Central Nervous System PDF

# Central Nervous System The nervous system is classified into: 1. **Central Nervous System** formed by brain and spinal chord 2. **Peripheral Nervous System** which includes: * Cranial nerves * Spinal nerves * Autonomic nervous system ## Brain The brain is the part of the central...

# Central Nervous System The nervous system is classified into: 1. **Central Nervous System** formed by brain and spinal chord 2. **Peripheral Nervous System** which includes: * Cranial nerves * Spinal nerves * Autonomic nervous system ## Brain The brain is the part of the central nervous system which lies within the cranial cavity. It consists of the following parts: 1. **Cerebrum** (made of two cerebral hemispheres) 2. **Cerebellum** 3. **Mid brain** 4. **Pons** 5. **Medulla oblangata** These three structures form the **Brain Stem** ### Cerebrum The cerebrum is the largest part of the brain. #### Parts The Cerebrum consists of two hemispheres namely right and left hemispheres. Theses two hemispheres are separated by: 1. **Falx cerebri**, a fold of dura mater, in the midline 2. **Corpus callosum**, a bridge of white matter, below #### Lobes Each cerebral hemisphere has four lobes. They are: 1. Frontal lobe 2. Temporal lobe 3. Parietal lobe 4. Occipital lobe ## Diagram Showing Different Parts of the Brain The surface of the hemispheres have some fissures. The four important fissures are: 1. **Central sulcus** 2. **Parieto-occipital sulcus** 3. **Callaso-marginal fissure** 4. **Sylvian fissure** ## Lobes and Fissures of the Brain ### Grey Matter The grey matter consists of nerve cells which constitute the surface of hemispheres. It is also called as cerebral cortex. It contains a number of folds or convolutions. These convolutions are separated by fissures described above. ### White Matter White matter is present in the interior of the hemispheres. It contains nerve fibres. ## Functions of Cerebrum 1. **Motor Functions** which include control of voluntary movements. 2. **Sensory Functions** which include: * Analysis of touch, temperature, pain, pressure, shape, etc * Governance of conditional reflexes * Control of intelligence, speech, memory, etc through higher centres of the cerebrum These functions are governed by two parts of the cerebral cortex namely **Motor cortex**, which controls voluntary movements of the body, and **Sensory cortex**, which deals with sensations like touch, pressure, temperature etc. The central sulcus divides the cerebral cortex to form these two parts. ## Specialized Structures of Grey Matter Deep in the cerebral hemispheres there are some collections of grey matter which form specialized structures. They are: 1. **Basal Ganglia** 2. **Thalamus** 3. **Hypothalamus** ### Basal Ganglia The basal ganglia are the following four pairs of grey matter: 1. caudate nuclei 2. lentiform nuclei 3. amygdaloid nuclei 4. claustrum The caudate nuclei and lentiform nuclei together form the **corpus striatum**. These nuclei have connections with each other and also with other brain centres. The basal ganglia control the fine movements of the body. ### Thalamus The thalamus is situated at the base of the cerebrum. It consists of two ovoid masses separated by the third ventricle. The thalamus is an important relay station for the incoming fibres of spinal cord and brain stem on their way to cerebral cortex. ### Hypothalamus The hypothalamus is present below the thalamus. It forms the floor and part of the lateral walls of the third ventricle. The functions of the thalamus are: 1. Synthesis of oxytocin and vasopressin (the two posterior pituitary hormones) 2. Production of releasing factors (like GH-releasing factor, FSH-releasing factor, and LH-releasing factor) 3. Regulation of body temperature 4. Co-ordination of autonomic nervous system 5. Control of appetite ## Cerebellum The cerebellum lies below the cerebrum. It is situated in the posterior cranial fossa behind the pons and medulla. The cerebellum also contains: * **Tentorium cerebelli**: a layer of dura mater which separates the cerebrum and cerebellum * **Cerebellum**: consists of two hemispheres * **Falx cerebelli**: a partition of dura mater which separates the two cerebellar hemispheres. It also contains: 1. An outer grey matter which has numerous convolutions. 