Central Nervous System

CENTRAL NERVOUS SYSTEM

Nervous System

Refers to the part of the body that coordinates its voluntary and involuntary actions and transmits signals between different parts It is the system that conducts stimuli from sensory receptors to the brain and spinal cord and that conducts impulses back to other parts of the body.

Autonomic Nervous System

The part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands, including digestion, respiration, perspiration, metabolism, and blood-pressure modulation.

Somatic Nervous System

The part of the peripheral nervous system that transmits signals from the central nervous system to skeletal muscle and from receptors of external stimuli, thereby mediating sight, hearing and touch.

Nerves are bundles of axons in the peripheral nervous system (PNS) that act as information highways to carry signals between the brain and spinal cord and the rest of the body.

The Central Nervous System

• Brain
• Spinal cord
• Peripheral Nervous system is divided into:
• Autonomic nervous system

Peripheral nervous system

Nerve tissue is made up of nerve cells called neurons or nerve fibres

• All neurons have the same physical parts.
• The cell body contains the nucleus and is essential for the continued life of the neuron.
• Dendrites are processes (extensions) that transmit impulses toward the cell body.

The one axon of a neuron transmits impulses away from the cell body.

It is the cell membrane of the dendrites, cell body, and axon that carries the electrical nerve impulse.

Axons and dendrites are collectively called Schwann cells in the peripheral nervous system and are covered by a membrane called myelin sheath.

Node of Ranvier is the spaces between adjacent Schwann cells or segments of myelin sheath.

Neurolema are the nuclei and cytoplasm of Schwann cells.

Synapse.

It is the small gap or space between the axon of one neuron and the dendrites/cell body of the next neuron.

The synaptic knob of presynaptic axon contains chemical neurotransmitters that are released to the synapse on the arrival of an impulse.

Synapses ensure one way impulse transmission The neurotransmitter diffuses across the synapse, combines with specific receptor sites on the cell membrane of the postsynaptic neuron, and there generates an electrical impulse that is, in turn, carried by this neuron’s axon to the next synapse.

Example of neurotransmitter is acetylcholine found at the neuromuscular junction.

Synape and synaptic transmission

The neurotransmitter diffuses across the synapse, combines with specific receptor sites on the cell membrane of the postsynaptic neuron, and there generates an electrical impulse that is, in turn, carried by this neuron’s axon to the next synapse, and so forth.

A chemical inactivator at the cell body or dendrite of the postsynaptic neuron quickly inactivates the neurotransmitter.

This prevents unwanted, continuous impulses, unless a new impulse from the first neuron releases more neurotransmitter.

Many synapses are termed excitatory, because the neurotransmitter causes the postsynaptic neuron to depolarize (become more negative outside as Na+ ions enter the cell) and transmit an electrical impulse to another neuron, muscle cell, or gland.

Some synapses, however, are inhibitory, meaning that the neurotransmitter causes the postsynaptic neuron to hyperpolarize (become even more positive outside as K+ions leave the cell or Cl- ions enter the cell) and therefore not transmit an electrical impulse.

Such inhibitory synapses are important, for example, for slowing the heart rate, and for balancing the excitatory impulses transmitted to skeletal muscles.

With respect to the skeletal muscles, this inhibition prevents excessive contraction and is important for coordination.

An example of a neurotransmitter is acetylcholine, which is found at neuromuscular junctions, in the CNS, and in much of the peripheral nervous system.

Acetylcholine usually makes a postsynaptic membrane more permeable to Na+ ions, which brings about depolarization of the postsynaptic neuron.

• Cholinesterase is the inactivator of acetylcholine.
• There are many other neurotransmitters, especially in the central nervous system.
• These include dopamine, GABA, norepinephrine, glutamate, and serotonin.

Each of these neurotransmitters has its own chemical inactivator.

Some neurotransmitters are reabsorbed into the neurons that secreted them; this process is called reuptake and also terminates the effect of the transmitter.

Types of neurons

• Neurons are classified in to three groups which are :
• 1.Sensory (afferent) neuron,
• 2.Motor (efferent) neuron
• 3.Interneuron.
• Sensory neurons

Carry impulses from receptors to the central nervous system.

Receptors detect external or internal changes and send the information to the CNS in the form of impulses by way of the afferent neurons.

The central nervous system interprets these impulses as a sensation.

Motor neurons

• Carry impulses from the central nervous system to effectors.
• The two types of effectors are muscles and glands.
• In response to impulses, muscles contract or relax and glands secrete.
• Interneurons

They are found entirely within the central nervous system.

They are arranged so as to carry only sensory or motor impulses, or to integrate these functions.

Some interneurons in the brain are concerned with thinking, learning, and memory.

Central nervous system (CNS) is a part of nervous system which is made up of brain and spinal cord

The Spinal cord

The spinal cord transmits impulses to and from the brain and is the integrating centre for the spinal cord reflexes The spinal cord extends from the foramen magnum of the occipital bone to the disc between the first and second lumbar vertebrae.

