Nervous System Anatomy and Function

1 Introduction

In any animal organism, information must circulate for the organism to function as a coordinated whole. Two systems are responsible for transmitting information: the nervous system and the hormonal system (or endocrine system).

• The nervous system, through nerve impulses circulating in nerve cells or neurons, allows for a relatively rapid adaptation of the organism to its environment (on the order of a fraction of a second).

• The hormonal system, through hormones circulating in the blood, has a slower action (from a second to several hours) but its effects are more lasting (from a few minutes to several days).

These two modes of communication can intervene separately; however, in many circumstances, they participate in the same activity to maintain the organism's life.

Example: Stress

In modern life, many situations can cause stress:

• Emotional constraints: violent fear, approaching exams, anger, etc.

• Physical aggressions: accident, intense physical exercise, trauma, etc.

• Sudden environmental changes: abrupt temperature variation, decreased oxygen supply, dehydration, etc.

The nervous and hormonal systems then work together to cope with stress. The body's reactions are an adaptation to fight or flee.

In all multicellular animals, the nervous system:

• Receives sensory information through receptor organs.

• Integrates and develops adapted responses in the nerve centers.

• Sends commands in the form of nerve impulses through motor nerves.

The human nervous system consists of a network of billions of nerve cells (neurons and glial cells). Neurons function to receive information (from other neurons or sensory cells), transform it into nerve impulses, and retransmit it to target cells (other neurons or muscle cells).

2 Evolution of Nervous Systems

By studying the nervous system of different animal species, three characteristic evolutionary steps can be identified:

• In the most primitive animals, neurons are not grouped into a localized nervous system but are scattered over the entire surface of the animal.

  • Diffuse nervous system.

• In more evolved invertebrates, the nervous system includes a kind of brain and ganglia (clusters of neurons) forming a ventral nerve chain.

  • Ventral nervous system.

• In vertebrates, the nervous system includes an encephalon and a spinal cord located above the digestive tract.

  • Dorsal nervous system.

3 Anatomy of the Nervous System

From an anatomical point of view, the vertebrate nervous system is subdivided into two parts:

1. The Central Nervous System (CNS): the encephalon and the spinal cord.

2. The Peripheral Nervous System (PNS): nerves and ganglia.

3.1 The Peripheral Nervous System

The Peripheral Nervous System (PNS) is the part of the nervous system located outside the CNS; it is mainly formed by nerves:

1. Cranial nerves: originating from the encephalon (12 pairs).

2. Spinal nerves: originating from the spinal cord (31 pairs).

A nerve is composed of neuron axons grouped into fascicles within a tissue called the perineurium. The nerve includes several fascicles associated by the epineurium, a connective tissue traversed by blood vessels.

Nerves can be sensory when they transmit information from sensory receptors (located in the skin, muscles, viscera) to the encephalon. They are motor when the command originates from the brain and they transmit the message to effector organs.

From a functional point of view, the PNS comprises two types of pathways: the sensory pathway and the motor pathway.

3.1.1 The Sensory Pathway (or Afferent)

It transports information or impulses from sensory receptors disseminated throughout the organism (skin, sense organs, skeletal muscle, joints, and viscera). The sensory pathway constantly informs the CNS about events occurring both inside (blood pressure, blood CO₂ concentration, ...) and outside the organism (through the 5 senses).

3.1.2 The Motor Pathway (or Efferent)

It is responsible for transmitting impulses from the CNS to effector organs, i.e., muscles and glands. These nerve impulses cause muscle contraction and gland secretion; in other words, they trigger a motor response adapted to the perceived event.

The motor pathway comprises two parts:

1. The Somatic Nervous System allows the transmission of nerve impulses from the CNS to skeletal muscles. It is often called the voluntary nervous system because it allows us to consciously control our skeletal muscles.

2. The Autonomic Nervous System (ANS) controls the activity of smooth muscles, cardiac muscle, and glands. We usually have no control over activities such as our heartbeats or the movement of food in our digestive tract, so we also refer to the ANS as the involuntary nervous system.

The ANS comprises two functional subdivisions:

• The sympathetic nervous system: mobilizes the body's systems in emergency situations (prepares the individual to react: increased heart rate...).

• The parasympathetic nervous system: is responsible for carrying out normal functions.

