Category: 6. Nervous system

Spinal ganglia of spinal nervesGanglia spinalia nervorum spinalium1/4Synonyms: Dorsal root ganglia of spinal nerves, Sensory

Ganglia (sing. ganglion) are clusters of neuronal cell bodies outside of the CNS, meaning that they are the PNS equivalents to subcortical nuclei of the CNS. Ganglia can be sensory or visceral motor (autonomic) and their distribution in the body is clearly defined.

Dorsal root ganglia are clusters of sensory nerve cell bodies located adjacent to the spinal cord, They are a component of the posterior root of a spinal nerve.

Autonomic ganglia are either sympathetic or parasympathetic. Sympathetic ganglia are found in the thorax and abdomen, grouped into paravertebral and prevertebral ganglia. Paravertebral ganglia lie on either side of vertebral column (para- means beside), comprising two ganglionic chains that extend from the base of the skull to the coccyx, called sympathetic trunks. Prevertebral ganglia (collateral ganglia, preaortic ganglia) are found anterior to the vertebral column (pre- means in front of), closer to their target organ. They are further grouped according to which branch of abdominal aorta they surround; celiac, aorticorenal, superior and inferior mesenteric ganglia.

Parasympathetic ganglia are found in the head and pelvis. Ganglia in the head are associated with relevant cranial nerves and are the ciliary, pterygopalatine, otic and submandibular ganglia. Pelvic ganglia lie close to the reproductive organs comprising autonomic plexuses for innervation of pelvic viscera, such as prostatic and uterovaginal plexuses.

Spinal nervesemerge from the segments of the spinal cord. They are numbered according to their specific segment of origin. Hence, the 31 pairs of spinal nerves are divided into 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal spinal nerve. All spinal nerves are mixed, containing both sensory and motor fibers.

Spinal nerves innervate the entire body, with the exception of the head. They do so by either directly synapsing with their target organs or by interlacing with each other and forming plexuses. There are four major plexuses that supply the body regions; 

• Cervical plexus (C1-C4) – innervates the neck 
• Brachial plexus (C5-T1) – innervates the upper limb 
• Lumbar plexus (L1-L4) – innervates the lower abdominal wall, anterior hip and thigh 
• Sacral plexus (L4-S4) – innervates the pelvis and the lower limb

The PNS consists of 12 pairs of cranial nerves, 31 pairs of spinal nerves and a number of small neuronal clusters throughout the body called ganglia.
Peripheral nerves can be sensory (afferent), motor (efferent) or mixed (both). Depending on what structures they innervate, peripheral nerves can have the following modalities;

• Special – innervating special senses (e.g. eye) and is found only in afferent fibers
• General – supplying everything except special senses
• Somatic – innervates the skin and skeletal muscles (e.g. biceps brachii)
• Visceral – supplies internal organs. 

Cranial nerves

Cranial nerves are peripheral nerves that emerge from the cranial nerve nuclei of the brainstem and spinal cord. They innervate the head and neck. Cranial nerves are numbered one to twelve according to their order of exit through the skull fissures. Namely, they are: olfactory nerve (CN I), optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), trigeminal nerve (CN V), abducens nerve (VI), facial nerve (VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), and hypoglossal nerve (XII). These nerves are motor (III, IV, VI, XI, and XII), sensory (I, II and VIII) or mixed (V, VII, IX, and X).

The central nervous system (CNS) consists of the brain and spinal cord. These are found housed within the skull and vertebral column respectively.

The brain is made of four parts; cerebrum, diencephalon, cerebellum and brainstem. Together these parts process the incoming information from peripheral tissues and generate commands; telling the tissues how to respond and function. These commands tackle the most complex voluntary and involuntary human body functions, from breathing to thinking.

The spinal cord continues from the brainstem. It also has the ability to generate commands but for involuntary processes only, i.e. reflexes. However, its main function is to pass information between the CNS and periphery. 

So nervous tissue, comprised of neurons and neuroglia, forms our nervous organs (e.g. the brain, nerves). These organs unite according to their common function, forming the evolutionary perfection that is our nervous system. 

The nervous system (NS) is structurally broken down into two divisions; 

• Central nervous system (CNS) – consists of the brain and spinal cord
• Peripheral nervous system (PNS) – gathers all neural tissue outside the CNS

Functionally, the PNS is further subdivided into two functional divisions; 

• Somatic nervous system (SNS) –  informally described as the voluntary system
• Autonomic nervous system (ANS) – described as the involuntary system. 

