a message in to the CNS. They are afferent, i.e. going toward the brain or spinal cord.
• interneuron (ie. association neuron, connecting neuron) - these neurons connect one neuron with another. For example in many reflexes interneurons connect the sensory neurons with the motor neurons.