Chapter 7: The nervous system Flashcards
What is the nervous system?
The nervous system detects and responds to changes inside and outside the body. Together with the endocrine system, it coordinates and controls vital aspects of body function and maintains homeostasis.
What does the nervous system consist of?
The nervous system consists of the brain, the spinal cord, and peripheral nerves. The structure and organization of the tissues that form these components enable rapid communication between all parts of the body.
How is the nervous system grouped?
The parts of the nervous system are grouped as follows:
- the central nervous system (CNS), consisting of the brain and the spinal cord
- the peripheral nervous system (PNS), consisting of all the nerves outside the brain and spinal cord.
Peripheral nervous system (PNS)
The peripheral nervous system (PNS) is the division of the nervous system containing all the nerves that lie outside of the central nervous system (CNS). The primary role of the PNS is to connect the CNS to the organs, limbs, and skin. These nerves extend from the central nervous system to the outermost areas of the body.
Somatic nervous system
Controls voluntary movement of skeletal muscles.
Autonomic nervous system
The autonomic nervous system, controlling involuntary processes such as heartbeat, peristalsis, and glandular activity. The autonomic nervous system has two divisions: sympathetic and parasympathetic.
Cells and tissues of the nervous system
There are two types of nervous tissue, neurons, and neuroglia. Neurons (nerve cells) are the working units of the nervous system that generate and transmit nerve impulses. Neurons are supported by connective tissue, collectively known as neuroglia, which is formed from different types of glial cells.
Neurons
The neuron is a nerve cell that is the basic building block of the nervous system.
Cell bodies
Cell bodies form the grey matter of the nervous system and are found at the periphery of the brain and in the center of the spinal cord. Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.
Axons and dendrites
Axons and dendrites are extensions of cell bodies and form the white matter of the nervous system. Axons are found deep in the brain and in groups, called tracts, at the periphery of the spinal cord. They are referred to as nerves or nerve fibers outside the brain and spinal cord.
Axons
Each nerve cell has only one axon, which begins at a tapered area of the cell body, the axon hillock. They carry impulses away from the cell body and are usually longer than the dendrites, sometimes if 100 cm.
Structure of an axon
The membrane of the axon is called the axolemma and it encloses the cytoplasmic extension of the cell body.
Myelinated neurons
The myelinated neurons are those neurons whose axons are covered with the layer of Schwann cell membranes called the myelin sheath. The myelin sheath plays the role in faster transmission of impulse or the action potential. The neurons with myelin sheath are found in the peripheral nervous system (Mostly in motor and sensory neurons).
Unmyelinated neurons
Unmyelinated neurons can be found in both the peripheral and central nervous systems in the group c nerve fibers, responsible for the transmission of secondary pain or itch. Unmyelinated nerve fibers can lose the nerve impulse during conduction.
Dendrites
These are the many short processes that receive and carry incoming impulses towards cell bodies. They have the same structure as axons but are usually shorter and branching. In motor neurons dendrites form part of synapses and in sensory neurons they form the sensory receptors that respond to specific stimuli.
The nerve impulse
The nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulses are mostly electrical signals along the dendrites to produce a nerve impulse or action potential. The action potential is the result of ions moving in and out of the cell.
The synapse and neurotransmitters
Synapse, also called neuronal junction, is the site of transmission of electric nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell (effector).
A chemical substance that is released at the end of a nerve fiber by the arrival of a nerve impulse and, by diffusing across the synapse or junction, causes the transfer of the impulse to another nerve fiber, a muscle fiber, or some other structure.
Nerves
A nerve consists of numerous neurons collected into bundles (bundles of nerve fibers in the central nervous system are known as tracts).
Each bundle has several coverings of protective connective tissue:
-endoneurium is a delicate tissue, surrounding each individual fiber, which is continuous with the septa that pass inwards from the perineurium
-perineurium is a smooth connective tissue, surrounding each bundle of fibers
-epineurium is the fibrous tissue that surrounds and encloses several bundles of nerve fibers. Most large nerves are covered by epineurium.