Exercise 1. Among the proposed terms, find the one that corresponds to each of the descriptions below:

A. Autonomic Nervous System
C. Peripheral Nervous System (PNS)
B. Central Nervous System (CNS)
D. Somatic Nervous System

• Division of the nervous system consisting of the brain and spinal cord. B. Central Nervous System (CNS)

• Division of the PNS that governs voluntary activities, such as those of skeletal muscles. D. Somatic Nervous System

• Division of the nervous system consisting of cranial and spinal nerves and ganglia. C. Peripheral Nervous System (PNS)

• Division of the PNS that governs the activities of the heart, smooth muscles, and glands; also called the involuntary nervous system. A. Autonomic Nervous System

• One of the two main divisions of the nervous system, which interprets information received from the outside and issues instructions. B. Central Nervous System (CNS)

• One of the two main divisions of the nervous system that serves as a communication network connecting all parts of the body to the CNS. C. Peripheral Nervous System (PNS)

3.2 The Central Nervous System

The Central Nervous System (CNS), composed of the encephalon and the spinal cord, is the integration center of the nervous system. It interprets incoming sensory information and develops motor responses based on experience, reflexes, and environmental conditions.

3.2.1 The Spinal Cord

The spinal cord, located inside the vertebral column, passes through the vertebral foramen of each vertebra. Two zones are distinguished within the spinal cord:

• The gray matter, butterfly-shaped, consists of neurons without a myelin sheath. It contains the cell bodies of motor neurons, the axons of sensory neurons, and interneurons.

• The white matter, peripheral, consists of myelinated axons.

On both sides, intervertebral foramina between the vertebrae allow the emergence of a pair of spinal nerves, composed of sensory and motor fibers.

Sensory fibers enter the spinal cord through the dorsal root of the spinal nerve, while motor fibers exit through the ventral root. The cell bodies of sensory neurons are located in a spinal ganglion.

The spinal cord receives information from the skin and muscles and transmits motor commands. It integrates simple responses to certain types of stimuli and transmits information to and from the encephalon.

Integration by the spinal cord usually takes the form of a reflex, i.e., an unconscious reaction to a given stimulus.

The Reflexes

Certain emergency situations require an extremely rapid reaction. For example: you are holding a pot of boiling water, and it splashes your arm; you drop the pot immediately and involuntarily even before feeling pain. This response is the consequence of a reflex, the activation of a motor neuron without passing through the encephalon. Reflexes occur in very specific neural pathways called reflex arcs, which include five essential elements.

1. A receptor on which the stimulus acts.

2. A sensory neuron, which carries afferent impulses to the spinal cord.

3. An integration center which, in simple reflex arcs, can consist of a single synapse between a sensory neuron and a motor neuron.

4. A motor neuron, which carries efferent impulses from the integration center to an effector organ (muscle or gland).

5. An effector, which responds to efferent impulses (muscle contraction or secretion).

Exercise 2. The reflex is a rapid and predictable motor response to a stimulus. It occurs in very specific neural pathways called reflex arcs. The speed of execution of a reflex is due to the transmission of information between a sensory neuron and a motor neuron that occurs at the level of the spinal column. Sensory information therefore does not immediately ascend to the brain, thus saving time.

Muscles are composed of special spindles called neuromuscular spindles that can inform the brain (they are connected to sensory neurons) about the length of the muscle. They are the basis of the patellar and Achilles reflexes. These reflexes are essential for maintaining muscle tone and thus posture. For example, the patellar reflex is a reflex that prevents our knees from buckling when we are standing. When our knees begin to bend and the quadriceps muscle (muscle forming the upper part of the thigh) lengthens, the information is detected by the neuromuscular spindle, which will cause a stretch reflex that will cause involuntary contraction of the quadriceps muscle.

It is possible to reproduce this reflex by applying pressure to the tendon, a structure that allows the muscle to attach to the bone. Indeed, percussion of the patellar tendon, in a sitting position, causes the quadriceps to stretch.

1. Label the patella and patellar tendon on the figure.

2. Label the extensor muscle (quadriceps).

3. What does the impact of the hammer cause? The impact of the hammer causes the patellar tendon to stretch, which in turn stretches the quadriceps muscle. This stretching is detected by neuromuscular spindles, initiating the patellar reflex.

4. Label the sensory and motor neurons. Also note the presence of the interneuron.

5. Indicate with + and - signs the neuronal parts that will be activated or inhibited.

3.2.2 The Encephalon

The encephalon, located at the upper end of the spinal cord, contains the centers that ensure more complex integration of body regulation, perception and movement, intellect, and emotions.

