The Nervous System PDF

**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. To this end the nervous system provides an immediate response while endo...

**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. To this end the nervous system provides an immediate response while endocrine activity is, usually, slower and more prolonged. Humans have only one nervous system, even though some of its subdivisions are referred to as separate systems. Thus, the central nervous system and the peripheral nervous system are subdivisions of the nervous system, instead of separate organ systems, as their names suggest. The **central nervous system (CNS)** consists of the brain and the spinal cord. The brain is located within the skull, and the spinal cord is located within the vertebral canal, formed by the vertebrae. The brain and spinal cord are continuous with each other at the foramen magnum. Screenshot\_20171017-202135.png  Screenshot\_20230815\_140259\~2.jpg  The **peripheral nervous system (PNS)** is external to the CNS. It consists of sensory receptors, nerves, ganglia, and plexuses. **Sensory receptors** are the endings of nerve cells or separate, specialized cells that detect temperature, pain, touch, pressure, light, sound, odors, and other stimuli. Sensory receptors **are located** in the skin, muscles, joints, internal organs, and specialized sensory organs, such as the eyes and ears. A **nerve** is a bundle of axons and their sheaths; it connects the CNS to sensory receptors, muscles, and glands. Twelve pairs of **cranial nerves** originate from the brain, and 31 pairs of **spinal nerves** originate from the spinal cord. A **ganglion** (pl. ***ganglia***; which means knot) is a collection of neuron cell bodies located outside the CNS. A **plexus** (plek\_sus; braid) is an extensive network of axons and, in some cases, neuron cell bodies, located outside the CNS. The PNS can be divided into two subcategories. The **sensory,** or **afferent, division** transmits electric signals, called **action potentials,** from the sensory receptors to the CNS. The cell bodies of sensory neurons are located in dorsal root ganglia near the spinal cord or in ganglia near the origin of certain cranial nerves. The **motor,** or **efferent, division** transmits action potentials from the CNS to effector organs, such as muscles and glands. The motor division is divided into the **somatic nervous system** and the **autonomic nervous system (ANS).** The somatic nervous system transmits action potentials from the CNS to skeletal muscles. Skeletal muscles are voluntarily controlled through the somatic nervous system. In turn the motor division is involved in activities that are: ** *voluntary*** *---the* somatic nervous system (movement of voluntary muscles) ** *involuntary*** *---* the autonomic nervous system (functioning of smooth and cardiac muscle and glands). The autonomic nervous system has two parts: ***sympathetic* and *parasympathetic.*** The cell bodies of somatic motor neurons are located within the CNS, and their axons extend through nerves to form synapses with skeletal muscle cells. A **synapse** is the junction of a nerve cell with another cell. The **neuromuscular junction**, the synapse between a neuron and a skeletal muscle cell. Nerve cells can also form synapses with other nerve cells, smooth muscle cells, cardiac muscle cells, and gland cells. In summary, the CNS **receives** sensory information **about** its internal and external environments from afferent/sensory nerves. The CNS **integrates** and **processes** this input and responds, when appropriate, by sending nerve impulses through motor nerves to the effector organs: muscles and glands. For example, responses to changes in the internal environment regulate essential involuntary body functions such as respiration and blood pressure; **responses** to changes in the external environment maintain posture and other voluntary activities. **Functions of the Nervous System.** The nervous system is involved in most body functions. Some of the major functions of the nervous system are; 1. 2. 3. 4. 5. **Cells and tissues of the nervous system** **A plexus:** an extensive network of axous and, in some cases, neuron cell bodies, located outside the CNS. **A nerve:** a bundle of axons and their sheaths; it connects the CNS to sensory receptors, muscles, and glands. **A ganglion:** a collection of neuron cell bodies located outside the CNS. **Nerves** A nerve consists of numerous neurones collected into bundles (bundles of nerve fibres in the central nervous system are known as *tracts*). For example large nerves such as the sciatic nerves (p. 169) contain tens of thousands of axons. Each bundle has several coverings of protective connective tissue: ** *endoneurium*** is a delicate tissue, surrounding each individual fibre, which is continuous with the septa that pass inwards from the perineurium ** *perineurium*** is a smooth connective tissue, surrounding each *bundle* of fibres ** *epineurium*** is the fibrous tissue which surrounds and encloses a number of bundles of nerve fibres. Most large nerves are covered by epineurium. Screenshot\_20230815\_110108\~2.jpg There are **two types** of nervous tissue, neurones and neuroglia*. **Neurones*** (nerve cells) are the working units of the nervous system that generate and transmit ***nerve impulses*.