Sensory and afferent nerves
Specialized endings of sensory neurons respond to different stimuli (changes) inside and outside the body.
sensory receptors
These originate from the skin. They are pain, touch, heat, and cold. Sensory nerve endings in the skin are fine branching filaments without myelin sheaths. When stimulated, an impulse is generated and transmitted by the sensory nerves to the brain where the sensation is perceived.
somatic, cutaneous, or common senses
These originate in muscles and joints. Impulses sent to the brain enable perception of the position of the body and its parts in space-maintaining posture and balance
Proprioceptors senses
These originate in muscles and joints. Impulses sent to the brain enable perception of the position of the body and its parts in space-maintaining posture and balance
Autonomic afferent nerves
These originate in internal organs, glands, and tissues, e.g., baroreceptors involved in the control of blood pressure, chemoreceptors involved in the control of respiration, and are associated with reflex regulation of involuntary activity and visceral pain.
Motor or efferent nerves
Motor nerves originate in the brain, spinal cord, and autonomic ganglia. They transmit impulses to the effector organs: muscles and glands. There are two types:
- somatic nerves – involved in voluntary and reflex skeletal muscle contraction
- autonomic nerves (sympathetic and parasympathetic) – involved in cardiac and smooth muscle contraction and glandular secretion.
Mixed nerves
In the spinal cord, sensory and motor nerves are arranged in separate groups or tracts. Outside the spinal cord, when sensory and motor nerves are enclosed within the same sheath of connective tissue, they are called mixed nerves.
Neuroglia
The neurons of the central nervous system are supported by non-excitable glial cells that greatly outnumber the neurons. Unlike nerve cells, which cannot divide, glial cells continue to replicate throughout life. There are four types: astrocytes, oligodendrocytes, ependymal cells, and microglia.
Astrocytes
Astrocytes are a sub-type of glial cells in the central nervous system. They are also known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. In humans, a single astrocyte cell can interact with up to 2 million synapses at a time.
Oligodendrocytes
These cells are smaller than astrocytes and are found in clusters around nerve cell bodies in grey matter, where they are thought to have a supportive function. They are found adjacent to and along the length of, myelinated nerve fibers. Oligodendrocytes form and maintain myelin like Schwann cells in peripheral nerves.
Ependymal cells
These cells form the epithelial lining of the ventricles of the brain and the central canal of the spinal cord. Those cells that form the choroid plexuses of the ventricles secrete the cerebrospinal fluid.
Microglia
The smallest and least numerous glial cells, these cells may be derived from monocytes that migrate from the blood into the nervous system before birth. They are found mainly in the area of blood vessels. They enlarge and become phagocytic, removing microbes and damaged tissue, in areas of inflammation and cell destruction.
Response of nervous tissue to injury
Damage to neuroses can either lead to rapid necrosis with sudden acute functional failure or slow atrophy with gradually increasing dysfunction. These changes are associated with:
- hypoxia and anoxia
- nutritional deficiencies
- trauma
- infections
- aging
Peripheral nerve regeneration
Peripheral nervous system regeneration. Neuroregeneration in the peripheral nervous system (PNS) occurs to a significant degree. After an injury to the axon, peripheral neurons activate a variety of signaling pathways which turn on pro-growth genes, leading to the reformation of a functional growth cone and regeneration.
Neuroglial damage (Astrocytes)
When these cells are damaged, their processes multiply forming a mesh or ‘scar’, which is thought to inhibit the regrowth of damaged CNS neurons.
Central nervous system
The central nervous system consists of the brain and the spinal cord. These essential structures are both well protected from damage and injury; the brain is enclosed within the skull and the spinal cord by the vertebrae that form the spinal column. Membranous coverings known as the meninges provide further protection.