Thalamus

Sensory signals, except olfactory signals, must pass through the thalamus to be directed to the appropriate cerebral areas and integrated into conscious sensory perceptions. The thalamus has thus been called the "gateway to consciousness." The thalamus filters out overly habitual signals that do not reach consciousness. During sleep, it disconnects all sensory perception by blocking the transmission of sensory impulses to the brain.

Hypothalamus

The hypothalamus controls the autonomic nervous system and regulates a large part of the hormonal system by producing hormones that act on the pituitary gland. It regulates fundamental bodily functions: thirst, hunger, thermoregulation, blood circulation, respiration, sexual activity, and circadian rhythm. It is an important interface between the brain and the body.

Pituitary Gland (Hypophysis)

The pituitary gland is subdivided into two different lobes. The posterior lobe, the neurohypophysis, receives hormones that have been produced by neurons in the hypothalamus and diffuses them further. The anterior lobe, the adenohypophysis, synthesizes hormones that act on other endocrine glands or influence growth and reproduction. The hormonal production of the pituitary gland is controlled by hormones from the hypothalamus.

Corpus Callosum

Approximately 200 million axons connect the two cerebral hemispheres via the corpus callosum and allow their coordination.

Cerebrum

The cerebrum, or telencephalon, is divided into two cerebral hemispheres. The folds of the cerebral cortex increase its surface area by three. Information transmitted by sensory cells is processed and integrated into conscious sensory perceptions in the telencephalon. This is where movement planning takes place. The various combinations between neurons enable higher functions, such as cognition, thought, speech, and memory.

Midbrain (Mesencephalon)

A passageway for information between the telencephalon and the spinal cord, the midbrain transmits sensory impulses it receives, including auditory ones, to the thalamus. It uses information from the eyes to control their movements. This is how it coordinates the reflex of turning the gaze towards an object that appears in the field of vision.

Pons

The pons is the relay center between the cerebrum and the cerebellum. It is also responsible for controlling sleep.

Cerebellum

The cerebellum, the center for coordination of movements, balance, and posture, constantly interacts with the motor centers of the telencephalon. During movement, it receives feedback information on the basis of which it controls and possibly corrects movements. For fast and precise movements, it uses learned motor programs.

The encephalon is subdivided into 4 parts:

• The cerebral hemispheres.

• The diencephalon (thalamus, hypothalamus, pituitary gland, and epithalamus).

• The brainstem (midbrain, pons, and medulla oblongata).

• The cerebellum.

Exercise 3. The encephalon:

1. Label:

2. Attribute these functions to the cerebellum, diencephalon, or brainstem:

Region	Functions
Brainstem	Regulation of cardiac activity
Diencephalon	Regulation of water balance and thirst
Brainstem	Control of respiration
Cerebellum	Coordination of movements
Diencephalon	Regulation of body temperature
Brainstem	Control of swallowing, salivation, coughing, and sneezing.
Diencephalon	Regulation of the sleep-wake cycle

3.2.3 The Cerebral Hemispheres

The cerebral hemispheres (left and right hemispheres) make up the upper part of the encephalon. They constitute approximately 83% of the encephalon's mass and are its most visible parts.

From an anatomical point of view, each hemisphere is divided into 4 lobes: frontal, parietal, temporal, and occipital.

A band of fibers called the corpus callosum connects the two hemispheres.

Each of the cerebral hemispheres has three fundamental regions:

• The gray matter constitutes the cerebral cortex (on the surface).

• The white matter (myelinated axons).

• The basal ganglia, which are clusters of neuron cell bodies distributed in the white matter.

From a functional point of view, two types of areas are recognized in the cortex: primary areas and associative areas. Primary areas receive sensory or motor information, and associative areas have an integration or association function for this information. They are responsible for thought in all its forms and memory.

Functions of associative areas include:

• Interpretation of sensory information.

• Association of perceptions with prior experiences.

• Memorization of information.

• Consciousness of perceptions.

Functional Zones of the Cortex

Attention: Do not confuse the sensory and motor regions of the cortex with sensory and motor neurons. All neurons in the cortex are interneurons.

Here are the main functional areas of the brain:

• Primary Motor Cortex: Neurons in this area control voluntary movements of skeletal muscles.

