Central Nervous System

CENTRAL NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM 1.

It includes the brain and spinal cord which are enclosed in cranium and vertebral column, respectively.

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Both brain and spinal cord are derived from the neural tube of the embryo. Prosencephalon becomes the thalamus, hypothalamus (diencephalon), cerebral cortex, corpus striatum, hippocampus and amygdala (telencephalon). Mesencephalon becomes midbrain. Rhombencephalon develops into the medulla (myelencephalon), pons and cerebellum (metencephalon).

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BRAIN (ENCEPHALON) •

It is a mass of nerve cells, enclosed within bony case called as cranium (brain box) constituting 98% weight of CNS, i.e., about 1400 gms (in adult man) made up of two components: (i) Gray matter consisting cell body (cyton) part of the neuron (ii) White matter the part with axon and dendron covered by myelin sheath. Embryonic division The anterior end of the neural tube first becomes distinct into three parts: Prosencephalon, Mesencephalon and rhombencephalon. Prosencephalon divides as telencephalon and diencephalons; the mesencephalon remains undivided and rhombencephalon divides as metencephalon and myelencephalon. The corresponding future (adult) parts of these five divisions are given below in the box.

COVERINGS OF BRAIN OR MENINGES There are three layers of connective tissue capsule to protect the brain and spinal cord: (i) Pia mater – the innermost thin, vascular membrane that remains close to nervous tissue. (ii) Arachnoid layer (or spider’s web) – the middle fibrous layer as network. (iii) Dura mater – the outermost tough, fibrous layer along cranial bones. Space between pia mater and arachnoid layer is called as subarachnoid space, between dura mater and arachnoid is subdural space and between dura mater and vertebral column is epidural space (which is absent is brain but present in spinal cord) VENTRICLES OF BRAIN • • • • • •

Cavity within brain is designated as ventricle lined with ependymal (ciliated) cells. Within cerebral hemispheres are ventricle I and II (or lateral ventricles or paracoel or telocoel). Both together open by a common foramen of Monro into the ventricle III (diocoel) of diencephalon. The ventricle III is connected with the ventricle IV (metacoel) of medulla oblongata through a narrow passage called Iter or aqueduct of Sylvius in the mid brain. Three small apertures, a median Foramen of Magendie and a pair of Lateral foramen of Luschka, are also present in the roof of ventricle IV through which the CSF can pass into meningeal spaces. Ventricles indirectly provide nutritive, excretory and defensive function. They also provide support to the nervous tissue and protect them against mechanical shock and contain cerebrospinal fluid. The plexuses of blood vessels (anterior and posterior choroids plexuses) that project into ventricles release most of the CSF which flows slowly towards ventricle IV by pressure and passes into the spinal cord.

PARTS OF ADULT BRAIN (HUMANS) Fore brain (or Pros encephalon) has 3 parts, olfactory lobe, cerebrum and diencephalon. Olfactory Lobes (or Rhinencephalon) • These are the pair of solid club-shaped structures present along the lower surface of cerebrum; each consists of an olfactory bulb and an olfactory tract to relay sense of smell to temporal lobes. Cerebrum (or big brain) The main and largest part of brain; divided by median fissure into two lateral halves called cerebral hemisphere. The outer layer cerebral cortex of grey matter has large number of gyri and sulci on the surface to increase it area. The dorsal thick roof is called as pallium which in humans is derived differently hence called as neopallium. In Human Brain

Cerebrum is largest part of the brain and is divided into four lobes each with areas that control respective functions as follows: (i) Frontal lobe. The anterior front part separated by central sulcus from parietal lobe. Its prefrontal or association area is the site for controlling thought, learning, intelligence and intellectual activities, translating perception and memories into plan and actions, reasoning, decision making, expression of emotions; motor area controls voluntary movements. Broca’s motor area is the centre for speech and premotor area control involuntary movements. (ii) Parietal lobe: Its somaesthetic area controls general sensation, pain, touch, temperature manual skills, etc. Gustatory area is the center for taste. (iii) Temporal lobe: Lies on both lateral sides below the parietal and frontal lobes separated by lateral fissure. Its auditory area is the site for hearing with the Wernicke’s area of understanding speech. It also controls smell and memory. (iv) Occipetal lobe: The hindermost part behind parietal lobe, its visual area is the site for vision. • Rolandic sulcus (or central sulcus) separates frontal and parietal lobes whereas Sylvian or lateral sulcus separates temporal lobe from parietal lobe. DIENCEPHALON • • • • • •

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The small posterior part of fore brain remains dorsally overlapped by cerebrum. Its roof is called epithalamus, having choroids plexus with blood vessels and on its dorsal furface there is epiphysis (or pineal body). The floor is hypothalamus with ventral hypophysis (or pituitary gland) attached through infundibulum. The thick lateral walls are the two optic thalami connected interiorly by middle or soft commissure (massa intermedia) and a Habenular commissure in the antero-dorsal part of the diocoel. In front of the hypophysis there is crossed tract of nerves called optic chiasma that connects the left optic lobe with right cerebrum and right optic lobe with left cerebrum.. Hypothalamus constitutes 1/300th part of total brain mass (just 4 gm) in man. It integrates and controls the autonomic (involuntary) functions, maintains homeostasis and equilibrium of the body. It is also the center for thirst, appetite, satiation, love, anger, sex, satisfaction, metabolism, thermoregulation, behaviors related to fighting, fleeing ,mating etc. Septum a part of hypothalamus is the center for sexual arousal.