2. An inner white matter ## Functions of Cerebellum 1. Co-ordination of muscular movements 2. To keep the muscles in a normal state of tension 3. Maintenance of balance and equilibrium ## Midbrain The midbrain is a short constricted segment of brain which forms a connection between cerebrum and other parts of the brain. It contains: 1. **Cerebral peduncles**: two thick bundles present ventrally. They are separated by a deep depression 2. **Cerebral aqueduct**: a narrow canal which passes through the centre of midbrain. This canal connects the cavity of the third and fourth ventricle. 3. **Superior colliculi**: a pair of rounded bodies present at the root of aqueduct. They are the reflex centres for hearing 4. **Inferior colliculi** (which is an interior pair) are also present at the root of aqueduct. They contain the reflex centres for hearing. 5. **Nuclei of 3rd and 4th cranial nerves** are present in the mid brain. ## Pons The pons lies under the cerebellum. It contains both grey matter and white matter. It forms a bridge between the two lobes of cerebellum. Nerve fibres pass through pons to midbrain above and medulla oblangata below. The pons contains nuclei for the 5th, 6th, 7th, and 8th cranial nerves. ## Medulla Oblangata The medulla oblangata is the part of the brain which forms the upward continuation of spinal cord. It lies on the base of skull just in front of foramen magnum. The medulla oblangata contains two ridges on the ventral surface called **pyramids**. The pyramids are formed by a number of nerve fibres which descend from cerebral hemispheres into the spinal cord. Before they enter into the spinal cord, the fibres of one side cross with those of the other side. This crossing of nerve fibres is called as **decussation of pyramids**. Because of this crossing, injury to one side of the brain produces paralysis of the muscles of opposite side. The medulla oblangata contains nuclei for 9th, 10th, 11th, and 12th cranial nerves. ## Spinal Cord ### Position The spinal cord is a part of the central nervous system which is present in the vertebral canal. The upper end of spinal cord commences at the level of foramen magnum. This upper end is continuous above with medulla oblangta. The lower end lies at the level of the first lumbar vertebra. So, it does not fill the lower part of the vertebral canal. The lower end of the spinal cord terminates in a bunch of nerves (resembling the tail of a horse) called as cauda equina. The longest of these nerves is called as filum terminale which ends at the coccyx. ### Enlargement The spinal cord shows two enlargements in its course. They are cervical and lumbar enlargements. Nerves to the arms and legs arise from these enlargements. ### Structure A cross section of the spinal cord shows the following structures: 1. White matter on the surface and grey matter in the interior (reverse of what is seen in the brain). 2. The grey matter is H-shaped. It divides the white matter into anterior, posterior, and lateral columns. 3. Anterior fissure separates the anterior columns of each side. 4. The H-shaped grey matter contains anterior and posterior horns. 5. The anterior horn cells are concerned with motor functions. Anterior root of spinal nerves arise from the anterior horn. 6. The posterior horn cells are sensory in function. The posterior root of spinal nerves enter the spinal cord through the posterior horns. 7. The posterior root contains a ganglion called a dorsal ganglion. 8. A minute canal is present in the centre of spinal cord. It is called as central canal. It contains cerebrospinal fluid and it is continuous above with the fourth ventricle. ## The Meninges The brain and spinal cord are protected by three coverings called meninges. They are 1. **Dura mater**, which forms the outer layer 2. **Arachnoid mater**, which forms the middle layer 3. **Pia mater**, which forms the inner layer ### Dura Mater The dura mater is the outermost fibrous membrane. It lines the interior of the skull. It contains two folds which project into cranial cavity. They are: 1. **Falx cerebri**: which lies in the midline. It separates the right and left cerebral hemispheres. 2. **Tentorium cerebelli**: which lies horizontally. It separates the cerebrum and cerebellum. ### Arachnoid Mater The arachnoid mater is a thin, transparent membrane. It lies between dura mater and pia mater. The space between arachnoid mater and pia mater is called as subarachnoid space. This space contains the cerebrospinal fluid. ### Pia Mater The pia mater is the innermost layer. It closely covers the brain and spinal cord. Also, it dips into the fissures of brain. It contains the minute blood vessels which supply the brain and spinal cord. ## Ventricles of Brain The ventricles are cavities in the brain which are filled with cerebrospinal fluid (CSF). The ventricles are four in number. They are: 1. The right and left lateral ventricles which lie in each cerebral hemisphere. 