DEVELOPMENT CLASSES
THE NERVOUS SYSTEM
Hand-Out
2009
Black Belt
DEVELOPMENT CLASS
THE NERVOUS SYSTEM
INTRODUCTION
The Nervous System transmits impulses from one part of the body to another, and also coordinates all the body’s activities.
For descriptive purposes the parts of the Nervous System are grouped as follows;
1. THE CENTRAL NERVOUS SYSTEM – The Brain and Spinal Cord
2. THE PERIPHERAL NERVOUS SYSTEM –
· 31 Pairs of Spinal Nerves
· 12 Pairs of Cranial Nerves
· The Autonomic Nervous System
NEURONES (see fig. 1)
Neurones are nerve cell bodies with two types of filaments:
· DENDRITES (which receive impulses)
· AXONS (which transmits impulses)
THE NERVOUS SYSTEM
DENDRITES
The Dendrites are the processes or nerve fibres which carry impulses towards nerve cells. They have the same structure as axons but are usually shorter and branching. They form synapses with dendrites of other neurons or terminate is specialized sensory receptors, such as those in the skin.
AXONS
Each nerve cell has only one axon, carrying nerve impulses away from the cell. They are usually longer than dendrites, sometimes as long as 100cm.
NERVE TYPES
Sensory (afferent) nerves
Motor (efferent) nerves
Mixed (afferent and efferent) nerves
TYPES OF NERVES (continued)
SENSORY (afferent) Nerves
These are the nerves that transmit impulses from the periphery of the body to the spinal cord. The impulses then pass to the brain or to the connector neurons of reflex arcs.
MOTOR (efferent) nerves
Motor nerves originate in the brain, spinal cord and autonomic ganglia. They are involved in;
1. Voluntary and reflex skeletal muscular contractions
2. Involuntary (autonomic) smooth muscle contractions and glandular
secretions.
GANGLIA
This is a group or string of neurons which lie outside the brain.
All neurons outside the brain and spinal cord are enclosed in a thin membrane called the neurilemma.
THE NERVOUS SYSTEM
SYNAPSE AND CHEMICAL TRANSMITTERS
There is always more than one neurone involved in the transmission of a nerve impulse from its origin to its destination. There is a space at the free end of the axon where it branches terminates in small swellings called boutons, which are in close proximity to the dendrites and the cell body of the next neurone. This space is called the SYNAPTIC CLEFT. At the ends of the boutons are minute synaptic vesicles, containing chemical transmitters which carry nerve impulses across the synaptic clefts.
CHEMICAL TRANSMITTERS
Noradrenaline
Acetylcholine
THE CENTRAL NERVOUS SYSTEM
The Central Nervous system consists of the BRAIN and SPINAL CORD.
THE BRAIN
The brain constitutes about one fiftieth (1/50) of the body weight and lies within the cranial cavity. The parts are;
· Cerebrum (great brain)
· Cerebellum (lesser or hind brain)
· The Brain Stem;
Mid-brain
Pons varolii
Medulla Oblongata
CEREBRUM
This is the largest and youngest part of the brain, and it is divided by a deep cleft called the Longitudinal Cerebral Fissure. This divides the cerebrum into the left and right cerebral hemispheres, which are connected deep within the brain by a mass of white matter (nerve fibres) called the Corpus Callosum. Each Hemisphere has four lobes, these are called by the names of the skull bones that they lie beneath.
FRONTAL
TEMPORAL
PAREITAL
OCCIPITAL
THE CENTRAL NERVOUS SYSTEM
THE CEREBRUM (continued)
The surface layer of the cerebrum is called the cortex and is composed of gray matter. It is corrugated like the kernel of a walnut. These corrugations are called convolutions and have the effect of increasing the surface area of the cortex allowing for a far greater number of cells. The white matter of the cerebrum is enclosed by the cortex and is made up of the myelinated axons of the neurones.
FUNCTIONS OF THE CEREBRUM
1. MENTAL ACTIVITES involved in memory, intelligence, sense of responsibility, thinking, reasoning, moral sense and learning.
2. SENSORY PERCEPTION, perception of pain, temperature, touch, sight, hearing, taste and smell.
3. Initiation and control of voluntary muscular contraction
THE NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM (continued)
Important masses of grey matter within the cerebral hemispheres act as relay stations;
BASAL NUCLEI
THALAMUS
HYPOTHALAMUS
BASAL NUCLEI
Lies deep within the cerebrum, is thought to influence skeletal muscle tone.
