Spinal cord

physiology of spinal cord

1. Physiology-3 Department of physiotherapy Central Nervous System BY Dr.Laraib jameel Rph Follow me on slideshare.net https://www.slideshare.net/

2. Spinal cord Situation and Extent Spinal cord lies loosely in the vertebral canal. It extends from foramen magnum where it is continuous with medulla oblongata Shape and Length Spinal cord is cylindrical in shape. Length of the spinal cord is about 45 cm in males and about 43 cm in females, with an average diameter of about 1.25 cm.

3. Spinal cord Coverings Spinal cord is covered by sheaths called meninges , which are membranous in nature . Meninges are dura mater, pia mater and arachnoid mater. These coverings continue as coverings of brain. Meninges are responsible for protection and nourishment of the nervous tissues.

4. Spinal cord Enlargements However , opposite to the attachments of the nerve roots, the spinal cord presents definite fusiform swellings called  cervical   and   lumbar enlargements  respectively. These enlargements are produced due to the presence of large number of large motor neurons in these regions to supply the musculature of the upper and lower limbs and associated girdles. The  cervical enlargement  extends from C5 to T1 spinal segments whereas  lumbar enlargement  extends from L2 to S3 spinal segments.

5. Conus Medullaris and Filum Terminale Below the lumbar enlargement, spinal cord rapidly narrows to a cone-shaped termination called conus medullaris . A slender non-nervous filament called filum terminale extends from conus medullaris downward to the fundus (hollow part) of the dural sac at the level of second sacral vertebra. Spinal Nerves Segments of spinal cord correspond to 31 pairs of spinal nerves in a symmetrical manner. 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal (final part of vertebrae at the base). A pair of spinal nerves leaves each segment of the spinal cord.

6. Segments : Like the vertebral column, the spinal cord is also segmented though the segments are not visible externally . The part of spinal cord to which a pair of spinal nerves (right and left) is attached is known as  spinal segment   Cervical (neck) Thoracic (chest) Lumbar (abdominal) Sacral (pelvic) Coccygeal (tailbone)

7. Spinal cord Internal Structure of the Spinal Cord The cross-section of the spinal cord shows that it consists of an inner core of grey matter, and a peripheral zone of white matter.

8. Grey Matter In cross-section of cord the grey matter is seen as H-shaped (or butterfly-shaped ) fluted column , extending throughout the length of the spinal cord . It is divided into symmetrical right and left comma-shaped masses which are connected across the midline by a  transverse grey commissure. The central canal of the cord passes through the centre of grey commissure. The canal is surrounded by  substantia gelatinosa centralis . The lateral comma-shaped mass of grey matter is further divided by a transverse grey commissure into a narrow elongated posterior horn , and a broad anterior horn.

9. Transverse sections of the spinal cord at different levels showing the arrangement of grey and white matters. The amount of the grey matter seen at a particular level is well correlated with the mass of tissue it supplies . It is, therefore, maximum in the regions of cervical and lumbar enlargements , which supply the limbs and their associated girdles. The horns are thus largest in the regions cervical and lumbar enlargements . The amount of white matter in the spinal cord undergoes progressive increase from below upwards .

10. Structure of the grey matter Like in other regions of the CNS, the grey matter of spinal cord consists of (a) nerve cells, (b) neuroglia (glial cells), and (c) blood vessels. Neurons in the grey matter of the spinal cord The nerve cells in the grey matter of spinal cord are multi-polar and can be classified into the following two ways: Structural classification •  Golgi type I , having long axons, which leave the grey matter and either join the anterior nerve roots or form the nerve tracts. •  Golgi type II , having short axons, which do not leave the grey matter and remain intrasegmental or intersegmental in position.

11. Functional classification •  Motor neurons:   These are present in the anterior and lateral horns. Types of motor neurons in the anterior grey columns: 1. Alpha ( α) neurons:   They are large multipolar cells (25  µ m or more in diameter) and supply the extrafusal skeletal muscle fibres . 2.  Gamma ( γ) neurons:   They are small multipolar cells (15–25  µ m in diameter) and supply the intrafusal muscle fibres of the neuromuscular spindles in skeletal muscles . Sensory neurons:   These are present in the posterior horn and involved in relay of sensory information to the different parts of the brain , forming  ascending tracts;  or to the other segments of spinal cord forming  intersegmental tracts . Interneurons :   These are small neurons present throughout the grey matter of the spinal cord. They connect different types of neurons, hence also called  association neurons . These are either inhibitory or excitatory, and concerned with integration of segmental activities.