2. The third ventricle which is present in the midline. The third ventricle communicates with the two lateral ventricles and also with the fourth ventricle. 3. The fourth ventricle lies in front of cerebellum and behind pons and medulla oblangata. It is continuous below with the central canal of spinal cord. ## Cerebrospinal Fluid (CSF) The CSF is a clear, colourless, alkaline fluid present in: 1. Subarachnoid space 2. Ventricles of brain 3. Central canal of spinal cord The CSF bathes the entire surface of the brain and spinal cord. ## Secretion of CSF The CSF is secreted by a network of blood vessels called **choroid plexus**. This plexus is situated in the lateral ventricles. ### Composition of CSF The CSF contains: * Water * Glucose * Proteins * Nitrogenous substances * Electrolytes (like sodium, potassium, and calcium chlorides) * A few cells (mostly lymphocytes) ## Functions of CSF 1. It protects the brain and spinal cord from shock. 2. It conveys nutrients to the brain and spinal cord. 3. It removes the waste products of brain and spinal cord. ## Peripheral Nervous System ### Cranial Nerves The cranial nerves are 12 pairs of nerves attached to the brain. These nerves may be: 1. Sensory: carrying impulses to the brain. 2. Motor: carrying impulses from the brain to the periphery 3. Mixed: containing both sensory and motor fibres. The following are the 12 pairs of cranial nerves: 1. **Olfactory Nerve**: (sensory) It is the nerve of smell 2. **Optic Nerve**: (sensory) It is the nerve of vision 3. **Oculomotor Nerve**: (motor) Supplies the muscles of eye ball 4. **Trochlear Nerve**: (motor) Supplies the muscles of eye ball 5. **Trigeminal Nerve**: (mixed) * Sensory fibres to face and forehead. * Motor fibres to muscles of mastication. 6. **Abducens Nerve**: (motor) It supplies the muscles of eye ball. 7. **Facial Nerve**: (motor) It supplies the muscles of expression. 8. **Auditory Nerve**: (sensory) It consists of two parts: * **Cochlear Nerve**: the nerve of hearing * **Vestibular Nerve**: the nerve of equilibrium and balance 9. **Glossopharyngeal Nerve**: (mixed) Sensory to tongue and motor to the pharyngeal muscles. 10. **Vagus Nerve**: (mixed) It is distributed two pharynx, larynx, lungs, heart, stomach, and intestines. 11. **Accessory Nerve**: (motor) It divides into two parts. One part joins the vagus and supplies pharynx and larynx. The other part supplies sternomastoid and trapezius muscles of neck. 12. **Hypoglossal Nerve**: (motor) It supplies the muscles of tongue ### Spinal Nerves There are 31 pairs of the spinal nerves which arise from the spinal cord. Each spinal nerve contains an anterior root and a posterior root. These two roots unite to form a single nerve trunk which comes out of the vertebral column. The posterior nerve root contains the posterior root ganglion (Fig. 62). One spinal nerve on each side corresponds with each segment of the vertebral column. ### Nerve Plexuses At certain regions of the spinal cord, some individual nerve trunks unite to form plexuses. The following are the four main plexuses: 1. **Cervical Plexus**: It is formed by the first four cervical nerves. Phrenic nerve is an important branch of cervical plexus and it supplies the diaphragm. 2. **Brachial Plexus**: It is formed by the lower four cervical nerves and first thoracic nerve. The important branches of brachial plexus are: * circumflex nerve * musculocutaneous nerve * radial nerve * ulnar nerve * median nerve 3. **Lumbar Plexus**: It is formed by the first four lumbar nerves. Its main branches are femoral nerve and obturator nerve. 