• The internal grey matter is shaped like the letter H
• Grey matter consists of the cell bodies of motor neurons and interneurons.
• The external white matter is made of myelinated axons and dendrites of interneurons.

Emerging from the spinal cord are spine nerves.

Spinal cord reflexes are those that do not depend directly on the brain, although the brain may inhibit or enhance them.

Central Nervous System

A reflex arc is the pathway that nerve impulses travel when a reflex is elicited, and there are five essential parts:

• 1.Receptors-detect a change (the stimulus) and generate impulses.
• 2.Sensory neurons-transmit impulses from receptors to the CNS.
• 3.Central nervous system-contains one or more synapses.
• 4.Motor neurons-transmit impulses from the CNS to the effector.

5.Effector-performs its characteristic action.

The Brain

The major parts of the brain are the medulla, pons, and midbrain (collectively called the brain stem), the cerebellum, the hypothalamus, the thalamus and the cerebrum.

• Ventricles are cavities in the brain where the cerebrospinal fluid is found.
• Medulla

The medulla extends from the spinal cord to the pons and is anterior to the cerebellum.

It contains cardiac centre, vasomotor centre and respiratory centre hence controls heart rate, diameter of blood vessels and breathing respectively.

Also there are reflex centres for coughing, sneezing, swallowing, and vomiting.

Pons

They bulge anteriorly from the upper part of the medulla.

Within the pons are two respiratory centres that work with those in the medulla to produce a normal breathing rhythm.

• Midbrain
• The midbrain extends from the pons to the hypothalamus
• Visual and auditory reflexes are integrated to the midbrain.

Cerebellum

It is concerned with movements like coordination, regulation of muscle tone, the appropriate trajectory and endpoint of movements, and the maintenance of posture and equilibrium.

Hypothalamus

• It is located superior to the pituitary gland and inferior to the thalamus

Their functions include:

Production of antidiuretic hormone and oxytocin which are stored in the posterior pituitary gland.

• Regulation of releasing hormone.
• Regulation of food intake.
• Regulation of body temperature
• Integration of the functioning of autonomic nervous system
• Stimulation f visceral response during emotions

Regulation of body rhythms like hormone secretion and sleep cycles.

Thalamus

• It is superior to the hypothalamus and inferior to the cerebrum.
• Many of the functions of the thalamus are concerned with sensation.
• Cerebrum

It is lined with grey matter on the surface called cerebral cortex The cerebral cortex has folds which enable us to read, speak, do long division, and write and other things done by human that animals cannot do because they lack the folds.

The cerebral cortex is divided into lobes which are frontal lobe, parietal lobe, temporal lobe, and occipital lobe.

Central Nervous System

Within the frontal lobes are the motor areas that generate the impulses for voluntary movement, like that of hands, face, mouth in speaking and others.

The general sensory areas in the parietal lobes receive impulses from receptors in the skin and feel and interpret the cutaneous sensations and also from stretch receptors in muscles for conscious muscle sense.

The olfactory areas in the temporal lobes receive impulses from receptors in the nasal cavities for the sense of smell.

The olfactory association area learns the meaning of odours such as the smell of sour milk, or fire.

Occipital lobe contains visual association areas which interpret what has seen and enable the thinking cerebrum to use the information.

Basal ganglia

These are paired masses of grey matter within the white matter of the cerebral hemispheres Their functions are certain subconscious aspects of voluntary movement, and they work with the cerebellum.

The basal ganglia help regulate muscle tone, and they coordinate accessory movements such as swinging the arms when walking or gesturing while speaking.

• Corpus callosum

It connects the brain hemispheres to know what is going on to each other.

This is especially important for people because for most of us, the left hemisphere contains speech areas and the right hemisphere does not.

Meninges

These are connective tissue that covers the brain and the spinal cord.

Between the membranes of meninges there is cerebrospinal fluid, the tissue fluid of the central nervous system.

• Examination of the fluid can be used to diagnose certain diseases.

Cranial nerves

These are nerves which emerge from the brain stem or other parts of the brain and are about twelve.

They carry impulses for functions involving the head.

The impulses for the senses of smell, taste, sight, hearing, and equilibrium are all carried by cranial nerves to their respective sensory areas in the brain.

AUTONOMIC NERVOUS SYSTEM

Autonomic nervous system (ANS) is a part of peripheral nervous system and consists of motor parts of spinal nerves and cranial nerves.

The peripheral nervous system relays information to and from the central nervous system Autonomic nervous system is made up of visceral motor neurons to the smooth muscles, cardiac muscles and glands of which muscles will either contract or relax and glands will either increase or decrease secretions.

• The ANS has two divisions which are sympathetic and parasympathetic.

Often, they function in opposition to each other.

Sympathetic division

The sympathetic division brings about widespread responses in many organs.

The sympathetic division is dominant in stressful situations, which include anger, fear, or anxiety, as well as exercise.