12. White Matter The white matter of the spinal cord surrounds the central ‘H-shaped mass of grey matter, and mainly consists of nerve fibers, the large proportion of them being myelinated , give it a white appearance’. Types of fibers in the white matter Functionally, the fibers in the white matter of spinal cord are divided into following three types: 1.   Sensory fibers:  These include: – The central processes of primary sensory neurons of the posterior root ganglia which enter the spinal cord and ascend or descend for varying lengths , and – The ascending fibers from the nuclei of spinal grey columns that convey sensory modalities to the higher centers. 2.   Motor fibers:  These include: – The descending fibers from higher centers ( supraspinal levels) to the spinal cord , and – The nerve fibers of anterior and lateral horn cells that go to the motor roots of the spinal nerves. 3.  Association fibers:   These fibers originate and end within the spinal cord , interconnecting the neurons of the same segment or of different segmental levels.

13. Function of spinal cord The central nervous system (CNS) controls most functions of the body and mind. It consists of two parts: the brain and the spinal cord. The brain is the center of our thoughts, the interpreter of our external environment, and the origin of control over body movement. Like a central computer, it interprets information from our eyes (sight), ears (sound), nose (smell), tongue (taste), and skin (touch), as well as from internal organs such as the stomach. The spinal cord is the highway for communication between the body and the brain. When the spinal cord is injured, the exchange of information between the brain and other parts of the body is disrupted.

14. The brain is the command center for your body, and the spinal cord is the pathway for messages sent by the brain to the body and from the body to the brain . the  peripheral nervous system is the network of nerves strands that branch off from the left and right sides of the spinal cord through openings between each vertebra on the spinal canal. These nerve pairs spread throughout your body to deliver commands from your brain and spinal cord to and from parts of your body The spinal cord is a complex cylinder of nerves that starts at the base of your brain and runs down the vertebral canal to the backbone. I t is part of the body’s collection of nerves, called the central nervous system, along with the brain In each of the spinal cord’s many segments lives a pair of roots that are made up of nerve fibers. These roots are referred to as the dorsa l (which is towards the back) and the ventral (which is away from the back) roots.

15. We depend on the spinal column to be the main support of our body. It allows us to stand upright , bend, and twist while protecting the spinal cord from injury . If the spinal cord is injured , it often causes issues like: Permanent changes in the body’s strength; Loss of sensation; and Loss of motor control or other functions . The spinal cord conducts   sensory information   from the peripheral nervous system (both somatic and  autonomic) to the brain conducts motor information  from the brain to our various effectors skeletal muscles cardiac muscle smooth muscle glands So it serves as a minor reflex center

16. Functions of spinal cord The Major Functions of the Spinal Cord Electrochemical communication.  Electrical currents travel up and down the spinal cord and across nerves, sending signals which allow different segments of the body to communicate with the brain. Walking.  While a person walks, a collection of muscle groups in the legs are constantly contracting and relaxing. The action of taking step after step may seem incredibly simple to us since we have been doing it all of our lives, but there are actually a lot of factors that have to be coordinated properly to allow this to happen. This central pattern generators in the spinal cord are made up of neurons which send signals to the muscles in the legs, making them relax or contract, and produce the alternating movements which occur when a person walks. Reflexes.   Reflexes are involuntary responses resulting from stimuli involving the brain, spinal cord, and nerves of the peripheral nervous system.

17. Tracts of spinal cord Tracts of the Spinal Cord The tracts are defined as collections of nerve fibres within the central nervous system, which have same origin, course and termination . They are sometimes referred to as  fasciculus  (= bundle) or  lemniscus  ( =   ribbon). T he spinal cord has numerous groups of nerve fibers going towards and coming from the brain . These have been collectively called the ascending and descending tracts of the spinal cord, respectively . The tracts are responsible for carrying sensory and motor stimuli to and from the periphery (respectively). Ascending tracts of the spinal cord Growing up, the impression was given that there were only five senses that humans can detect. These were sight, smell, sound, taste, and touch. However, it is clear that touch can be further expanded to include pain, thermal changes, pressure, light (crude) touch, vibration, two-point discrimination, and proprioception. Sight, sound, smell, and taste are special afferent stimuli that are conveyed through their respective  cranial nerves . However, the other tactile modalities are transmitted through the ascending tracts of the spinal cord. There are eight known ascending tracts conveying a variety of sensory stimuli that are discussed below.