• Broca's and Wernicke's Areas: Located only in the left hemisphere, these two areas are involved in language. People with lesions affecting Broca's area can understand language, but they have difficulty speaking (and sometimes cannot write or use sign language). In contrast, people with Wernicke's area lesions are able to speak, but their words are meaningless, and their speech is incoherent. They also have great difficulty understanding language.

• Primary Somatosensory Cortex: Neurons in this gyrus receive messages from somatic receptors in the skin and proprioceptors (receptors sensitive to locomotion, spatial position, and muscle tone) in skeletal muscles, joints, and tendons.

• Somatosensory Association Cortex: Its main function is to integrate the various somatosensory information (temperature, pressure, etc.) transmitted to it via the primary somatosensory cortex, and then to derive a global meaning from it. Thus, when you put your hand in your pants pocket or handbag, your somatosensory association cortex consults memories of stored sensory experiences and identifies the objects your hand encounters, such as coins or keys.

• Primary Visual Cortex: It receives visual information from the retina.

• Visual Association Areas: They communicate with the primary visual cortex and interpret visual stimuli (color, shape, and movement) based on previous visual experiences. It is thanks to them that we can recognize a flower or a face.

• Primary Auditory Cortex: Sound waves stimulate auditory receptors in the inner ear and trigger the transmission of nerve impulses to the primary auditory cortex, which decodes their amplitude, rhythm, and intensity.

• Auditory Association Area: It allows the perception of sound stimuli, which we interpret as speech, a cry, music, thunder, noise, etc. It seems that memories of sounds are stored there.

Exercise 4. For each of the patients below, identify the affected cortical area.

Symptom	Affected Cortical Area
Following a shock to the cortex, the individual is affected by functional blindness.	Primary Visual Cortex
Following a shock to the cortex, the person cannot recognize an object by touching it but only by looking at it.	Somatosensory Association Cortex
Following a shock to the cortex, the person is unable to perform a movement (paralysis of skeletal muscles).	Primary Motor Cortex
Following a shock to the cortex, the person is able to see but does not understand what they are looking at.	Visual Association Areas

The associative areas therefore suggest that our brain does not act like a camera or an audio recorder that retransmits visual and auditory information as is. Indeed, our brain interprets visual and auditory information, which can cause differences in perception of the same reality between two different individuals. Optical illusion perfectly illustrates this subtlety.

Mapping of the Primary Somatosensory Cortex and Motor Cortex

In the somatosensory cortex and motor cortex, neurons are arranged according to the body part that transmits sensory stimuli to them or receives motor commands from them.

The portion of the cortex dedicated to each body part is not related to the size of that part, but rather to the importance of the neural control required (for the motor cortex) or to the number of sensory neurons that extend the axons of that part (for the somatosensory cortex).

This is why the portion dedicated to the face and hands is much larger than that devoted to the trunk, reflecting the significant involvement of facial muscles in communication.

Exercise 5. The peripheral nervous system. Color in blue the neurons of the sensory pathway, and in red those of the motor pathway. Complete the diagram with the following legends:

• Somatic Nervous System (SNS)

• Autonomic Sympathetic Nervous System (SNAs)

• Autonomic Parasympathetic Nervous System (SNAp)

3.2.4 Protection of the Central Nervous System

In addition to the skull and vertebrae, the central nervous system is protected by the meninges and by the cerebrospinal fluid.

The Vertebrae

The vertebral column is composed of stacked vertebrae between which are discs that act as "shock absorbers." The discs are composed of a gelatinous central nucleus surrounded by a fibrous ring that attaches the two vertebrae to each other.

Exercise 6. Herniated disc:

From adolescence, intervertebral discs become less flexible and more brittle. When the annulus fissures, the nucleus substance exits the vertebral column. This disc overflow can then cause compression of a nerve root or the spinal cord. While herniated discs are possible at all levels of the vertebral column, they are much more common at the lumbar and cervical levels. What are the possible consequences?

Possible consequences of a herniated disc include:

• Pain: Localized pain at the site of the herniation, or radiating pain along the path of the compressed nerve (e.g., sciatica if the sciatic nerve is compressed).

• Numbness or tingling: A sensation of pins and needles or loss of sensation in the area supplied by the affected nerve.

• Muscle weakness: Weakness in the muscles controlled by the compressed nerve.

• Loss of reflexes: Diminished or absent reflexes in the affected area.