** Neurones are supported by connective tissue, collectively known as ***neuroglia*,** which is formed from different types of ***glial cells***. There are vast numbers of both cell types, **1 trillion** (**10^12^**) glial cells and **10 times fewer** (**10^11^**) neurones. Non neural cells are called glial cells, or neuroglia (nerve glue), and they support and protect neurons and perform other functions. G.C accounts for over half of the brains weight with a ratio of 10 to 50 : 1. **Types of Neurons** Neurons are classified according to their function or structure. The functional classification is based on the direction in which action potentials are conducted. - - - The structural classification scheme is based on the number of processes that extend from the neuron cell body. The three major categories of neurons are - - -  Each neurone consists of a *cell body* and its processes, one *axon* and many ***dendrites***. Neurones are commonly referred to as nerve cells. Bundles of axons bound together are called *nerves*. Neurones cannot divide, and for survival they need a continuous supply of oxygen and glucose. Unlike many other cells, neurones can synthesise chemical energy (ATP) only from glucose. Neurones generate and transmit electrical impulses called ***action potentials**.* The initial strength of the impulse is maintained throughout the length of the neurone. Some neurones initiate nerve impulses while others act as 'relay stations' where impulses are passed on and sometimes redirected. Nerve impulses can be initiated in response to stimuli from: outside the body, e.g. touch, light waves inside the body, e.g. a change in the concentration of carbon dioxide in the blood alters respiration; a thought may result in voluntary movement. Transmission of nerve signals is both electrical and chemical. The action potential travelling down the nerve axon is an electrical signal, but because nerves do not come into direct contact with each other, the signal between a nerve cell and the next cell in the chain is nearly always chemical. **Neuronal Cell Body (Soma)** Nerve cells vary considerably in size and shape but they are all too small to be seen by the naked eye. Cell bodies form the ***grey matter*** of the nervous system and are found at the periphery of the brain and in the centre of the spinal cord. Groups of cell bodies are called ***nuclei*** in the central nervous system and ***ganglia*** in the peripheral nervous system. An important exception is the basal ganglia (nuclei) situated within the cerebrum. Each neuron cell body contains the following; - - - - - - - - - Screenshot\_20171017-202303.png **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 fibres*** outside the brain and spinal cord. **Dendrites** Dendrites are short, often highly branched cytoplasmic extensions that are tapered from their basis at the neuron cell body to their tips. 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. Many dendrites surfaces have small extensions called dendritic spines, where axons of other neurons form synapses with the dendrites. Dendrites are input part of the neuron. In motor neurones dendrites form part of synapses and in sensory neurones they form the sensory receptors that respond to specific stimuli. **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 as long as 100 cm. Axons terminate by branching to form small extensions with enlarged ends called presynaptic terminals. Numerous small vesicles containing neurotransmitters are present in the presynaptic terminals. The presynaptic terminals release the neurotransmitters which are chemicals that cross the synapse to stimulate or inhibit the postsynaptic cell (effector organ). **Structure of an axon.** The membrane of the axon is called the ***axolemma*** and it encloses the cytoplasmic extension of the cell body. ***Myelinated neurones*** Large axons and those of peripheral nerves are surrounded by a ***myelin sheath***. This consists of a series of ***Schwann cells*** arranged along the length of the axon. Each one is wrapped around the axon so that it is covered by **a number of concentric layers** of Schwann cell plasma membrane. Between the layers of plasma membrane is a small amount of fatty substance called ***myelin*.** The outermost layer of the Schwann cell plasma membrane is the ***neurilemma*.** There are tiny areas of exposed axolemma between adjacent Schwann cells, called ***nodes of Ranvier***, which assist the **rapid transmission** of nerve impulses in myelinated neurones. ***Unmyelinated neurones*** Postganglionic fibres and some small fibres in the central nervous system are ***unmyelinated*.** In this type a number of axons are embedded in **one** Schwann cell. The adjacent Schwann cells are in close association and there is no exposed axolemma. The **speed of transmission** of nerve impulses is **significantly slower** in unmyelinated fibres. **The synapse and neurotransmitters** A synapse is a specialized junction; functional membrane -- to -- membrane contact between the processes of two nerve cell (neurons) or between a neuron and an effector organ; muscle cell, gland cell, or sensory receptor; where impulse (or action potentials) are transmitted. (A.K.A Neuromuscular junction). There is always more than one neurone involved in the transmission of a nerve impulse from its origin to its destination, whether it is sensory or motor. There is no physical contact between two neurones. The point at which the nerve impulse passes from the ***presynaptic neurone*** to the ***postsynaptic neurone*** is the *synapse*. At its free end, the axon of the presynaptic neurone breaks up into minute branches that terminate in small swellings called ***synaptic knobs*,** or terminal boutons. These are in close proximity to the dendrites and the cell body of the postsynaptic neurone. The space between them is the ***synaptic* *cleft*.** Synaptic knobs contain spherical membrane bound *synaptic vesicles*, which store a chemical, the *neurotransmitter* that is released into the synaptic cleft.  