The Nervous System Divisions PDF

**The nervous system is divided into two anatomical divisions:** **(i) Central nervous system (CNS) which consists of:** - **Brain** - **Spinal cord** **(ii) Peripheral nervous system (PNS; neurons outside the CNS) which consists of:** - **Afferent division -- neurons which bring signals...

**The nervous system is divided into two anatomical divisions:** **(i) Central nervous system (CNS) which consists of:** - **Brain** - **Spinal cord** **(ii) Peripheral nervous system (PNS; neurons outside the CNS) which consists of:** - **Afferent division -- neurons which bring signals from the periphery to the CNS** - **[Efferent] (motor) division - neurons carrying signals away from the brain and spinal cord to the peripheral tissues** **The [efferent] division is further divided into two major subdivisions:** **[Somatic]**: **The somatic nervous system differs from the autonomic system in that it consist of a single myelinated motor neuron, originating in the CNS**; **the neuron travels directly to skeletal muscle *[without the mediation of ganglia]*. The somatic nervous system is under *[voluntary control]*, whereas the autonomic nervous system is an involuntary system.** **[Autonomic]: This is also called the visceral, vegetative, or *[involuntary nervous system]*.** It is **largely autonomous (independent) in that its activities are not under direct conscious control** and it is **concerned primarily with visceral (organs in the abdominal cavity) functions that are necessary for life such as cardiac output, blood flow to various organs, digestion, etc** **ANS is further subdivided into three systems, i.e:** - **Sympathetic nervous system** - **Parasympathetic nervous system** - **Enteric nervous system** *[a) Anatomy of the Autonomic Nervous System]* **There are two types of efferent ANS neurons responsible for transmitting nerve impulses from the CNS to the effector organs:** **1) Preganglionic neurons:** **They emerge from the brainstem or spinal cord and make a synaptic connection in ganglia (an aggregation of nerve cell bodies located in the peripheral nervous system)**. **Their cell bodies are located within the CNS**. **2) Postganglionic neurons** **The latter neurons have cell bodies originating in the ganglia**. **They are generally non-myelinated and terminate on effector organs, such as smooth muscles of the viscera, cardiac muscle, and the exocrine glands.** *[b) Functions of the sympathetic nervous system]* **It prepares the body in response to stressful situations, such as trauma, fear, hypoglycemia, cold, or exercise.** **Stimulation of the sympathetic division increases the heart rate and blood pressure thus mobilising energy stores of the body. Also increase the blood flow to skeletal muscles and the heart, while diverting the blood flow from the skin and internal organs.** **"Fight or flight" response (in emergency) triggered both by direct sympathetic activation of** **the effector organs, and by stimulation of the adrenal medulla to release adrenaline (A) and lesser amounts of noradrenaline (NA).** **NA & A enter the bloodstream and promote responses in effector organs that contain adrenergic receptors throughout the body.** *[c) Functions of the parasympathetic nervous system]* **The parasympathetic division maintains essential bodily functions required for life such as the conservation of energy & maintenance of organ function during periods of minimal activity. It usually acts to oppose or balance the actions of the sympathetic division**. It is **generally dominant over the sympathetic system in \"rest and digest" situations.** **The parasympathetic division slows the heart rate, lowers the BP, stimulates the GIT movements & excretion, aids absorption of nutrients, protects the retina from excessive light, & empties the urinary bladder & rectum.** *[d) Innervation by the ANS]* **Most organs in the body are innervated by both divisions of the autonomic nervous system (sympathetic & parasympathetic). Despite this dual innervation, one system usually predominates in controlling the activity of a given organ** **e.g. in the heart, the parasympathetic is the predominant factor for controlling the heart rate.** **Organs receiving only sympathetic innervation:** - **some effector organs, such as the adrenal medulla, blood vessels, and sweat glands, receive innervation only from the sympathetic system** *[e) Chemical signalling between cells]* **Chemical communication between cells (including those of the ANS) occurs through the use of chemical mediators which are classified as follows:** - **Local hormones**: **Secreted by most cells in the body**. **They act locally and are rapidly destroyed; they do not enter the bloodstream** **e.g. histamine secreted by the mast cells during inflammation.** - **Hormones**: **Secreted by the endocrine glands into the bloodstream distributed throughout the body.** - **Neurotransmitters: Released from the nerve terminals - triggered by the arrival of the action potential at the nerve ending. They are responsible for the communication between:** - **the nerve cells (ganglion), and nerve cell and the effector organ (neuroeffector)** - **They rapidly diffuse across the synaptic cleft or gap (synapse) and combine with (bind to) specific receptors on the postsynaptic (target) cell. Like local hormones, they are rapidly destroyed** *[f) Neurotransmitter Chemistry of the ANS]* **Autonomic nerves are classified on the basis of their primary transmitter molecules:** - **Acetylcholine (Ach)** - **Noradrenaline (NA)** **Nerves that synthesise and release Ach are referred to as cholinergic neurons (fibers),** **while those synthesising and secreting NA are referred to as noradrenergic (or simply "adrenergic") fibers.** *1. Cholinergic transmission* **Ach is a NT at autonomic ganglia in both the sympathetic** and **parasympathetic nervous systems.** **Also a NT at the adrenal medulla where it modulates the sympathetic nervous system.** **Also act at autonomic neuroeffector junction (between postganglionic nerves and the effector organs) in the parasympathetic nervous system. In the somatic nervous system, transmission at the neuromuscular junction (that is, between nerve fibers and voluntary muscles) is also cholinergic.** *2. **Adrenergic fibers constitute both NA and A*** **NA is a transmitter at autonomic neuroeffector junction (between postganglionic nerves and the effector organs) in the sympathetic system, with the exception of the sweat glands** **where the neurotransmitter is Ach.** **Both NA and A are also synthesised and released by the adrenal medulla following cholinergic stimulation to control (increase) BP.** *[g) Autonomic Receptors]* **All neurotransmitters are too hydrophilic to penetrate the cell membranes lipid bilayer of target-cell**. **Instead, their signal is mediated by binding to specific receptors on the cell surface of target organs** **There are two main types of receptors in the ANS:** - **Cholinergic receptors** - **Adrenergic receptors** ***1. Cholinergic receptors*** **The primary ACh (cholinergic) receptor subtypes were named after the alkaloids originally used in their identification:** - **Muscarine - Muscarinic receptors (M)** - **Nicotine -- Nicotinic receptors (N).** ***2. Adrenergic receptors*** **Adrenoceptor or simply adrenergic receptors respond to catecholamines such as NA, A, & DA. The adrenoceptors can be subdivided into two main divisions on the basis of their agonist and antagonist selectivity:** - **Alpha-adrenergic receptors (α-ARs)** - **Further subdivided into α~1~-& α~2~-ARs** - **The rank order of potency of agonists for α-ARs is A \> NA ≥ isoproterenol** - **Beta-adrenergic receptors (β-ARs)** - **Further subdivided into β~1~-, β~2~- & β~3~-ARs** - **The rank order of potency of agonists for β-ARs is isoproterenol \> A ≥ NA**