The meninges
The brain and spinal cord are surrounded by three layers of tissue, the meninges, lying between the skull and the brain, and between the vertebral foramina and the spinal cord. Named from outside inwards they are the:
- dura mater
- arachnoid mater
- pia mater
Dura mater
Dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. The other two meningeal layers are the arachnoid mater and the pia mater.
arachnoid mater
A fine, delicate membrane, the middle one of the three membranes or meninges that surround the brain and spinal cord, is situated between the dura mater and the pia mater.
Pia mater
The delicate innermost membrane enveloping the brain and spinal cord
Ventricles of the brain and cerebrospinal fluid
The brain contains four irregular-shaped cavities, or ventricles, containing cerebrospinal fluid (CSF). They are:
- right and left lateral ventricles
- third ventricle
- fourth ventricle.
The lateral ventricles
The lateral ventricles are the largest in the series of four interconnecting fluid-filled cavities within the brain
The third ventricle
The third ventricle is a cavity situated below the lateral ventricles between the two parts of the thalamus. It communicates with the fourth ventricle by a canal, the cerebral aqueduct.
The fourth ventricle
The fourth ventricle is a diamond-shaped cavity situated below and behind the third ventricle, between the cerebellum and pons. It is continuous below the central canal of the spinal cord and communicates with the subarachnoid space by foramina in its roof. Cerebrospinal fluid enters the subarachnoid space through these openings and through the open distal end of the central canal of the spinal cord.
Cerebrospinal fluid
Clear watery fluid which fills the space between the arachnoid membrane and the pia mater.
Functions of cerebrospinal fluid
CSF supports and protects the brain and spinal cord by maintaining a uniform pressure around these vital structures and acting as a cushion or shock absorber between the brain and the skull.
It keeps the brain and spinal cord moist and there may be an exchange of nutrients and waste products between CSF and the interstitial fluid of the brain. CSF is thought to be involved in the regulation of breathing as it bathes the surface of the medulla where the central respiratory chemoreceptors are located.
Brain
An organ of soft nervous tissue contained in the skull of vertebrates, functioning as the coordinating center of sensation and intellectual and nervous activity. Its parts are: (Diencephalon) -cerebrum -thalamus -hypothalamus (Brain stem) -midbrain -pons -medulla oblongata -cerebellum
Blood supply and venous drainage
The circulus arteriosus and its contributing arteries play a vital role in maintaining a constant supply of oxygen and glucose to the brain when the head is moved and if a contributing artery is narrowed.
Venous blood from the brain drains into the Dural venous sinuses and then downwards into the internal jugular veins
Cerebrum
The principal and most anterior part of the brain in vertebrates, located in the front area of the skull and consisting of two hemispheres, left and right, separated by a fissure. It is responsible for the integration of complex sensory and neural functions and the initiation and coordination of voluntary activity in the body.
Cerebral tracts
Tracts are neural pathways that are in the brain and spinal cord (central nervous system).
Tracts are formed by neurons synapsing onto one another, and these neurons can be classified as first-order, second-order, and third-order neurons depending on their location and order within the tract.
Basal ganglia
The basal ganglia are groups of cell bodies that lie deep within the brain and form part of the extrapyramidal tracts. They act as relay stations with connections to many parts of the brain including motor areas of the cerebral cortex and thalamus. Their functions include initiation and fine control of complex movement and learned coordinated activities, such as posture and walking. If control is inadequate or absent, movements are jerky, clumsy and uncoordinated.
Functions of the cerebral cortex
There are three main types of activity associated with the cerebral cortex:
- higher-order functions, i.e., the mental activities involved in memory, sense of responsibility, thinking, reasoning, moral decision making, and learning
- sensory perception, including the perception of pain, temperature, touch, sight, hearing, taste, and smell
- initiation and control of skeletal muscle contraction and therefore voluntary movement.
Functions areas of the cerebral cortex
There are different types of functional areas:
-motor, which directs skeletal (voluntary) muscle movements
-sensory, which receive and decode sensory impulses enabling sensory perception
-association, which are concerned with the integration and processing of complex mental functions such as intelligence, memory, reasoning, judgment, and emotions.