Neurotransmitters are synthesised by nerve cell bodies, actively transported along the axons and stored in the synaptic vesicles. They are released by exocytosis in response to the action potential and diffuse across the synaptic cleft. They act on specific receptor sites on the postsynaptic membrane. Their action is **short lived**, because immediately they have acted on the postsynaptic cell such as a muscle fibre, they are either inactivated by enzymes or taken back into the synaptic knob. Some important drugs mimic, neutralise (antagonise) or prolong neurotransmitter activity. Neurotransmitters usually have an excitatory effect on postsynaptic receptors but they are sometimes **inhibitory**. There are more than 50 neurotransmitters in the brain and spinal cord including **noradrenaline (norepinephrine), adrenaline (epinephrine), dopamine, histamine, serotonin, gamma aminobutyric acid (GABA) and acetylcholine.** Other substances, such as enkephalins, endorphins and substance P, have specialised roles in, for example, transmission of pain signals. Somatic nerves carry impulses directly to the synapses at skeletal muscles, the *neuromuscular junctions* stimulating contraction. In the autonomic nervous system, efferent impulses travel along two neurons (preganglionic and postganglionic) and across two synapses to the effector tissue, i.e. cardiac muscle, smooth muscle and glands, in both the sympathetic and the parasympathetic divisions. Screenshot\_20171017-203724.png  **Neuroglia/Glial Cells** The neurones of the central nervous system are supported by non-excitable *glial cells* that greatly outnumber the neurones. Unlike nerve cells, which cannot divide, glial cells continue to replicate throughout life. There are two main types: **Glial Cells of the CNS and Glial Cells of the PNS** **Glial Cells of the CNS** The neurones of the central nervous system are supported by non-excitable *glial cells* that greatly outnumber the neurones. Unlike nerve cells, which cannot divide, glial cells continue to replicate throughout life. Glial cells are the major supporting cells in the CNS; they participate in the formation of a permeability barrier between the blood and the neurons, phagocytize foreign substances, produce cerebrospinal fluid, and form myelin sheaths around axons. There are four types of CNS glial cells, each with unique structural and functional characteristics; ***Astrocytes, Ependymal Cells, Microglia, Oligodendrocytes.*** **Astrocytes** These cells form the main supporting tissue of the central nervous system. They are star shaped with fine branching processes and they lie in a mucopolysaccharide ground substance. At the free ends of some of the processes are small swellings called *foot processes*. Astrocytes are found in large numbers **adjacent to** blood vessels with their foot processes forming a sleeve round them. This means that the blood is separated from the neurones by the capillary wall and a layer of astrocyte foot processes which together constitute the ***blood--brain*** ***barrier*.** The blood--brain barrier is a selective barrier that protects the brain from potentially toxic substances and chemical variations in the blood, e.g. after a meal. Oxygen, carbon dioxide, glucose and other lipid-soluble substances, e.g. alcohol, quickly cross the barrier into the brain. Some large molecules, many drugs, inorganic ions and amino acids pass more slowly, if at all, from the blood to the brain. Screenshot\_20171017-202420.png **Oligodendrocytes** These cells are smaller than astrocytes and are found in clusters round 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 fibres. 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 cerebrospinal fluid. Screenshot\_20171017-202545.png **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. **Glial Cells of the PNS** **Schwann cells,** or **neurolemmocytes** (noo r-o¯-lem\_mo¯-sı¯tz), are glial cells in the PNS that wrap around axons. If a Schwann cell wraps many times around an axon, it forms a myelin sheath. Unlike oligodendrocytes, however, each Schwann cell forms a myelin sheath around a portion of only one axon. **Satellite cells** surround neuron cell bodies in sensory ganglia. They provide support and nutrition to the neuron cell bodies, and they protect neurons from heavy metal poisons, such as lead and mercury, by absorbing them and reducing their access to the neuron cell bodies.  **ELECTRIC SIGNALS** Like computers, humans depend on electric signals to communicate and process information. The electric signals produced by cells are called **action potentials.** They are an important means by which cells transfer information from one part of the body to another. For example, such as light, sound, and pressure, act on specialized sensory cells in the eye, ear, and skin to produce action potentials, which are conducted from these cells to the spinal cord and brain. Action potentials originating within the brain and spinal cord are conducted to muscles and certain glands to regulate their activities. The **synapse** (sin\_aps), which is the junction between two cells, is where two cells communicate with each other. The cell that transmits a signal toward a synapse is called the **presynaptic cell,** and the cell that receives the signal is called the **postsynaptic cell.** The average presynaptic neuron synapses with about 1000 other neurons, but the average postsynaptic neuron has up to 10,000 synapses. Some postsynaptic neurons in the part of the brain called the cerebellum have up to 100,000 synapses. There are two types of synapses: electrical and chemical. **Brain** The brain is a large organ weighing around 1.4 kg that lies within the cranial cavity. Its parts are cerebrum} the **diencephalon** thalamus } the **diencephalon** hypothalamus } **the diencephalon** midbrain } **the brain stem** pons } **the brain stem** medulla oblongata } **the brain stem** cerebellum Screenshot\_20230815\_110246\~2.jpg **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 also if a contributing artery is narrowed. The brain receives about 15% of the cardiac output, approximately 750 mL of blood per minute. Autoregulation keeps blood flow to the brain constant by adjusting the diameter of the arterioles across a wide range of arterial blood pressure (about **65--140 mmHg**) with changes occurring only outside these limits. Venous blood from the brain drains into the ***dural venous sinuses*** and then downwards into the ***internal*** ***jugular veins*.