4. **Sacral Plexus**: It is formed by the 4th and 5th lumbar nerves and the five sacral nerves. Its main branch is sciatic nerve ## Sensation and Sensory Path ### Sensation Sensations can be classified into two types: 1. **Special Sensation**: which can be felt by specialised organs e.g. sensation of smell, taste, sight, hearing, etc 2. **General Sensation**: which can be felt by all parts of the body. General sensations can again be classified into two types: * **Superficial Sensation**: (like pain, touch, and temperature which are felt by the skin) * **Deep Sensations**: (like pain, touch, and deep pain) felt by muscles, joints, and other organs. ### Sensory Organs The organs which receive the sensory stimuli can be classified into: 1. **Exteroceptive Organs**: which receive sensations produced by stimuli which occur outside the body e.g. smell, taste, sight, and temperature 2. **Interoceptive Organs**: which receive sensations produced from stimuli inside the body. * **Enteroceptive organs**: which receive stimuli from visceral organs (e.g. thirst and hunger). * **Proprioceptive organs**: which receive stimuli from muscles and joints (e.g. touching the mouth after closing the eyes. The muscles and joints can identify the mouth or any other organ even in the absence of vision). ### Sensory Path The following is the path of sensory impulses: 1. The peripheral nerves carry superficial sensations (from skin) and deep sensations (from muscles and joints) towards the spinal cord. 2. These sensory fibres enter the posterior horn of grey matter of spinal cord through posterior nerve root. 3. Then the fibres of deep and superficial sensation travel by different paths as follows: * Fibres carrying superficial sensation travel upwards in the anterior column of spinal cord. * Fibres carrying deep sensations travel upwards in the posterior column of spinal cord. 4. Both these fibres cross each other and go to the opposite side of the spinal cord. This crossing occurs at the level of medulla oblangata. 5. The impulses are then conveyed to sensory areas of brain through brain stem, thalamus, and white matter of brain. ### Motor Path The motor path for voluntary movements consists of two neurons: 1. **Upper motor neuron**: extends from the pyramidal cells of motor cortex of the brain to the anterior horn cells of spinal cord. The path is as follows: * Impulses commence from the pyramidal cells of the motor cortex (situated in front of central sulcus). * The fibres pass through internal capsule and pons and reach the medulla oblangata. * These fibres cross each other in the medulla oblangata * Then they travel through the lateral column of spinal cord and terminate in the anterior horn cells. 2. **Lower motor neuron**: extends from the anterior horn to the peripheral nerve supplying the muscle. The path is as follows: * The fibres from anterior horn cells reach the anterior nerve root. * The fibres of anterior nerve root unite with the incoming fibres of posterior nerve root, and form the spinal nerve. * The motor nerves emerging from the spinal nerves supply the muscle. ## Reflex Action Reflex action occurs independent of will and it is concerned with involuntary movements. It is a defence mechanism manifesting as a quick and automatic motor response for a sensory stimulus. ### Reflex Arc A reflex arc consists of structures which are involved in the production of a reflex action. These structures are: 1. **A sensory organ**: which receives the sensory stimulus. 2. **A sensory nerve**: which arises from the sensory organ. Through a peripheral nerve, it terminates at the posterior nerve root of the spinal cord. 3. **The spinal cord**: which transmits the impulses to the motor organ such as muscle. 4. **A motor nerve**: which commences from the anterior horn cell of spinal cord. This nerve transmits the impulses to the motor organ such as muscle. ### Importance of Reflex Action The impulses of reflex action are carried only to the spinal cord and not to the motor cortex of the brain. The impulses from the spinal cord are then conveyed to the motor organ. So the response is quick and immediate. ## Autonomic Nervous System The Autonomic Nervous System (Visceral, Vegetative, or Involuntary nervous system) is widely distributed throughout the body. It controls tissues (e.g. smooth muscles, heart, and glands) which are not under voluntary control. ### Autonomic Nervous System Divisions The autonomic nervous system consists of two divisions: 1. **Sympathetic Nervous System**: (Thoraco-Lumbar Outflow.) 2. **Parasympathetic Nervous System**: (Cranio-Sacral Outflow) The sympathetic and parasympathetic mostly exhibit mutual antagonism. ## Functions of the Autonomic Nervous System The parasympathetic nervous system is mostly concerned with the vegetative functions, e.g. motility and secretions of the gastrointestinal tract. This system is essential for the normal existence of the organism. The sympathetic nervous system is concerned with preparing the organism for emergency (fight or flight). Unlike the parasympathetic, the sympathetic nervous system is not essential for normal existence. ## Structure of the Autonomic Nervous System Both the sympathetic and parasympathetic nervous systems consist of: 1. **Preganglionic nerve**: which originates from the spinal cord for the sympathetic nervous system and from the brain stem or sacral spinal cord for the parasympathetic nervous system. 