In general, areas of the cortex lying anterior to the central sulcus are associated with motor functions, and those lying posterior to it are associated with sensory functions
The primary motor area
The primary motor cortex, located just in front of the central sulcus, is the area that provides the most important signal to produce skilled movements. Electrical stimulation of this area results in focal movements of muscle groups on the opposite side of the body, depending on the area stimulated
Motor speech (Broca’s) area
This is situated in the frontal lobe just above the lateral sulcus and controls the muscle movements needed for speech. It is dominant in the left hemisphere in right-handed people and vice versa.
The somatosensory area
The primary somatosensory area in the human cortex is in the postcentral gyrus of the parietal lobe. This is the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus at this location.
The auditory (hearing) area
This lies immediately below the lateral sulcus within the temporal lobe. The nerve cells receive and interpret impulses transmitted from the inner ear by the cochlear (auditory) part of the vestibulocochlear nerves (8th cranial nerves).
The olfactory (smell) area
This lies deep within the temporal lobe where impulses from the nose, transmitted via the olfactory nerves (1st cranial nerves), are received and interpreted.
The tasting area
This lies just above the lateral sulcus in the deep layers of the somatosensory area. Here, impulses from sensory receptors in taste buds are received and perceived as taste.
The visual area
This lies behind the parieto-occipital sulcus and includes the greater part of the occipital lobe. The optic nerves (2nd cranial nerves) pass from the eye to this area, which receives and interprets the impulses as visual impressions.
Association areas
These are connected to each other and other areas of the cerebral cortex by association tracts and some are outlined below. They receive, coordinate and interpret impulses from the sensory and motor cortices permitting higher cognitive abilities
The premotor area
This lies in the frontal lobe immediately anterior to the motor area. The neurons here coordinate movement initiated by the primary motor cortex, ensuring that learned patterns of movement can be repeated.
Sensory speech (Wernicke’s) area
This is situated in the temporal lobe adjacent to the parietal-occipitotemporal area. It is here that the spoken word is perceived, and comprehension and intelligence are based. Understanding language is central to higher mental functions as they are language-based. This area is dominant in the left hemisphere in right-handed people and vice versa.
The prefrontal area
This extends anteriorly from the premotor area to include the remainder of the frontal lobe. It is a large area and is more highly developed in humans than in other animals. Intellectual functions controlled here include perception and comprehension of the passage of time, the ability to anticipate consequences of events, and the normal management of emotions
The parieto-occipitotemporal area
This lies behind the somatosensory area and includes most of the parietal lobe. Its functions are thought to include spatial awareness, interpreting written language, and the ability to name objects. It has been suggested that objects can be recognized by touch alone because of the knowledge from experience (memory) retained in this area.
Diencephalon
This connects the cerebrum and the midbrain. It consists of several structures situated around the third ventricle, the main ones being the thalamus and hypothalamus, which are considered here. The pineal gland and the optic chiasma are situated there.
Thalamus
The thalamus is a small structure within the brain located just above the brain stem between the cerebral cortex and the midbrain and has extensive nerve connections to both. The primary function of the thalamus is to relay motor and sensory signals to the cerebral cortex.
Hypothalamus
A region of the forebrain below the thalamus which coordinates both the autonomic nervous system and the activity of the pituitary, controlling body temperature, thirst, hunger, and other homeostatic systems, and involved in sleep and emotional activity.
The brain stem (midbrain)
The midbrain is the area of the brain situated around the cerebral aqueduct between the cerebrum above and the pons below. It consists of nuclei and nerve fibers (tracts), which connect the cerebrum with lower parts of the brain and with the spinal cord. The nuclei act as relay stations for the ascending and descending nerve fibers and have important roles in auditory and visual reflexes.
Pons
The part of the brainstem that links the medulla oblongata and the thalamus.
There are nuclei within the pons that act as relay stations and some of these are associated with the cranial nerves
Medulla oblongata
The medulla oblongata or simply medulla is a long stem-like structure that makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing.