18. TRACTS IN SPINAL CORD Groups of nerve fibers passing through spinal cord are known as tracts of the spinal cord. The spinal tracts are divided into two main groups . They are: 1. Short tracts 2. Long tracts. 1. Short Tracts Fibers of the short tracts connect different parts of spinal cord itself . Short tracts are of two types : i . Association or intrinsic tracts , which connect adjacent segments of spinal cord on the same side ii. Commissural tracts , which connect opposite halves of same segment of spinal cord. 2. Long Tracts Long tracts of spinal cord, which are also called projection tracts , connect the spinal cord with other parts of central nervous system. Long tracts are of two types: i . Ascending tracts , which carry sensory impulses from the spinal cord to brain ii . Descending tracts , which carry motor impulses from brain to the spinal cord.

19. Naming of tracts Transverse section of spinal cord at mid-cervical (neck) region showing main descending (motor) tracts in the left half and ascending (sensory) tracts in the right half of the spinal cord. The tracts are named after the names of masses of grey matter connected by them . The name usually consists of two components (or terms), the first term denotes the origin and second the termination of the tract. For example, a tract arising in cerebral cortex and terminating in the spinal cord is called corticospinal tract, similarly a tract arising in the spinal cord and terminating in the thalamus is called spinothalamic tract . he  lateral spinothalamic tract  refers to a cluster of nerve fibers traveling within the lateral funiculus of the spinal cord, which originated within the cord and will terminate within the  thalamus .

20. Function of ascending tract Ascending Tracts The ascending tracts conduct the impulses from the periphery to the brain through the cord. The important ascending tracts fall into the following three types: 1. Those concerned with  pain and temperature sensations and crude touch , e.g. lateral and anterior spinothalamic tracts. 2. Those concerned with  fine touch and conscious proprioceptive sensations , e.g. fasciculus gracilis and fasciculus cuneatus . 3. Those concerned with  unconscious proprioception and muscular coordination , e.g. anterior and posterior spinocerebellar tracts.

21. Reflex action and reflexes DEFINITION AND SIGNIFICANCE OF REFLEXES Reflex: A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus Reflex activity is the response to a peripheral nervous stimulation that occurs without our consciousness . It is a type of protective mechanism and it protects the body from irreparable damages. For example, when hand is placed on a hot object , it is withdrawn immediately. When a bright light is thrown into the eyes, eyelids are closed and pupil is constricted to prevent the damage of retina by entrance of excessive light into the eyes.

22. Reflex arc REFLEX ARC Reflex arc is the anatomical nervous pathway for a reflex action. A simple reflex arc includes five components. 1. Receptor: Receptor is the end organ, which receives the stimulus . When receptor is stimulated, impulses are generated in afferent nerve. 2. Afferent Nerve: Afferent or sensory nerve transmits sensory impulses from the receptor to center . 3. Center: Center receives the sensory impulses via afferent nerve fibers and in turn, it generates appropriate motor impulses . Center is located in the brain or spinal cord. 4. Efferent Nerve: Efferent or motor nerve transmits motor impulses from the center to the effector organ. 5. Effector Organ: Effector organ is the structure such as muscle or gland where the activity occurs in response to stimulus. ( Afferent and efferent nerve fibers may be connected directly to the center. In some places, one or more neurons are interposed between these nerve fibers and the center . Such neurons are called( connector neurons or internuncial neurons or interneurons .)

23. Reflexes CLASSIFICATION OF REFLEXES Reflexes are classified by six different methods depending upon various factors . 1. DEPENDING UPON WHETHER INBORN OR ACQUIRED REFLEXES i . Inborn Reflexes or Unconditioned Reflexes Unconditioned reflexes are the natural reflexes, which are present since the time of birth , hence the name inborn reflexes. Such reflexes do not require previous learning , training or conditioning. Best example is the secretion of saliva when a drop of honey is kept in the mouth of a newborn baby for the first time. The baby does not know the taste of honey, but still saliva is secreted . ii. Acquired Reflexes or Conditioned Reflexes Conditioned or acquired reflexes are the reflexes that are developed after conditioning or training . These reflexes are not inborn but, acquired after birth. Such reflexes need previous learning, training or conditioning. Example is the secretion of saliva by sight, smell, thought or hearing of a known edible substance.