• Motor deficits: In severe cases, difficulty with movement or even paralysis.

• Bladder or bowel dysfunction: If the herniation affects nerves controlling these functions (e.g., cauda equina syndrome, a medical emergency).

The Meninges

The meninges consist of three membranes that surround the brain and spinal cord. They contain cerebrospinal fluid and function to protect the central nervous system and blood vessels. They are named, from outside to inside: the dura mater - the arachnoid mater - the pia mater.

Inflammation of the meninges is called meningitis. Of viral or bacterial origin, meningitis is diagnosed by analyzing a sample of cerebrospinal fluid, obtained by lumbar puncture. If meningitis spreads to the central nervous system, it can lead to encephalitis.

The Cerebrospinal Fluid

The cerebrospinal fluid (CSF) (or cerebrospinal fluid) found inside and around the encephalon and spinal cord forms a watery cushion for the organs of the CNS. By floating in this fluid, the encephalon avoids collapsing under its own weight. In addition, the CSF protects the encephalon and spinal cord from shocks and other traumas.

Exercise 7. Protection of the spinal cord:

The spinal cord is protected by different vertebrae named, from top to bottom, cervical (C), thoracic (T), lumbar (L), and sacrum (S). It is known that the nerves located between C₁ and C₆ contain motor neurons responsible for controlling smooth muscles enabling respiratory functions. There are also motor nerves responsible for regulating cardiac activity. Nerves between C₆ and T₁₂ contain motor neurons responsible for controlling skeletal muscles of the upper body (from the neck to the hip). Nerves located between T₁₂ and L₅ contain motor neurons that control skeletal muscles of the lower body. Finally, nerves located below L₅ notably contain motor neurons that control the urinary system (micturition) and reproductive system (erection).

The figure opposite schematizes complete cross-sections of the spinal cord. Explain the consequences of these different lesions.

• Lesion above C₂: A lesion above C₂ would likely result in paralysis of all muscles below the neck, including those involved in respiration. This would lead to complete quadriplegia and require mechanical ventilation for survival, as the motor neurons controlling respiratory smooth muscles (C₁-C₆) would be affected. Cardiac activity regulation could also be severely impaired.

• Lesion just below C₆: A lesion just below C₆ would affect motor control of the upper body muscles from the neck down to the hip (C₆-T₁₂). This would result in paralysis or significant weakness in the arms, hands, and trunk muscles, while respiratory and cardiac functions (controlled by nerves above C₆) would likely be preserved. This typically leads to quadriplegia with some arm function retained.

• Lesion just below T₁₂: A lesion just below T₁₂ would primarily affect motor control of the lower body muscles (T₁₂-L₅). This would result in paraplegia, meaning paralysis of the legs and lower trunk, while arm and upper body functions would be preserved. Urinary and reproductive functions (controlled by nerves below L₅) might also be affected if the lesion extends lower.

• Lesion just below L₅: A lesion just below L₅ would primarily affect the motor neurons controlling the urinary and reproductive systems. This could lead to bladder and bowel dysfunction (incontinence or retention) and sexual dysfunction, while motor control of the legs might be partially or fully preserved depending on the exact level and extent of the lesion.

Exercise 8. By observing a cross-section of the spinal cord, it is noted that the initial part of each spinal nerve (rachidian) consists of two roots, a ventral (or anterior) root, which is motor, and a dorsal (or posterior) root, which is sensory. These two nerve trunks fuse at each level to form the nerve. The dorsal roots have a swelling called the spinal ganglion, which contains the cell bodies of sensory neurons.

1. Using the information above, label this diagram:

2. In the following 3 cases, the nerve innervates a hind leg. Indicate the consequences of a section of this nerve on the sensitivity and/or motor function of the affected leg.

• The section is performed at the level of the spinal nerve: A section of the entire spinal nerve (which contains both sensory and motor fibers) would result in a complete loss of both sensation (anesthesia) and motor function (paralysis) in the hind leg it innervates.

• The section is performed at the level of the dorsal root: A section of the dorsal root (which contains only sensory fibers) would result in a complete loss of sensation (anesthesia) in the hind leg, but motor function would remain intact as the motor fibers in the ventral root are unaffected.

• The section is performed at the level of the ventral root: A section of the ventral root (which contains only motor fibers) would result in a complete loss of motor function (paralysis) in the hind leg, but sensation would remain intact as the sensory fibers in the dorsal root are unaffected.