Decussation (crossing) the pyramids
The medulla, motor nerves descending from the motor area in the cerebrum to the spinal cord in the pyramidal (corticospinal) tracts cross from one side to the other. This means that the left hemisphere of the cerebrum controls the right half of the body and vice versa. These tracts are the main pathway to skeletal (voluntary) muscles.
Sensory decussation
Some of the sensory nerves ascending to the cerebrum from the spinal cord cross from one side to the other in the medulla. Others decussate lower down in the spinal cord.
The cardiovascular center (CVC)
This area controls the rate and force of cardiac contraction. It also controls blood pressure. Within the CVC, other groups of nerve cells forming the vasomotor center control the diameter of the blood vessels, especially the small arteries and arterioles. The vasomotor center is stimulated by the arterial baroreceptors, body temperature, and emotions such as sexual excitement and anger.
The respiratory center
This area controls the rate and depth of respiration. From here, nerve impulses pass to the phrenic and intercostal nerves which stimulate contraction of the diaphragm and intercostal muscles, thus initiating inspiration. It functions in close association with the pnuemotaxic and apneustic centers in the pons
Reflex centers
Irritants present in the stomach or respiratory tract stimulate the medulla oblongata, activating the reflex centers. Vomiting, coughing and sneezing are protective reflexes that attempt to expel irritants.
Reticular formation
The reticular formation is a collection of neurons in the core of the brain stem, surrounded by neural pathways that conduct ascending and descending nerve impulses between the brain and the spinal cord. It has a vast number of synaptic links with other parts of the brain and is therefore constantly receiving ‘information’ being transmitted in ascending and descending tracts.
Cerebellum
The cerebellum is situated behind the pons and immediately below the posterior portion of the cerebrum occupying the posterior cranial fossa. It is ovoid in shape and has two hemispheres, separated by a narrow median strip called the vermis. Grey matter forms the surface of the cerebellum, and the white matter lies deeply.
Spinal cord
The spinal cord is the elongated, almost cylindrical part of the central nervous system, which is suspended in the vertebral canal surrounded by the meninges and cerebrospinal fluid. The spinal cord is continuous above with the medulla oblongata and extends from the upper border of the atlas (first cervical vertebra) to the lower border of the 1st lumbar vertebra
Grey matter
The arrangement of grey matter in the spinal cord resembles the shape of the letter H, having two posterior, two anterior and two lateral columns. The area of grey matter lying transversely is the transverse commissure and it is pierced by the central canal, an extension from the fourth ventricle, containing cerebrospinal fluid. The nerve cell bodies may belong to:
- sensory neurons, which receive impulses from the periphery of the body
- lower motor neurons, which transmit impulses to the skeletal muscles
- connector neurons, also known as interneurons linking sensory and motor neurons, at the same or different levels, which form spinal reflex arcs.
Posterior columns of grey matter
These are composed of cell bodies that are stimulated by sensory impulses from the periphery of the body. The nerve fibers of these cells contribute to the white matter of the cord and transmit the sensory impulses upwards to the brain.
Anterior columns of grey matter
These are composed of the cell bodies of the lower motor neurons that are stimulated by the upper motor neurons or the connector neurons linking the anterior and posterior columns to form reflex arcs.
The posterior root (spinal) ganglia are formed by the cell bodies of the sensory nerves.
White matter
The white matter of the spinal cord is arranged in three columns or tracts: anterior, posterior, and lateral. These tracts are formed by sensory nerve fibers ascending to the brain, motor nerve fibers descending from the brain, and fibers of connector neurons.
Tracts are often named according to their points of origin and destination, e.g., spinothalamic, corticospinal.
Sensory nerve tracts in the spinal cord
Neurons that transmit impulses towards the brain are called sensory (afferent, ascending). There are two main sources of sensation transmitted to the brain via the spinal cord.