There are synapses between preganglionic and postganglionic neurons One preganglionic neuron often synapses with many postganglionic neurons to many effectors. This anatomic arrangement has physiological importance.

Parasympathetic division

The parasympathetic division dominates in relaxed (non-stress) situations to promote normal functioning of several organ systems.

In the parasympathetic division, one preganglionic neuron synapses with just a few postganglionic neurons to only one effector. With this anatomic arrangement, much localized (one organ) responses are possible.

Central Nervous System

In autonomic pathways there are two synapses: one between preganglionic and postganglionic neurons, and the second between postganglionic neurons and visceral effectors.

Acetylcholine is the transmitter released by all preganglionic neurons, both sympathetic and parasympathetic and is inactivated by cholinesterase in postganglionic neurons.

Parasympathetic postganglionic neurons all release acetylcholine at the synapses with their visceral effectors.

Most sympathetic postganglionic neurons release the transmitter norepinephrine at the synapses with the effector cells.

Norepinephrine is inactivated by either catechol-Methyltransferase (COMT) or monoamine oxidase (MAO), or it may be removed from the synapse by reuptake.

Keypoints

Autonomic nervous system (ANS) is a part of peripheral nervous system and consists of motor parts of spinal nerves and cranial nerves.

The sympathetic division is dominant in stressful situations while parasympathetic division dominates in relaxed (non-stress) situations In their functions, sympathetic and parasympathetic always oppose each other.

Common disorders of Nervous system

Nervous system disorders are the disorders of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body.

• The common nervous system disorders:
• Psychosis
• Depression
• Schizophrenia
• Epilepsy
• Multiple sclerosis
• Parkinson’s disease
• Alzheimer’s disease

Hydrocephalus

Parkinson’s disease

This is a disorder of the basal ganglia The cause is unknown, and though there is a genetic component in some families, it is probably not the only factor The disease usually begins after the age of 60 Neurons in the basal ganglia that produce the neurotransmitter dopamine begin to degenerate and die, and the deficiency of dopamine causes specific kinds of muscular symptoms.

Tremor, or involuntary shaking, of the hands is probably the most common symptom.

The accessory movements regulated by the basal ganglia gradually diminish, and the affected person walks slowly without swinging the arms.

Alzheimer’s disease

This is the genetic disorder of the nervous system.

It is due to presence of abnormal fibrous proteins in the cells of cerebral cortex in areas important for memory and reasoning.

The symptoms includes memory lapse and slight personality changes.

As the disease progresses, there is total loss of memory, reasoning ability, and personality, and those with advanced disease are unable to perform even the simplest tasks or self-care.

Neuroleptic disorders are mixtures of symptoms, especially anxiety and depressive ones.

• Psychosis and Depression
• They are due to imbalance of chemicals in the brain
• Psychosis is mostly presented by;
• Hallucinations
• Delusions
• Emotional withdraw

Lack of attention to personal hygiene

Multiple sclerosis

• Refers to autoimmune demyelinating disease

It involves deterioration of the myelin sheath of neurons in the central nervous system.

Without the myelin sheath, the impulses of these neurons are short-circuited and do not reach their proper destinations, and the neuron axons are damaged and gradually die.

The first symptoms usually appear between the ages of 20 and 40 years, and the disease may progress either slowly or rapidly.

Schizophrenia

It is defined by a group of characteristic positive and negative symptoms of deterioration in social, occupational, or interpersonal relationships.

• It distorts thinking and perception capabilities.
• It is not an excess of dopamine.
• Epilepsy
• Epilepsy is a recurrent seizures of not more than 24 hours

Seizure is a clinical manifestation of synchronized electrical discharges of neurons.

Some seizures can be originated outside the brain due to fever, infection, syncope, head trauma, hypoxia, toxins and cardiac arrhythmias.

Status epileptics (SE) is the continuous convulsion lasting longer than 30 minutes or occurrence of serial convulsions between which there is no return of consciousness Seizures can be due to cerebral damage or cerebral malformation like hydrocephalus They are mostly presented by convulsions.

Hydrocephalus

• Is the accumulation of excessive CSF within the ventricular system.
• Mostly is divided into two categories,which are;
• 1.Impaired flow of CSF (Non communicating)
• 2.Impaired resorption of CSF (Communicating)
• 3.Rare instances of tumours in choroid plexus

Overproduction of CSF may be responsible.

When hydrocephalus develops in infancy before closure of the cranial sutures, there is enlargement of the head.

Hydrocephalus developing after fusion of the sutures is associated with expansion of the ventricles and increased intracranial pressure, without a change in head circumference.

If there is an obstacle to the flow of CSF within the ventricular system, then a portion of the ventricles enlarges while the remainder does not.

Neural tube defects

• These are congenital anomalies of the spinal cord and their vertebrae
• These includes;
• 1.Spina bifida
• 2.Meningiocele

3.Meningiomyocele