3.3 The Cells of the Nervous System

The cells that primarily compose the nervous system, central or peripheral, are neurons. The figure below represents the relationship between gray and white matter, nerves, and neurons:

3.3.1 The Neurons

Neurons are nerve cells that transmit messages along nerve pathways. They are the functional units of the nervous system and are specialized in transmitting messages between different regions of the body.

A neuron has three distinct parts:

1. The cell body (soma): relatively large, it contains the nucleus and several other cellular organelles.

2. The dendrites: extensions of the cell body. They transmit messages towards the neuron's cell body.

3. The axon: often a single, long extension. It conducts messages from the cell body to other cells.

Dendrites are generally numerous and highly branched (their name comes from the Greek dendron "tree"). They increase the neuron's receptive surface and thus allow good reception of messages from other neurons.

The axon can be very long. For example, the axons of the sciatic nerve in the leg extend from the spinal cord to a muscle in the foot (~1 meter)!

In some vertebrates, axons are surrounded by supporting cells, called Schwann cells. These cells form an insulating protective sheath comparable to an electrical cable sheath.

The axon can have ramifications, and each of them has hundreds or thousands of axon terminals. These transmit information to other cells by releasing chemical messengers called neurotransmitters.

Types of Neurons

There are three main categories of neurons, depending on the type of cells with which they form synapses:

1. Sensory neurons: transmit information received from sensory receptors to the CNS.

2. Motor neurons: transmit commands from the CNS to effector cells.

3. Interneurons: neurons located in the CNS that integrate sensory and motor messages. They only connect with other neurons.

The "morphology" of these 3 types of neurons differs.

Sensory Neurons and Type of Stimulus

Sensory neurons can be classified according to the type of stimulus they detect:

• Mechanoreceptors react to touch, pressure, vibrations, or stretching.

• Thermoreceptors react to changes in temperature.

• Photoreceptors react to light.

• Chemoreceptors are sensitive to dissolved chemical substances.

• Nociceptors react to potentially harmful stimuli, and the information they transmit is interpreted as pain by the brain.

Sensory Neurons and Location

Sensory neurons can also be classified according to their location:

• Exteroceptors are sensitive to stimuli from the environment. This is why most exteroceptors are located on or near the body surface.

• Interoceptors, or visceroceptors, react to stimuli produced in the internal environment, i.e., in the viscera and vessels.

• Proprioceptors, like interoceptors, react to internal stimuli, but they are found only in skeletal muscles, tendons, joints, ligaments, and the connective tissue covering bones and muscles. Proprioceptors constantly inform the brain about our body's movements by measuring the degree of stretching of tendons and muscles.

Exercise 9. For each of the descriptions below, determine the nature of the sensory neuron.

Function of the sensory neuron	According to the stimulus	According to the localization
Baroreceptors detect a change in pressure in blood vessels.	Mechanoreceptor	Interoceptor
Sensory neuron capable of detecting changes in O2 concentration in the blood.	Chemoreceptor	Interoceptor
Sensory neuron detecting a change in external temperature.	Thermoreceptor	Exteroceptor
Sensory neuron detecting extreme heat.	Nociceptor	Exteroceptor
Sensory neuron detecting a stretch of the patellar tendon.	Mechanoreceptor	Proprioceptor
Sensory neuron detecting salty taste.	Chemoreceptor	Exteroceptor

3.3.2 Examples of Neuronal Connections

Sensory Information or Ascending Tract

Sensory information travels from the periphery to the CNS. In the following example, three neurons are useful for transmitting sensory information. The receptor studied is the temperature receptor located on the soles of the feet.

1. A first sensory neuron (PNS) triggers an action potential when the temperature rises. Note that the cell bodies of sensory neurons are located in the ganglion. The first synapse (connection between two neurons) is located in the gray matter of the spinal cord.

2. From there, an interneuron (CNS) ascends to the thalamus (gateway for sensory pathways to the hemispheres) where the second synapse is located.

3. Finally, a last interneuron (CNS) transmits the information to the primary somatosensory cortex.

Remark: The integration of information occurs in the opposite zone (left hemisphere) of the involved organ (located in the right part of the body). Indeed, a neuron in this pathway crosses from one side of the CNS to the other. This particularity is called decussation.