2. **Ganglion**: which receives the preganglionic nerve. 3. **Postganglionic nerve**: which arises from the ganglion and synapses with the effector organ. 4. **Effector organ**: which is the target tissue, such as smooth muscle, cardiac muscle, or gland, that is innervated by the postganglionic nerve. ## Sympathetic Nervous System ### Preganglionic Fibers * The preganglionic fibers of the sympathetic nervous system arise from the lateral horn cells of the spinal cord, typically from the first thoracic to third lumbar segment. * They pass through anterior nerve roots of the spinal nerves. * They then travel for a short distance in the spinal nerve. * From the spinal nerve, they are communicated to the ganglia of the sympathetic chain through **white rami communicantes**. The sympathetic chain consists of a series of ganglia present on either side of the vertebral column. These ganglia are connected with each other by means of fibers. ### Postganglionic Fibers * Postganglionic fibers are formed by **grey rami communicantes** which arise from ganglia of the sympathetic chain. * They enter into the spinal nerve of the same level and reach the organ which they supply. * Some of the fibers may ascend to higher ganglia or descend to lower ganglia of the sympathetic chain. They come out from a ganglion of appropriate level through grey rami communicates and enter into spinal nerves. ## Parasympathetic Nervous System ### Preganglionic Fibers * Preganglionic fibers of the parasympathetic nerves arise from cells present in: * The midbrain * The medulla * The sacral portion of the spinal cord. * From the midbrain, the fibers emerge through the oculomotor nerve. * From the medulla, they emerge through facial, glassopharyngeal, and vagus nerves. * At the sacral portion of spinal cord, they arise from the anterior column of the 2nd, 3rd, and 4th lumbar segments. They then pass through anterior roots of the corresponding spinal nerves. * These nerves (which have three different origins) end in a ganglion each. These parasympathetic ganglia are very near to the structure which these nerves supply. ### Postganglionic Fibers Postganglionic fibers arise from the ganglia and then reach the structures which these nerves supply. ## Chemical Transmitters of Autonomic Nerves ### Parasympathetic System The transmitter in the preganglionic parasympathetic nerve is **acetylcholine (Ach)**, which is liberated at the ganglion. * Acetylcholine is also the transmitter in the postganglionic parasympathetic nerve, where it is liberated at the postganglionic parasympathetic nerve ending. ### Sympathetic System * Similar to preganglionic parasympathetic nerves, **acetylcholine** is the transmitter in preganglionic sympathetic nerves also. * But in postganglionic sympathetic nerves, **noradrenaline** is the transmitter. It is liberated at the postganglionic sympathetic nerve ending. ## Functions of Autonomic Nervous System The following table summarizes the functions of the sympathetic and parasympathetic nervous systems. The mutual antagonistic effects of these two can be noted. | **Sympathetic System** | **Parasympathetic System** | | :-------------------------------------------------------- | :------------------------------------------------------------------ | | Dilatation of the pupil of the eye | Constriction of the pupil | | Dilatation of Bronchi | Constriction of bronchi | | Constriction of blood vessels | Dilatation of blood vessels | | Increase in the rate and force of the heart | Decrease in the rate and force of the heart | | Decrease in glandular secretions | Increase in glandular secretions | | Decrease in gastrointestinal motility | Increase in gastrointestinal motility | ### Adrenergic Receptors The receptors present at the postganglionic sympathetic nerve ending are called "adrenergic receptors." The a **adrenergic receptors** are classified as: 1. **Alpha receptors**: The action on which is excitatory (except intestine). 2. **Beta receptors**: The action on which is inhibitory (except the heart). These receptors are of great importance in pharmacology.