**  **Cerebrum** This is the largest part of the brain and it occupies the anterior and middle cranial fossae. It is divided by a deep cleft, the ***longitudinal* *cerebral fissure***, into *right* and *left **cerebral hemispheres***, each containing one of the lateral ventricles. Deep within the brain, the hemispheres are connected by a mass of white matter (nerve fibres) called the ***corpus callosum*.** The falx cerebri is formed by the dura mater. It separates the two cerebral hemispheres and penetrates to the depth of the corpus callosum. The superficial part of the cerebrum is composed of nerve cell bodies (grey matter), forming the *cerebral cortex*, and the deeper layers consist of nerve fibres (axons, white matter). The cerebral cortex shows many infoldings or furrows of varying depth. The exposed areas of the folds are the *gyri* (convolutions) and these are separated by ***sulci* (fissures**). These convolutions greatly increase the surface area of the cerebrum. For descriptive purposes each hemisphere of the cerebrum is divided into *lobes* which take the names of the bones of the cranium under which they lie: ** frontal** ** parietal** ** temporal** ** occipital.** The boundaries of the lobes are marked by deep sulci. These are the *central*, *lateral* and *parieto-occipital sulci* The **frontal lobe** is important in voluntary motor function, motivation, aggression, the sense of smell, and mood. The **parietal lobe** is the major center for the reception and evaluation of most sensory information, except for smell, hearing, and vision. The frontal and parietal lobes are separated by the central sulcus. The **occipital lobe** functions in the reception and integration of visual input and is not distinctly separate from the other lobes. The **temporal lobe** receives and evaluates input for smell and hearing and plays an important role in memory. Its anterior and inferior portions are referred to as the "psychic cortex," and they are associated with such brain functions as abstract thought and judgment. The temporal lobe is separated from the rest of the cerebrum by a **lateral fissure.** Screenshot\_20230815\_161721\~2.jpg The gray matter on the outer surface of the cerebrum is the **cortex ,** and clusters of gray matter deep inside the brain are nuclei. The cerebral cortex contains a number of neuron types, named largely for their shape, such as fusiform cells, stellate cells, and pyramidal cells. These cells are distributed in layers within the cerebral cortex. The thickness of the cortex is not uniform throughout the cerebrum but ranges from two or three layers in the most "primitive" parts of the cortex to six layers in the more "advanced" regions. The white matter of the brain between the cortex and nuclei is the **cerebral medulla.** This term should not be confused with the medulla oblongata; ***medulla*** is a general term meaning **the center of a structure.** The cerebral medulla consists of tracts that connect areas of the cerebral cortex to each other or to other parts of the CNS. **Interior of the cerebrum** \[**Cerebral tracts and basal ganglia\]** The surface of the cerebral cortex is composed of grey matter (nerve cell bodies). Within the cerebrum the lobes are connected by masses of nerve fibres, or *tracts*, which make up the white matter of the brain. The afferent and efferent fibres linking the different parts of the brain and spinal cord are as follows. ** *Association* (*arcuate*) *tracts*** are most numerous and connect different parts of a cerebral hemisphere by extending from one gyrus to another, some of which are adjacent and some distant. ** *Commissural tracts*** connect corresponding areas of the two cerebral hemispheres; the largest and most important commissure is the ***corpus callosum*.** ** *Projection tracts*** connect the cerebral cortex with grey matter of lower parts of the brain and with the spinal cord, e.g. the internal capsule.  The ***internal capsule*** is an important projection tract that lies deep within the brain between the basal ganglia and the thalamus. Many nerve impulses passing to and from the cerebral cortex are carried by fibres that form the internal capsule. Motor fibres within the internal capsule form the *pyramidal tracts* (corticospinal tracts) that cross over (decussate) at the medulla oblongata and are the main pathway to skeletal muscles. **The meninges** The brain and spinal cord are completely 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. Screenshot\_20230815\_162353\~2.jpg The dura and arachnoid maters are separated by a potential space, the *subdural space*. The arachnoid and pia maters are separated by the ***subarachnoid space***, containing ***cerebrospinal fluid*.** **Dura mater** The cerebral dura mater consists of two layers of dense fibrous tissue. The outer layer takes the place of the periosteum on the inner surface of the skull bones and the inner layer provides a protective covering for the brain. There is only a potential space between the two layers except where the inner layer sweeps inwards between the cerebral hemispheres to form the ***falx cerebri*;** between the cerebellar hemispheres to form the ***falx cerebelli*;** and between the cerebrum and cerebellum to form the ***tentorium cerebelli*.