THE LIMBIC (OR LIP-LIKE) SYSTEM • • • •

This is the site of emotions, anger, fear, pleasure and instinctive behavior. It includes interconnected parts of frontal lobe, temporal lobe, thalami and hypothalamus. It is also associated with other related parts (motor areas) of the brain that control the responses like sweating, blushing, sobbing and laughing. Amygdala is almond-shaped bulge of neurons attached to both the tips of the fork (of limbic system). It is like defence castle to control the moods (anger and range). Hippocampus is the swollen lower lip of limbic form, this controls the signals of smell and memories, converts information of short term memory into long term memory i.e, for learning.

BRAIN STEM • • •

In human it includes the area between thalamus and spinal cord as mid brain, medulla oblongata, pons varoli. All the nerve fibres which relay signals of afferent input and efferent output between spinal cord and the cerebrum, pass through it. 10 out of 12 cranial nerves, emerge from here. A core of tissue called reticular formation running through entire brain stem is composed of highly branched neurons (called nuclei).It receives and integrates information from many different pathways.

MID BRAIN (OR MESSENCEPHALON) • •

The small part, also remains covered by cerebrum. It mainly consists of dorsal optic lobes. But, in humans there are four smaller solid lobes corpora quadrigemina, the anterior pair is called superior colliculi and posterior pair as inferior colliculi, responsible for visual and auditory reflexes respectively. A pair of tract of nerve called crura cerebri (or cerebral peduncles) is situated on ventral side connects the cerebral cortex with cerebellum and controls semi-automatic movement.

HIND BRAIN (OR RHOMBENCEPHALON) • The posterior part of brain has two main parts cerebellum and medulla oblongata. Cerebellum (or little brain) It constitutes about 1/8th of the size of cerebrum situated below occipital lobe connected with ventral brain stem by three peduncles, with the lateral halves, cerebellar hemispheres. ‰ Its outer surface (cortex of gray matter) has a large number of gyri and sulci. Medullar solid part is filled with white matter called arbor vitae (tree of life). ‰ It acts as executive of the cerebrum to run the initiated voluntary muscle actions. MEDULLA OBLONGATA ‰

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It is the posterior most rhomboidal part with large number of sensory and motor nerve connections. On its dorsal surface there is a colored part, posterior choroids plexus with network of blood vessels fused with piamater. Outside the cranium it extends out as spinal cord with large number of sensory and motor nerves crossing from one side to other. Reticular formation connect the thalami with major nerves in spinal cord hence acts as gatekeeper to consciousness.

PONS VAROLI It is situated on ventral side of the cerebellum. Carries impulses from one hemisphere of cerebellum to the other. Helps in coordination of muscle movement of two sides of the body. Possesses pontine (or pneumotaxic) center, the component of respiratory control.

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SPINAL CORD • • • • • •

It is a cylindrical, cord-like, uniform extension of medulla oblongata, (42-45 cm long and 2 cm thick in man) runs from neck to lumbar region through the neural canal of vertebral column, it ends in a thin prolongation called filum terminale. It stops growing beyond 4-5 years of age. Here white matter is outside and H-shaped grey matter is inside. The spinal canal with CSF is lined with ciliated ependymal cells. Sensory neurons enter the dorsal root and have their cell bodies in the dorsal root ganglion, close to the spinal cord. In the dorsal horn of grey where they synapse with interneurons. The interneurons synapse with motor neurons in the ventral horn and leave the spinal cord via the ventral root. From the thorax region downwards, a lateral horn is present between dorsal and ventral horns that contain the cell bodies of the preganglionic autonomic neurons.

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Ascending tracts carry sensory information to the brain and descending tracts relay motor information to the spinal cord. It acts as a coordinating centre for reflexes like knee-jerk response, contraction of the urinary bladder

CEREBROSPINAL FLUID (CSF) • • • • •

Clear, colorless fluid similar to plasma with much less protein and cholesterol, about 80-150 ml in amount, secreted by (ACP & PCP) i.e., Anterior choroids plexus and Posterior choroids plexus. Protects CNS as shock absorbing medium. Provides buoyancy to brain, (net wt. Of brain is reduced from 1.4 kg to 0.18 kg) hence pressure at the base is reduced. Excretion of waste products. Carries nutrient, respiratory gases and hormones from blood to the remote parts of brain.