- ) The skin. Sensory receptors (nerve endings) in the skin are stimulated by pain, heat, cold, and touch, including pressure. The nerve impulses generated are conducted by three neurons to the sensory area in the opposite hemisphere of the cerebrum where the sensation and its location are perceived
- ) The tendons, muscles, and joints. Sensory receptors are specialized nerve endings in these structures, called proprioceptors, and they are stimulated by a stretch. Together with impulses from the eyes and the ears, they are associated with the maintenance of balance and posture, and with the perception of the position of the body in space. These nerve impulses have two destinations:
- by a three-neuron system, the impulses reach the sensory area of the opposite hemisphere of the cerebrum
Motor nerve tracts in the spinal cord
Neurons that transmit nerve impulses away from the brain are motor (efferent or descending) neurons. Stimulation of the motor neurons results in:
- contraction of skeletal (voluntary) muscle, or
- contraction of smooth (involuntary) muscle, cardiac muscle, and the secretion by glands controlled by nerves of the autonomic nervous system.
Voluntary muscle movement
The contraction of muscles that move the joints is, in the main, under conscious (voluntary) control, which means that the stimulus to contract originates at the level of consciousness in the cerebrum. However, skeletal muscle activity is regulated by output from the midbrain, brain stem, and cerebellum. This involuntary activity is associated with coordination of muscle activity, e.g., when very fine movement is required and in the maintenance of posture and balance.
Efferent nerve impulses are transmitted from the brain to other parts of the body via bundles of nerve fibers (tracts) in the spinal cord. The motor pathways from the brain to the muscles are made up of two neurons
The upper motor neuron
They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles to contract or relax.
The lower motor neuron
Is the efferent neuron of the peripheral nervous system (PNS) that connects the central nervous system (CNS) with the muscle to be innervated.
Upper motor neurons
These have their cell bodies in the brain at a level below the cerebrum, i.e. in the midbrain, brain stem, cerebellum, or spinal cord. They influence muscle activity that maintains posture and balance, coordinates skeletal muscle movement, and controls muscle tone.
Spinal reflexes
These consist of three elements:
-sensory neurons
-connector neurons (or interneurons) in the spinal cord
-lower motor neurons.
In the simplest reflex arc, there is only one of each type of neuron above. A reflex action is an involuntary and immediate motor response to a sensory stimulus. Many connectors and motor neurons may be stimulated by afferent impulses from a small area of skin
Stretch reflexes
The stretch reflex or myotatic reflex refers to the contraction of a muscle in response to its passive stretching. When a muscle is stretched, the stretch reflex regulates the length of the muscle automatically by increasing its contractility if the stretch is within the physiological limits.
autonomic reflexes
These include the pupillary light reflex when the pupil immediately constricts, in response to bright light, preventing retinal damage
Peripheral nervous system
The nervous system outside the brain and spinal cord.
Most of the nerves of the peripheral nervous system are composed of sensory fibers that transmit afferent impulses from sensory organs to the brain, or motor nerve fibers that transmit efferent impulses from the brain to the effector organs, e.g., skeletal muscles, smooth muscle and glands.
Spinal nerves
Spinal nerves are the major nerves of the body. A total of 31 pairs of spinal nerves control motor, sensory, and other functions. These nerves are located at the cervical, thoracic, lumbar, sacral, and coccygeal levels.
Nerves roots
The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina.
The anterior nerve root consists of motor nerve fibers, which are the axons of the lower motor neurons from the anterior column of grey matter in the spinal cord and, in the thoracic and lumbar regions, sympathetic nerve fibers, which are the axons of cells in the lateral columns of grey matter.
The posterior nerve root consists of sensory nerve fibers. Just outside the spinal cord, there is a spinal ganglion (posterior, or dorsal, root ganglion), consisting of a little cluster of cell bodies. Sensory nerve fibers pass through these ganglia before entering the spinal cord. The area of skin whose sensory receptors contribute to each nerve is called a dermatome.
Branches
Immediately after emerging from the intervertebral foramen, spinal nerves divide into branches, or rami: a ramus communicans, a posterior ramus and an anterior ramus.
The rami communicantes are part of preganglionic sympathetic neurons of the autonomic nervous system.