** Venous blood from the brain drains into venous sinuses between the two layers of dura mater. The ***superior sagittal* *sinus*** is formed by the falx cerebri, and the tentorium cerebelli forms the ***straight* and *transverse sinuses.*** **Arachnoid mater** This is a layer of fibrous tissue that lies between the dura and pia maters. It is separated from the dura mater by the ***subdural space*** that contains a small amount of serous fluid, and from the pia mater by the ***subarachnoid space*,** which contains *cerebrospinal fluid*. The arachnoid mater passes over the convolutions of the brain and accompanies the inner layer of dura mater in the formation of the falx cerebri, tentorium cerebelli and falx cerebelli. It continues downwards to envelop the spinal cord and ends by merging with the dura mater at the level of the 2^nd^ sacral vertebra. **Pia mater** This is a delicate layer of connective tissue containing many minute blood vessels. It adheres to the brain, completely covering the convolutions and dipping into each fissure. It continues downwards surrounding the spinal cord. Beyond the end of the cord it continues as the ***filum* *terminale***, pierces the arachnoid tube and goes on, with the dura mater, to fuse with the periosteum of the coccyx. **Ventricles of the brain and the cerebrospinal fluid 7.5** 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** These cavities lie within the cerebral hemispheres, one on each side of the median plane just below the corpus callosum. They are separated from each other by a thin membrane, the septum lucidum, and are lined with ciliated epithelium. They communicate with the third ventricle by ***interventricular foramina*.** **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 with 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 (CSF)** Cerebrospinal fluid is secreted into each ventricle of the brain by ***choroid plexuses*.** These are vascular areas where there is a proliferation of blood vessels surrounded by ependymal cells in the lining of ventricle walls. CSF passes back into the blood through tiny diverticula of arachnoid mater, called ***arachnoid villi*** (arachnoid granulations), which project into the venous sinuses. The movement of CSF from the subarachnoid space to venous sinuses depends upon the difference in pressure on each side of the walls of the arachnoid villi, which act as one-way valves. When CSF pressure is higher than venous pressure, CSF is pushed into the blood and when the venous pressure is higher the arachnoid villi collapse, preventing the passage of blood constituents into the CSF. There may also be some reabsorption of CSF by cells in the walls of the ventricles. Screenshot\_20230815\_110155\~2.jpg **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. **Functional areas of the cerebral cortex** The main functional areas of the cerebral cortex have been identified but it is unlikely that any area is associated exclusively with only one function. Except where specially mentioned, the different areas are active in both hemispheres; however, there is some variation between individuals. There are different types of functional area: motor, which direct skeletal (voluntary) muscle movements sensory, which receive and decode sensory impulses enabling sensory perception association, which are concerned with integration and processing of complex mental functions such as intelligence, memory, reasoning, judgement 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.  **Motor areas of the cerebral cortex** ***The primary motor area.*** This lies in the frontal lobe immediately anterior to the central sulcus. The cell bodies are pyramid shaped (**Betz's cells**) and they control skeletal muscle activity. **Two neurones** involved in the pathway to skeletal muscle. The first, the ***upper motor neurone,*** descends from the motor cortex through the internal capsule to the medulla oblongata. Here it crosses to the opposite side and descends in the spinal cord. At the appropriate level in the spinal cord it synapses with a second neurone (the ***lower motor neurone***), which leaves the spinal cord and travels to the target muscle. It terminates at the motor end plate of a muscle fibre. This means that the motor area of the right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa. Damage to either of these neurones may result in paralysis. Screenshot\_20230815\_110337\~2.jpg ***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. **Sensory areas of the cerebral cortex** ***The somatosensory area.*** This is the area immediately behind the central sulcus. Here sensations of pain, temperature, pressure and touch, awareness of muscular movement and the position of joints (proprioception) are perceived. The somatosensory area of the right hemisphere receives impulses from the left side of the body and vice versa. The size of the cortical areas representing different parts of the body is proportional to the extent of sensory innervation, e.g. the large area for the face is consistent with the extensive sensory nerve supply by the three branches of the trigeminal nerves (5^th^ cranial nerves).  ***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). Screenshot\_20230815\_110549\~2.jpg ***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 taste 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 and, although depicts some of the areas involved, their functions are much more complex. ***The premotor area.*** This lies in the frontal lobe immediately anterior to the motor area. The neurones here coordinate movement initiated by the primary motor cortex, ensuring that learned patterns of movement can be repeated. For example, in tying a shoelace or writing, many muscles contract but the movements must be coordinated and carried out in a particular sequence. Such a pattern of movement, when established, is described as ***manual dexterity*.