24. 2. DEPENDING UPON SITUATION – ANATOMICAL CLASSIFICATION In this method, reflexes are classified depending upon the situation of the center. i . Cerebellar Reflexes: Cerebellar reflexes are the reflexes which have their center in cerebellum . ii. Cortical Reflexes: Cortical reflexes are the reflexes that have their center in cerebral cortex . iii. Midbrain Reflexes: Midbrain reflexes are the reflexes which have their center in midbrain. iv. Bulbar or Medullary Reflexes: Bulbar or medullary reflexes are the reflexes which have their center in medulla oblongata . v. Spinal Reflexes: Reflexes having their center in the spinal cord are called spinal reflexes. Depending upon the segments involved , spinal reflexes are divided into three groups : a. Segmental spinal reflexes b . Intrasegmental spinal reflexes c. Suprasegmental spinal reflexes.

25. DEPENDING UPON PURPOSE – PHYSIOLOGICAL CLASSIFICATION In this method, reflexes are classified depending upon the purpose (functional significance). i . Protective Reflexes or Flexor Reflexes Protective reflexes are the reflexes which protect the body from nociceptic (harmful) stimuli. These reflexes are also called withdrawal reflexes or flexor reflexes. Protective reflexes involve flexion at different joints hence the name flexor reflexes . ii. Antigravity Reflexes or Extensor Reflexes Antigravity reflexes are the reflexes that protect the body against gravitational force. These reflexes are also called the extensor reflexes because, the extensor muscles contract during these reflexes resulting in extension at joints.

26. 4. DEPENDING UPON THE NUMBER OF SYNAPSE Depending upon the number of synapse in reflex arc, reflexes are classified into two types: i . Monosynaptic Reflexes: Reflexes having only one synapse in the reflex arc are called monosynaptic reflexes. Knee jerk is the best example for monosynaptic reflex and it is elicited due to the stimulation of muscle spindle. ii. Polysynaptic Reflexes Reflexes having more than one synapse in the reflex arc are called polysynaptic reflexes. Flexor reflexes (withdrawal reflexes) are the polysynaptic reflexes

27. 5. DEPENDING UPON WHETHER SOMATIC OR VISCERAL REFLEXES i . Somatic Reflexes Somatic reflexes are the reflexes, for which the reflex arc is formed by somatic nerve fibers. These reflexes involve the participation of skeletal muscles . And there may be flexion or extension at different joints during these reflexes. ii. Visceral or Autonomic Reflexes Visceral or autonomic reflexes are the reflexes, for which at least a part of reflex arc is formed by autonomic nerve fibers. These reflexes involve participation of smooth muscle or cardiac muscle. Visceral reflexes include pupillary reflexes, gastrointestinal reflexes, cardiovascular reflexes, respiratory reflexes, etc. Some reflexes like swallowing, coughing or vomiting are considered as visceral reflexes. However, these reflexes involve some participation of skeletal muscles also.

28. 6. DEPENDING UPON CLINICAL BASIS Depending upon the clinical basis, reflexes are classified into four types : i . Superficial reflexes ii. Deep reflexes iii. Visceral reflexes iv . Pathological reflexes. SUPERFICIAL REFLEXES Superficial reflexes are the reflexes, which are elicited from the surface of the body. Superficial reflexes are of two types: mucus membrane reflexes and skin reflexes. 1. MUCOUS MEMBRANE REFLEXES Mucous membrane reflexes arise from the mucus membrane

29. Superficial mucous membrane reflexes REFLEX STIMULUS RESPONSE AFFERENT NERVE CENTER EFFERENT NERVE Corneal reflex Irritation of cornea Blinking eye (eyelid closure) 5 th cranial nerve pons 7 th cranial nerve Conjunctival reflex Irritation of conjunctiva Blinking of eye 5 th cranial nerve pons 7 th cranial nerve Nasal reflex Irritation of nasal mucus membrane sneezing 5 th cranial nerve Motor nucleus of V cranial nerve 10 th cranial nerve and upper cervical nerves Superficial mucous membrane reflexes