The posterior rami pass backwards and divide into smaller medial and lateral branches to supply skin and muscles of relatively small areas of the posterior aspect of the head, neck and trunk.
The anterior rami supply the anterior and lateral aspects of the neck, trunk and the upper and lower limbs
Plexuses
A network of nerves or vessels in the body.
There are five large plexuses of mixed nerves formed on each side of the vertebral column. They are the:
-cervical plexuses
-brachial plexuses
-lumbar plexuses
-sacral plexuses
-coccygeal plexuses
Cervical plexus
The cervical plexus is a plexus of the anterior rami of the first four cervical spinal nerves which arise from the C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medial side and vertebral.
Brachial plexus
The brachial plexus is the network of nerves that sends signals from your spinal cord to your shoulder, arm and hand. A brachial plexus injury occurs when these nerves are stretched, compressed, or in the most serious cases, ripped apart or torn away from the spinal cord.
Lumbar plexus
The lumbar plexus is a network of nerve fibers that supply the skin and musculature of the lower limb. It is in the lumbar region, within the substance of the psoas major muscle and anterior to the transverse processes of the lumbar vertebrae.
Sacral plexus
The sacral plexus is a nerve plexus that provides motor and sensory nerves for the posterior thigh, most of the lower leg and foot, and part of the pelvis. It is part of the lumbosacral plexus and emerges from the lumbar vertebrae and sacral vertebrae
Coccygeal plexus
The coccygeal plexus is a very small plexus formed by part of the 4th and 5th sacral and the coccygeal nerves. The nerves from this plexus supply the skin around the coccyx and anal area.
Thoracic nerves
The thoracic nerves do not intermingle to form plexuses. There are 12 pairs and the first 11 are the intercostal nerves. They pass between the ribs supplying them, the intercostal muscles and overlying skin. The 12th pair comprises the subcostal nerves. The 7th–12th thoracic nerves also supply the muscles and the skin of the posterior and anterior abdominal walls
cranial nerves
There are 12 pairs of cranial nerves originating from nuclei in the inferior surface of the brain, some sensory, some motor, and some mixed. Their names suggest their distribution or function, which, in the main, is generally related to the head and neck. They are numbered using Roman numerals according to the order they connect to the brain, starting anteriorly.
I olfactory nerves
These are the nerves of the sense of smell. Their sensory receptors and nerve fibers originate in the upper part of the mucous membrane of the nasal cavity, pass upwards through the cribriform plate of the ethmoid bone and then pass to the olfactory bulb. The nerves then proceed backward as the olfactory tract, to the area for the perception of smell in the temporal lobe of the cerebrum
II optic nerves
The optic nerve (CN II) is the second cranial nerve, responsible for transmitting the special sensory information for vision. It is developed from the optic vesicle, an out pocketing of the forebrain.
III oculomotor nerves (motor)
These nerves arise from nuclei near the cerebral aqueduct. They supply:
Four of the six extrinsic muscles, which move the eyeball, i.e., the superior, medial and inferior recti and the inferior oblique muscle
The intrinsic (intraocular) muscles:
–ciliary muscles, which alter the shape of the lens, changing its refractive power
–circular muscles of the iris, which constrict the pupil
The levator palpebrae muscles, which raise the upper eyelids.
IV trochlear nerves (motor)
These nerves arise from nuclei near the cerebral aqueduct. They supply the superior oblique muscles of the eyes.
VI abducens nerves (motor)
The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves.
VII facial nerves (mixed)
These nerves are composed of both motor and sensory nerve fibers, arising from nuclei in the lower part of the pons. The motor fibers supply the muscles of facial expression. The sensory fibers convey impulses from the taste buds in the anterior two-thirds of the tongue to the taste perception area in the cerebral cortex
VIII vestibulocochlear nerves
These nerves are composed of two divisions, the vestibular nerves and cochlear nerves.
The vestibular nerves arise from the semicircular canals of the inner ear and convey impulses to the cerebellum. They are associated with the maintenance of posture and balance.