Although divided structurally into central and peripheral parts, the nervous system divisions are actually interconnected with each other. Axon bundles pass impulses between the brain and spinal cord. These bundles within the CNS are called afferent and efferent neural pathways or tracts. Axons that extend from the CNS to connect with peripheral tissues belong to the PNS. Axons bundles within the PNS are called afferent and efferent peripheral nerves.

Cerebral cortexCortex cerebri1/4Synonyms: Brain cortex, Cortical grey matter

The white color of myelinated axons is distinguished from the gray colored neuronal bodies and dendrites. Based on this, nervous tissue is divided into white matter and gray matter, both of which has a specific distribution; 

• White matter comprises the outermost layer of the spinal cord and the inner part of the brain.
• Gray matter is located in the central part of the spinal cord, outermost layer of the brain (cerebral cortex), and in several subcortical nuclei of the brain deep to the cerebral cortex.

Glial cellsNeuroglia1/5Synonyms: Neuroglia

Glial cells, also called neuroglia or simply glia, are smaller non-excitatory cells that act to support neurons. They do not propagate action potentials. Instead, they myelinate neurons, maintain homeostatic balance, provide structural support, protection and nutrition for neurons throughout the nervous system. 

This set of functions is provided for by four different types of glial cells;

• Myelinating glia produce the axon-insulating myelin sheath. These are called oligodendrocytes in the CNS and Schwann cells in the PNS. Remember these easily with the mnemonic “COPS” (Central – Oligodendrocytes; Peripheral – Schwann)
• Astrocytes (CNS) and satellite glial cells (PNS) both share the function of supporting and protecting neurons. 
• Other two glial cell types are found in CNS exclusively; microglia are the phagocytes of the CNS and ependymal cells which line the ventricular system of the CNS. The PNS doesn’t have a glial equivalent to microglia as the phagocytic role is performed by macrophages.

Most axons are wrapped by a white insulating substance called the myelin sheath, produced by oligodendrocytes and Schwann cells. Myelin encloses an axon segmentally, leaving unmyelinated gaps between the segments called the nodes of Ranvier. The neural impulses propagate through the Ranvier nodes only, skipping the myelin sheath. This significantly increases the speed of neural impulse propagation. 

The morphology of neurons makes them highly specialized to work with neural impulses; they generate, receive and send these impulses onto other neurons and non-neural tissues. 

SynapseSynapsis1/2

There are two types of neurons, named according to whether they send an electrical signal towards or away from the CNS;

• Efferent neurons (motor or descending) send neural impulses from the CNS to the peripheral tissues, instructing them how to function. 
• Afferent neurons (sensory or ascending) conduct impulses from the peripheral tissues to the CNS. These impulses contain sensory information, describing the tissue’s environment.

The site where an axon connects to another cell to pass the neural impulse is called a synapse. The synapse doesn’t connect to the next cell directly. Instead, the impulse triggers the release of chemicals called neurotransmitters from the very end of an axon. These neurotransmitters bind to the effector cell’s membrane, causing biochemical events to occur within that cell according to the orders sent by the CNS.

Two basic types of cells are present in the nervous system; 

• Neurons
• Glial cells

Neurons

Neurons, or nerve cell, are the main structural and functional units of the nervous system. Every neuron consists of a body (soma) and a number of processes (neurites). The nerve cell body contains the cellular organelles and is where neural impulses (action potentials) are generated. The processes stem from the body, they connect neurons with each other and with other body cells, enabling the flow of neural impulses. There are two types of neural processes that differ in structure and function; 

• Axons are long and conduct impulses away from the neuronal body. 
• Dendrites are short and act to receive impulses from other neurons, conducting the electrical signal towards the nerve cell body.

Every neuron has a single axon, while the number of dendrites varies. Based on that number, there are four structural types of neurons; multipolar, bipolar, pseudounipolar and unipolar. 

The nervous system is a network of neurons whose main feature is to generate, modulate and transmit information between all the different parts of the human body. This property enables many important functions of the nervous system, such as regulation of vital body functions (heartbeat, breathing, digestion), sensation and body movements. Ultimately, the nervous system structures preside over everything that makes us human; our consciousness, cognition, behaviour and memories.

The nervous system consists of two divisions; 

• Central nervous system (CNS) is the integration and command center of the body
• Peripheral nervous system (PNS) represents the conduit between the CNS and the body. It is further subdivided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). 

Definition	A network of neurons that sends, receives and modulates neural impulses between different body parts.
Divisions	Central nervous system Peripheral nervous system
Central nervous system	Brain and spinal cord
Peripheral nervous system	Spinal and cranial nerves. Functional divisions: – Somatic nervous system – Autonomic nervous system; sympathetic, parasympathetic and enteric divisions