30. 2. CUTANEOUS REFLEXES OR SKIN REFLEXES Cutaneous reflexes are elicited from skin by the stimulation of cutaneous receptors. Details of these reflexes are given in Table DEEP REFLEXES Deep reflexes are elicited from deeper structures beneath the skin like tendon . These reflexes are otherwise known as tendon reflexes. Details of these are given in Table V ISCERAL REFLEXES Visceral reflexes are the reflexes arising from pupil and visceral organs . Following are the visceral reflexes : 1. Pupillary reflexes 2 . Oculocardiac reflex 3 . Carotid sinus reflex. PUPILLARY REFLEXES: Pupillary reflexes are the reflexes in which, the size of pupil is altered . EXAMPLE OF PUPILLARY REFLEX IS: Light Reflex When retina of the eye is stimulated by a sudden flash of light, constriction of pupil occurs. It is called light reflex. ii. Accommodation Reflex While eyes are fixed on a distant object and if another object is brought in front of the eye (near the eye) the vision shifts form far object to near object. During that time some changes occur in the eyes. Changes during accommodation reflex are: a. Constriction of pupil b. Convergence of eyeball c. Increase in anterior curvature of lens.

31. Superficial cutaneous reflexes Reflex Stimulus Response Central spinal segment involved Scapular (shoulder blade)reflex Irritation of skin at the interscapular space Contraction of scapular muscles and drawing in of scapula C5 to T1 Lower abdominal reflex Stroking the abdominal wall at umbilical and iliac level contraction of abdominal muscle and movement of umbilicus towards the site of stroke T10 to T12 Gluteal reflex Stroking the skin over glutei Contraction of glutei L4 to S1,2

32. Deep reflexes REFLEX STIMULUS RESPONSE Center – spinal segments involved Jaw jerk Tapping middle of the chin with slightly opened mouth Closure of mouth Pons – V cranial nerve Biceps jerk Percussion of biceps tendon Flexion of forearm C5, C6 Triceps jerk Percussion of triceps tendon Extension of forearm C6 to C8

33. OCULOCARDIAC REFLEX: Oculocardiac reflex is the reflex, in which heart rate decreases due to the pressure applied over eyeball. CAROTID SINUS REFLEX: Carotid sinus reflex is the decrease in heart rate and blood pressure caused by pressure over carotid sinus in neck due to tight collar. Carotid artery supplies blood to heart, neck & face Carotid sinus- consist of baroreceptor, which monitor blood pressure.

34. PROPERTIES OF REFLEXES 1. ONE WAY CONDUCTION (BELL-MAGENDIE LAW) During any reflex activity, impulses are transmitted in only one direction through the reflex arc as per Bell- Magendie law. The impulses pass from receptors to center and then from center to effector organ. 2. REACTION TIME Reaction time is the time interval between application of stimulus and the onset of reflex. It depends upon the length of afferent and efferent nerve fibers, velocity of impulse through these fibers and central delay. Central delay is the delay at the synapse. It is also called synaptic delay. 3. SUMMATION : Summation in reflex action is of two types: i . Spatial Summation When two afferent nerve fibers supplying a muscle are stimulated separately with subliminal stimulus, there is no response. But the muscle contracts when both the nerve fibers are stimulated together with same strength of stimulus. It is called spatial summation. ii. Temporal Summation When one nerve fiber is stimulated repeatedly with subliminal stimuli, these stimuli are summed up to give response in the muscle. It is called temporal summation.

35. RECRUITMENT Recruitment is defined as the successive activation of additional motor units with pr When an excitatory nerve is stimulated for a long time, there is a gradual increase in the response of reflex activities. It is due to the activation of more and more motor neurons. Recruitment is similar to the effect of temporal summation . Indefinite increase in response does not produce unlimited recruitment. A plateau is reached . Thus, t here is a limit to the number of motor neurons, which are recruited . So, beyond certain limit, the prolongation of stimulation does not increase the response Pogressive increase in force of muscular contraction

36. REBOUND PHENOMENON Reflex activities can be forcefully inhibited for some time. But, when the inhibition is suddenly removed , the reflex activity becomes more forceful than before inhibition. It is called rebound phenomenon. Reason for this state of over excitation is not known FATIGUE When a reflex activity is continuously elicited for a long time, the response is reduced slowly and at one stage, the response does not occur. This type of failure to give response to the stimulus is called fatigue. Center or the synapse of the reflex arc is the first seat of fatigue.