The cochlear nerves originate in the spiral organ (of Corti) in the inner ear and convey impulses to the hearing areas in the cerebral cortex where sound is perceived.
IX glossopharyngeal
Known as the ninth cranial nerve (CN IX), is a mixed nerve that carries afferent sensory and efferent motor information. It exits the brainstem out from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve.
X vagus nerves
The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with the parasympathetic control of the heart, lungs, and digestive tract. The vagus nerves are normally referred to in the singular.
XI accessory nerves
• These nerves arise from nuclei in the medulla oblongata and in the spinal cord. The fibers supply the sternocleidomastoid and trapezius muscles. Branches join the vagus nerves and supply the pharyngeal and laryngeal muscles in the neck.
XII hypoglossal nerves
These nerves arise from nuclei in the medulla oblongata. They supply the muscles of the tongue and muscles surrounding the hyoid bone and contribute to swallowing and speech.
The preganglionic neuron
This has its cell body in the lateral column of grey matter in the spinal cord between the levels of the 1st thoracic and 2nd or 3rd lumbar vertebrae. The nerve fiber of this cell leaves the cord by the anterior root and terminates at a synapse in one of the ganglia either in the lateral chain of sympathetic ganglia or passes through it to one of the prevertebral ganglia. Acetylcholine is the neurotransmitter at sympathetic ganglia.
The postganglionic neuron
A neuron of the autonomic nervous system whose cell body lies in an autonomic ganglion and whose axon terminates in a visceral effector (smooth or cardiac muscle or glands).
Sympathetic ganglia
The sympathetic ganglia or paravertebral ganglia are autonomic ganglia, of the sympathetic nervous system. Ganglia are 20,000 to 30,000 afferent and efferent nerve cell bodies that run along on either side of the spinal cord. … The cell bodies create long sympathetic chains that are on either side of the spinal cord.
Prevertebral ganglia
Prevertebral ganglia are midline structures located anterior to the aorta and vertebral column and are represented by the celiac ganglia, aortic–renal ganglia, and the superior and inferior mesenteric ganglia.
Parasympathetic nervous system
The parasympathetic nervous system is responsible for the body’s rest and digestion response when the body is relaxed, resting, or feeding. It basically undoes the work of sympathetic division after a stressful situation. The parasympathetic nervous system decreases respiration and heart rate and increases digestion.
The preganglionic neuron
A neuron of the autonomic nervous system whose cell body lies in the central nervous system and whose axon terminates in a peripheral ganglion, synapsing with postganglionic neurons.
Sympathetic stimulation
Sympathetic stimulation is the activation of the sympathetic nervous system.
Respiratory system
This causes smooth muscle relaxation and therefore dilation of the airways (bronchodilation), especially the bronchioles, allowing a greater amount of air to enter the lungs at each inspiration, and increases the respiratory rate. In conjunction with the increased heart rate, the oxygen intake and carbon dioxide output of the body are increased to deal with ‘fight or flight’ situations.
Eye (sympathetic stimulation)
This causes contraction of the radiating muscle fibers of the iris, dilating the pupil. Retraction of the levator palpebrae muscles occurs, opening the eyes wide and giving the appearance of alertness and excitement. The ciliary muscle that adjusts the thickness of the lens is slightly relaxed, facilitating distant vision.
Skin (sympathetic stimulation)
Increases sweat secretion, leading to more loss of heat generated by increased skeletal muscle activity.
Contracts the arrector pili (the muscles in the hair follicles of the skin), giving the appearance of ‘goose flesh.
Constricts the peripheral blood vessels, increasing blood supply available to active organs, e.g., the heart and skeletal muscle.
Afferent impulses from viscera
Sensory fibers from the viscera travel with autonomic fibers and are sometimes called autonomic afferents. The impulses they transmit are associated with:
- visceral reflexes, usually at an unconscious level, e.g., cough, blood pressure (baroreceptors)
- sensation of, e.g., hunger, thirst, nausea, sexual sensation, rectal and bladder distension
- visceral pain.