THALAMUS
This consists of two masses of nerve cells and fibres situated within the cerebral hemispheres just below the corpus Callosum, one on each side of the third ventricle. Sensory input from the skin, vicera and special sense organs are transmitted to the thalamus before redistribution to the cerebrum.
HYPOTHALAMUS
Composed of a number of groups of nerve cells. It is situated below and in front of the thalamus, immediately above the Pituitary gland. The hypothalamus also controls the Autonomic Nervous System, i.e. control of hunger, thirst, body temperature, heart and blood vessels and defensive reactions, such as those associated with fear and rage.
THE BRAIN STEM
The Mid-brain, Pons varolii and Medulla Oblongata make up the Brain Stem and are the oldest part of the Brain.
THE MID-BRAIN
The Mid-Brain is situated between the cerebrum above (superior) and the pons varolii below (inferior). It acts as a relay station for ascending and descending nerve fibres.
THE NERVOUS SYSTEM
THE CENTRAL NERVOUS SYSTEM
THE PONS VAROLII
The pons also acts as a relay station. It consists mainly of nerves which forms a bridge between the two hemispheres, sometimes called ‘The Bridge of Nerve Matter’.
THE MEDULLA OBLONGATA
Situated between the Pons Varolii above (superior) and the Spinal Cord below (inferior). It is shaped like a pyramid with its base upwards and lies just within the cranium above the Foramen Magnum.
THE NERVOUS SYSTEM
THE MEDULLA OBLOGATA (continued)
The ‘VITAL CENTRES’ lie within the Medulla Oblongata. These are groups of cells associated Autonomic Reflex Activity. These are;
THE CARDIAC CENTRE
Controls the rate and force of contraction of the heart. Sympathetic (quicken heart rate) and Parasympathetic (slowing heart rate) nerve fibres originating in the medulla pass to the heart.
THE RESPIRATORY CENTRE
Controls the rate, rhythm and depth of respiration. This center is stimulated by Excess Carbon Dioxide and, to a lesser extent by Low Oxygen levels in it’s blood supply (see Hypoxic Drive) and by nerve impulses from the chemoreceptors in the carotid bodies.
THE VASOMOTOR CENTRE
Controls the diameter of the blood vessels. Vasomotor impulses reach the blood vessels through the Autonomic nervous system. Stimulation may cause either constriction or dilatation of the blood vessels depending on the site.
THE REFLEX CENTRE
Irritants present in either the Digestive or Respiratory tract to initiate the reflex action of sneezing, vomiting, coughing and also swallowing stimulate this.
THE DECUSSATION OF THE PYRAMIDS
This is where the majority of the descending motor nerves from the motor area of the cerebrum to the spinal cord cross from one side to the other. This means the left hemisphere controls the right side of the body.
THE CEREBELLUM
The cerebellum or hind brain is situated behind (posterior) the Pons Varolii and immediately below (inferior) the posterial portion of the cerebrum occupying the posterial cranial fossa. Like the larger cerebrum above it has two hemispheres, separated by a narrow median called the Vermis
THE NERVOUS SYSTEM
THE CEREBELLUM
FUNCTIONS
Coordination of voluntary muscular muscle movement, posture and balance. Cerebellular activites are carried out below the level of concousness, i.e., not under voluntary control. Damage to the cerebellum results in clumsy uncoordinated muscular movements, staggering gait and inability to carry out smooth, steady, precise movements.
SPINAL CORD
Elongated, almost cylindrical part of the nervous system, which is suspended in the vertebral canal surrounded by the Meninges and cerebrospinal fluid. It is approximately 45cm in length in an adult and is about the thickness of a little finger (pinkie). Except for cranial nerves, the spinal cord is the nervous tissue link between the brain and the rest of the body. Cord terminates at the lower border of the 1st Lumbar vertebrae.
THE NERVOUS SYSTEM
FUNCTIONS OF THE SPINAL CORD
To relay impulses;
· entering and leaving at the same level
· transmitting up and down the cord to other levels
· to and from the brain
THE MENINGES
Three layers of tissue completely cover the brain and spinal cord,
DURA MATER - Tough fibrous outermost layer.
ARACHNOID MATER - This is a delicate membrane which encloses the
Arachnoid space, which contains cerebrospinal
Fluid, connective tissue and blood vessels.
PIA MATER - This is a very delicate inner membrane which closely covers
the brain and spinal cord. It supports a network of fine
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