Motor Response or Descending Tract

Let's take the example of skeletal muscle contraction. An interneuron originates from the primary motor cortex (the cell body is located in the cortex) and descends to the spinal cord. Decussation is observed at this level. The neuron synapses in the ventral horn of the spinal cord. Finally, a motor neuron, whose cell body is located in the gray matter of the spinal cord, innervates the skeletal muscle.

Exercise 10. There is a second motor pathway called the lateral corticospinal tract. It begins with an interneuron whose cell body is located in the primary motor cortex (indicated by the arrow). This interneuron is myelinated and descends to the spinal cord. Decussation is observed between the medulla oblongata and the cervical spinal cord. The first synapse is located in the lumbar spinal cord and connects the 1^st neuron with an interneuron entirely located in the gray matter. This second neuron synapses with a motor neuron whose cell body is located in the ventral horn.

Represent the lateral corticospinal tract on the diagram below.

3.3.3 The Gliocytes (Glial Cells - Neuroglia)

All neurons are closely associated with cells of much smaller size: gliocytes (or neuroglia). Gliocytes are about 10 times more numerous than neurons in the CNS and constitute about half of the brain's mass. Most primary CNS tumors are gliomas, meaning tumors of the neuroglia. This particularity would stem from the almost unlimited division capacity of gliocytes, unlike neurons and most cells. Indeed, neurons become amitotic once mature, meaning they are incapable of dividing.

There are 4 main types of gliocytes:

• Astrocytes are the most abundant and versatile. Their numerous star-shaped extensions adhere to neurons and cover capillaries. They support neurons and anchor them to their nutrient supply: blood capillaries.

They are involved in exchanges between capillaries and neurons and determine capillary permeability. Astrocytes thus form the blood-brain barrier, which limits the access of most substances to the CNS. This allows strict control of the extracellular chemical environment of the CNS.

• Microglia, or microglial cells, are small ovoid cells with relatively long thorny extensions. Their extensions touch neighboring neurons and monitor their integrity.

When microglia detect that certain neurons are damaged or show abnormalities, they migrate towards them. If foreign microorganisms are present or neurons die, microglia transform into macrophages; they then phagocytize (internalize and destroy) microorganisms and debris from dead neurons. The protective role of microglia is of great importance because immune system cells do not have access to the CNS.

• Oligodendrocytes are less branched than astrocytes. They are aligned along the thick axons of the CNS, and their cytoplasmic extensions wrap tightly around them, thus forming insulating envelopes called myelin sheaths.

• Schwann cells: The PNS also has neurons with myelin sheaths. However, these are formed by Schwann cells. Along the axon, approximately every millimeter, adjacent Schwann cells are separated by gaps of 1 to 2 µm: the Nodes of Ranvier.

Exercise 11. Check the correct answer:

1. A sensory nerve is a nerve:

   • - déférent

   • - afférent

   • - efférent

2. A motor nerve is a nerve:

   • - déférent

   • - afférent

   • - efférent

3. Motor neurons transmit information:

   • - des muscles vers le tronc cérébral

   • - du cerveau vers les muscles

   • - des muscles vers le cerveau

4. Sensory neurons transmit information:

   • - des muscles vers le tronc cérébral

   • - du cerveau vers les muscles

   • - des muscles vers le cerveau

5. When a neuron transmits information to the spinal cord which then triggers a muscular response, it is:

   • - un stimulus

   • - un reflex

   • - une impulsion

6. The excitation of a sensory nerve ending is:

   • - un stimulus

   • - un reflex

   • - une impulsion

7. A motor response is made by:

   • - un stimulus

   • - un reflex

   • - une impulsion

8. Respiration, circulation, digestion are controlled by:

   • - le système nerveux central

   • - la moelle épinière

   • - le système nerveux autonome

9. The vegetative or autonomic nervous system is composed of:

   • - des systèmes sympathique et parasympathique

   • - du système sympathique

   • - du système parasympathique

10. The parasympathetic system generally allows:

    • - la mise au repos de l'organisme

    • - la stimulation de l'organisme

    • - la stimulation et la mise au repos de l'organisme

11. The vegetative system:

    • - échappe au contrôle volontaire

    • - est sous le contrôle volontaire

    • - est sous le contrôle volontaire et involontaire

12. The sympathetic system:

    • - ralentit le rythme cardiaque

    • - accélère le rythme cardiaque

    • - n'a aucune action sur la fréquence cardiaque