** ***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.  ***Sensory speech (Wernicke's) area.*** This is situated in the temporal lobe adjacent to the parieto-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 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 recognised by touch alone because of the knowledge from past 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** This consists of two masses of grey and white matter situated within the cerebral hemispheres just below the corpus callosum, one on each side of the third ventricle. Sensory receptors in the skin and viscera send information about touch, pain and temperature, and input from the special sense organs travels to the thalamus where there is recognition, although only in a basic form, as refined perception also involves other parts of the brain. It is thought to be involved in the processing of some emotions and complex reflexes. The thalamus relays and redistributes impulses from most parts of the brain to the cerebral cortex. Screenshot\_20230815\_154449\~2.jpg  **Hypothalamus** The hypothalamus is a small but important structure which weighs around **7 g** and consists of a number of nuclei. It is situated below and in front of the thalamus, immediately above the ***pituitary gland*.** The hypothalamus is linked to the posterior lobe of the pituitary gland by nerve fibres and to the anterior lobe by a complex system of blood vessels. Through these connections, the hypothalamus controls the output of hormones from both lobes of the pituitary gland. Other functions of the hypothalamus include control of: ** the autonomic nervous system** ** appetite and satiety** ** thirst and water balance** ** body temperature** ** emotional reactions, e.g. pleasure, fear, rage** ** sexual behaviour and child rearing** ** sleeping and waking cycles.** **Brain stem** The medulla oblongata, pons, and midbrain constitute the **brainstem**. The brainstem connects the spinal cord to the remainder of the brain and is responsible for many essential functions. Damage to small brainstem areas often causes death because many reflexes essential for survival are integrated in the brainstem, whereas relatively large areas of the cerebrum or cerebellum may be damaged without life-threatening consequences. **Pons** The pons is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists mainly of nerve fibres (white matter) that form a bridge between the two hemispheres of the cerebellum, and of fibres passing between the higher levels of the brain and the spinal cord. There are nuclei within the pons that act as relay stations and some of these are associated with the cranial nerves. Others form the *pneumotaxic* and *apnoustic centres* that operate in conjunction with the respiratory centre in the medulla oblongata to control respiration. The anatomical structure of the pons differs from that of the cerebrum in that the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface. **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 fibres (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 fibres and have important roles in auditory and visual reflexes. **Medulla oblongata** The medulla oblongata, or simply the medulla, is the most interior region of the brain stem. Extending from the pons above, it is continuous with the spinal cord below. It is about 2.5 cm long and lies just within the cranium above the foramen magnum. Its anterior and posterior surfaces are marked by central fissures. The outer aspect is composed of white matter, which passes between the brain and the spinal cord, and grey matter, which lies centrally. Some cells constitute relay stations for sensory nerves passing from the spinal cord to the cerebrum. The *vital centres*, consisting of groups of cell bodies (nuclei) associated with autonomic reflex activity, lie in its deeper structure. These are the: cardiovascular centre respiratory centre reflex centres of vomiting, coughing, sneezing and swallowing. **Reticular formation** The reticular formation is a collection of neurones 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. **Functions** The reticular formation is involved in: coordination of skeletal muscle activity associated with voluntary motor movement and the maintenance of balance **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. **Functions** The cerebellum is concerned with the coordination of voluntary muscular movement, posture and balance. Cerebellar activity is not under voluntary control. The cerebellum controls and coordinates the movements of various groups of muscles ensuring smooth, even, precise actions. It coordinates activities associated with the ***maintenance of* *posture, balance* and *equilibrium*.** The sensory input for these functions is derived from the muscles and joints, the eyes and the ears. *Proprioceptor impulses* from the muscles and joints indicate their position in relation to the body as a whole; impulses from the eyes and the semicircular canals in the ears provide information about the position of the head in space. The cerebellum integrates this information to regulate skeletal muscle activity so that balance and posture are maintained. The cerebellum may also have a role in learning and language processing. Damage to the cerebellum results in clumsy uncoordinated muscular movement, staggering gait and inability to carry out smooth, steady, precise movements. **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. It is approximately 45 cm long in adult males, and is about the thickness of the little finger. A specimen of cerebrospinal fluid can be taken using a procedure called *lumbar* *puncture*. Screenshot\_20171017-201941.png Except for the cranial nerves, the spinal cord is the nervous tissue link between the brain and the rest of the body. Nerves conveying impulses from the brain to the various organs and tissues descend through the spinal cord. At the appropriate level they leave the cord and pass to the structure they supply. Similarly, sensory nerves from organs and tissues enter and pass upwards in the spinal cord to the brain. Some activities of the spinal cord are independent of the brain and are controlled at the level of the spinal cord by *spinal reflexes*. To facilitate these, there are extensive neurone connections between sensory and motor neurones at the same or different levels in the cord. The spinal cord is incompletely divided into two equal parts, anteriorly by a short, shallow *median fissure* and posteriorly by a deep narrow septum, the *posterior* *median septum*. A cross-section of the spinal cord shows that it is composed of grey matter in the centre surrounded by white matter supported by neuroglia. Figure 7.28 shows the parts of the spinal cord and the nerve roots on one side. The other side is the same.  **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 neurones*, which receive impulses from the periphery of the body *lower motor neurones*, which transmit impulses to the skeletal muscles *connector neurones*, also known as interneurones linking sensory and motor neurones, 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 fibres 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 neurones that are stimulated by the upper motor neurons or the connector neurones 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 fibres ascending to the brain, motor nerve fibres descending from the brain and fibres of connector neurones. Tracts are often named according to their points of origin and destination, e.g. spinothalamic, corticospinal. **Spinal reflexes.** These consist of three elements: sensory neurones connector neurones (or interneurones) in the spinal cord lower motor neurones. Screenshot\_20171017-203152.png In the simplest *reflex arc* there is only one of each type of the neurones above. A *reflex action* is an involuntary and immediate motor response to a sensory stimulus. Many connector and motor neurones may be stimulated by afferent impulses from a small area of skin. For example, the pain impulses initiated by touching a very hot surface with the finger are transmitted to the spinal cord by sensory fibres in mixed nerves. These stimulate many connector and lower motor neurons in the spinal cord, which results in the contraction of many skeletal muscles of the hand, arm and shoulder, and the removal of the finger. Reflex action happens very quickly; in fact, the motor response may occur simultaneously with the perception of the pain in the cerebrum. Reflexes of this type are invariably protective but they can occasionally be inhibited. For example, if a precious plate is very hot when lifted every effort will be made to overcome the pain to prevent dropping it!  **Stretch reflexes.** Only two neurones are involved. The cell body of the lower motor neurone is stimulated directly by the sensory neurone, with no connector neurone in between. The *knee jerk* is one example, but this type of reflex can be demonstrated at any point where a stretched tendon crosses a joint. By tapping the tendon just below the knee when it is bent, the sensory nerve endings in the tendon and in the thigh muscles are stretched. This initiates a nerve impulse that passes into the spinal cord to the cell body of the lower motor neurone in the anterior column of grey matter on the same side. As a result the thigh muscles suddenly contract and the foot kicks forward. This is used as a test of the integrity of the reflex arc. This type of reflex also has a protective function -- it prevents excessive joint movement that may damage tendons, ligaments and muscles. **Autonomic reflexes.** These include the pupillary light reflex when the pupil immediately constricts, in response to bright light, preventing retinal damage. Screenshot\_20230815\_154720\~2.jpg  **Peripheral nervous system** This part of the nervous system consists of: 31 pairs of spinal nerves that originate from the spinal cord 12 pairs of cranial nerves, which originate from the brain the autonomic nervous system. Most of the nerves of the peripheral nervous system are composed of sensory fibres that transmit afferent impulses from sensory organs to the brain, or motor nerve fibres that transmit efferent impulses from the brain to the effector organs, e.g. skeletal muscles, smooth muscle and glands. Screenshot\_20171017-202232.png **Spinal nerves** Thirty-one pairs of spinal nerves leave the vertebral canal by passing through the intervertebral foramina formed by adjacent vertebrae. They are named and grouped according to the vertebrae with which they are associated: ** 8 cervical** ** 12 thoracic** ** 5 lumbar** ** 5 sacral** ** 1 coccygeal.** Although there are only seven cervical vertebrae, there are eight nerves because the first pair leaves the vertebral canal between the occipital bone and the atlas (first cervical vertebra) and the eighth pair leaves below the last cervical vertebra. Thereafter the nerves are given the name and number of the vertebra immediately *above*. The lumbar, sacral and coccygeal nerves leave the spinal cord near its termination, at the level of the 1st lumbar vertebra, and extend downwards inside the vertebral canal in the subarachnoid space, forming a sheaf of nerves which resembles a horse's tail, the ***cauda equina*.** These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level, depending on their destination.  **Nerve roots** The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina. Each nerve is formed by the union of a *motor* (anterior) and a *sensory* (posterior) *nerve root* and is, therefore, a *mixed nerve*. Thoracic and upper lumbar (L1 and L2) spinal nerves have a contribution from the sympathetic part of the autonomic nervous system in the form of a *preganglionic fibre* (neurone). **Autonomic nervous system** The autonomic or involuntary part of the nervous system controls involuntary body functions. Although stimulation does not occur voluntarily, the individual can sometimes be conscious of its effects, e.g. an increase in their heart rate. The autonomic nervous system is separated into two divisions: ***sympathetic*** (thoracolumbar outflow) ** *parasympathetic*** (craniosacral outflow). Screenshot\_20230815\_110654\~2.jpg The two divisions work in an integrated and complementary manner to maintain involuntary functions and homeostasis. Such activities include coordination and control of breathing, blood pressure, water balance, digestion and metabolic rate. Sympathetic activity predominates in stressful situations as it equips the body to respond when exertion and exercise is required. Parasympathetic activity is increased (and sympathetic activity is normally lessened) when digestion and restorative body activities predominate. There are similarities and differences between the two divisions. Some similarities are outlined in this section before the descriptions of the two divisions below. As with other parts of the nervous system, the effects of autonomic activity are rapid. The effector organs are: *smooth muscle*, which controls the diameter of smaller airways and blood vessels *cardiac muscle*, which controls the rate and force of cardiac contraction *glands* that control the volumes of gastrointestinal secretions. The ***efferent (motor) nerves*** of the autonomic nervous system arise from the brain and emerge at various levels between the midbrain and the sacral region of the spinal cord. Many of them travel within the same nerve sheath as peripheral nerves to reach the organs they innervate. Each division has two efferent neurones between the central nervous system and effector organs. These are: *the preganglionic neurone* *the postganglionic neurone.* The cell body of the preganglionic neurone is in the brain or spinal cord. Its axon terminals synapse with the cell body of the postganglionic neurone in an *autonomic ganglion* outside the CNS. The postganglionic neurone conducts impulses to the effector organ. **Sympathetic nervous system** Since the preganglionic neurones originate in the spinal cord at the thoracic and lumbar levels, the alternative name of 'thoracolumbar outflow' is apt. **Parasympathetic nervous system** Like the sympathetic nervous system, two neurones (preganglionic and postganglionic) are involved in the transmission of impulses to the effector organs. The neurotransmitter at both synapses is acetylcholine. **Functions of the autonomic nervous system** The autonomic nervous system is involved in many complex involuntary reflex activities which, like the reflexes described in earlier sections, depend not only onsensory input to the brain or spinal cord but also on motor output. In this case the reflex action is rapid contraction, or inhibition of contraction, of involuntary (smooth and cardiac) muscle or glandular secretion. These activities are coordinated subconsciously. Sometimes sensory input does reach consciousness and may result in temporary of sympathetic stimulation. It is said that sympathetic stimulation mobilises the body for **'fight or flight'**. The effects of stimulation on the heart, blood vessels and lungs (see below) enable the body to respond by preparing it for exercise. Additional effects are an increase in the metabolic rate and increased conversion of glycogen to glucose. During exercise, e.g. fighting or running away, when oxygen and energy requirements of skeletal muscles are greatly increased, these changes enable the body to respond quickly to meet the increased energy demand. ***Parasympathetic stimulation*** has a tendency to slow down cardiac and respiratory activity but it stimulates digestion and absorption of food and the functions of the genitourinary systems. Its general effect is that of a **'peace maker'**, allowing digestion and restorative processes to occur quietly and peacefully. Normally the two systems function together, maintaining a regular heartbeat, normal temperature and an internal environment compatible with both physiological needs and the immediate external surroundings. **Introduction of Needles into the Subarachnoid Space** Several clinical procedures involve the insertion of a needle into the subarachnoid space inferior to the level of the second lumbar vertebra. The needle is introduced into either the L3/L4 or the L4/L5 intervertebral space. The needle does not contact the spinal cord because it extends only approximately to the second lumbar vertebra of the vertebral column, but the subarachnoid space extends to level S2 of the vertebral column. Nor does the needle damage the nerve roots of the cauda equina located in the subarachnoid space because the needle quite easily pushes them aside. In **spinal anesthesia,** or spinal block, drugs that block action potential transmission are introduced into the subarachnoid space to prevent pain sensations in the lower half of the body. A **spinal tap** is the removal of CSF from the subarachnoid space. A spinal tap may be performed to examine the CSF for infectious agents (meningitis), for the presence of blood (hemorrhage), or for the measurement of CSF pressure. A radiopaque substance may also be injected into this area, and a **myelogram** (radiograph of the spinal cord) may be taken to visualize spinal cord defects or damage.