37. Production of CSF CSF: Cerebrospinal fluid is a clear, watery fluid that surrounds the brain and the spinal cord. It is an ultra-filtrate of blood plasma and is contained within the subarachnoid space and the central canal of the spinal cord .

38. Contents of CSF   CSF Blood pH 7.33 7.41 Osmolarity 295 mOsm/L 295 mOsm/L Glucose (fasting) 2.5 – 4.5 mmol/L 3.0 – 5.0 mmol/L Protein 200 – 400 mg/L 60 – 80 g/L Sodium 144 – 152 mmol/L 135 – 145 mmol/L Potassium 2.0 – 3.0 mmol/L 3.8 – 5.0 mmol/L Chloride 123 -128 mmol/L 95 – 105 mmol/L Calcium 1.1 – 1.3 mmol/L 2.2 – 2.6 mmol/L Urea 2.0 – 7.0 mmol/L 2.5 – 6.5 mmol /

39. Cerebrospinal Fluid Circulation and Absorption CSF is formed within the ventricles by small, delicate tufts of spe­cialized tissue called the choroid plexus. The solid arrows in the draw­ing,  Cerebrospinal fluid (CSF) Circulatory Pathway, show the major pathway of CSF flow . Beginning in the lateral ventricles , CSF flows through two passageways into the third ventricle. From the third ventricle it flows down a long, narrow passageway (the aqueduct of Sylvius ) into the fourth ventricle. From the fourth ventricle it passes through three small openings (foramina) into the subarachnoid space surrounding the brain and spinal cord . CSF is absorbed through blood vessels over the surface of the brain back into the bloodstream. Some absorption also occurs through the lymphatic system . Once in the bloodstream, it is carried away and filtered by our kidneys and liver in the same way as are our other body fluids.

40. The ventricular system is the major pathway for the flow of CSF . CSF also flows directly from the ventricles into the brain tissue sur­rounding them. This is shown by the broken arrows .   Here the CSF passes through the spaces between the cells to where it eventually enters the subarachnoid space.  Our bodies produce approximately a pint (500 ml) of CSF daily, continuously replacing CSF as it is absorbed. Under normal conditions there is a delicate balance between the amount of CSF that is pro­duced and the rate at which it is absorbed. Hydrocephalus occurs when this balance is disrupted.  Hydrocephalus: is a pathological condition of abnormal accumulation of CSF caused by increased CSF production, blockage of flow, or decreased absorption. The ventricles distend to accommodate elevated CSF volumes, potentially causing damage to the brain by pressing its tissue against the boney skull. Hydrocephalus may be congenital or acquired.

41. when compared to plasma , CSF has a higher concentration of sodium, chloride, and magnesium but a lower concentration of potassium and calcium . Unlike plasma, CSF has only trace amounts of cells, protein, and immunoglobulins . No cells can pass through the blood-CSF barrier , although small numbers of white blood cells are usually introduced to the CSF indirectly. The normal cell count of CSF is generally lower than 5 cells/ml .   Despite changes in blood composition and flow, the composition of CSF is kept constant, which provides a stable intraventricular environment , critical for maintaining normal neuronal function

42. CSF Clearance CSF gets drained into the superior sagittal venous sinus through the arachnoid villi. The pressure gradient between the subarachnoid space and the venous sinus results in the fluid moving through the arachnoid villi . because there is no appreciable barrier between the CSF and the extracellular space of the brain (ECSB), the blood-CSF barrier also serves to regulate the environment of the brain. Larger substances such as cells, protein, and glucose are not allowed passage, whereas ions and small molecules such as vitamins and nutrients can pass into the CSF relatively easily.

43. Functions of CSF The CSF has many functions: Buoyancy  – the brain weighs ~1400g, but due to the presence of CSF creating a bath, it only has a net weight of 50g. The brain otherwise is only supported within the arachnoid space by blood vessels and nerve roots which are fragile structures. Protection  – CSF acts as a shock absorber preventing damage from occurring to the brain when the cranium is jolted/hit. Homeostasis  – regulates the distribution of metabolites surrounding the brain keeping the environment ideal to prevent any damage to the nervous system. Clearing waste  – waste products produced by the brain move into the CSF which then clears out through the arachnoid granulations into the venous sinus so it can be absorbed into the bloodstream.