Introduction to the Nervous System

Prof. Vajira WeerasingheProfessor of Physiology

Faculty of MedicineUniversity of Peradeniya

(This lecture is available at www.slideshare.net/vajira54)

Why study nervous system?

Neurological diseases are disabling and debilitating and very little treatment

is available

Migraine (severe headache)

Stroke or paralysis Parkinsonism

Alzheimer’s disease (Memory loss)

Muscle wasting

Why study nervous system?

Activities such as walking and running and various forms of exercise and sports require

proper functioning of the nervous system

Controlling system

Nervous system is the main controlling system of the body QuickElectrical

Endocrine system also controls body functions SlowChemical

Nerve impulses

• It controls other organs by sending electrical messages called nerve impulses

• It is also the main communication system (like the telecommunication system)

What does it do ?Functions of the nervous system

1. Receives sensory messages from the external environment

2. Organizes information and integrates it with already stored information

3. Uses integrated information to send out messages to muscles and glands, producing organized movement and secretions

4. Provides the basis for conscious experience

Functional Subdivisions

• Sensory functions feeling, eg. pain

• Motor functionsmovement, eg. walking

• Integrative functionseg. reflexes

• Autonomic functionscontrol of blood pressure

• Higher functionsmemory, learning

Anatomical Subdivisions• Central Nervous system

Brain and spinal cord

• Peripheral Nervous systemCranial Nerves & Peripheral Nerves

• Autonomic systemsympathetic & parasympathetic

Brain

• Brain contains the main controlling centres

• It is divided into several lobes by several grooves

Brain - Lobes

• Frontal lobe Located in the frontPerforms motor functions

• Parietal lobe Located laterallyPerforms main sensory

functions

Brain - Lobes

• Occipital lobe Located in the backPerforms visual functions

• Temporal lobe Located laterallyPerforms hearing functions

White matter and grey matter

• In the nervous system Cell bodies are collected as

grey matter

Nerve fibres are collected as white matter

• In the brain grey matter is located in the

peripheral cell layer

White matter consists of central nerve fibre tracts

White matter and grey matter

• With nerve fibre tracts each lobe is connected to different structures of the body

• Frontal lobe is connected to the muscles

• Parietal lobe is connected to the sensory organs of the skin

• Occipital lobe is connected to the eye

• Temporal lobe is connected to the ear

Spinal cord

• Nerve fibre tracts starting from the brain then continue as spinal cord

• Divided into following areasCervicalThoracicLumbarSacral

Spinal cord

• Functions

To transmit impulses to and from the brain to the periphery

To cause reflex action

To process information

Peripheral nervous system

• Cranial nervesThere are 12 cranial nerves which

connects brain & brainstem with different organs of the head and neck region

• Spinal nerves There are several pairs connecting

different segments of the spinal cord with peripheral organs

Cranial nerves I-Olfactory nerve - smell

II-Optic nerve - vision

III-Occulomotor nerve - eye movements

IV-Trochlear nerve - eye movements

V-Trigeminal nerve - mastication

VI-Abducens nerve - eye movements

VII-Facial nerve - facial expressions

VIII-Vestibulocochlear nerve - hearing, balance

IX-Glossopharyngeal nerve - taste

X-Vagus nerve - parasympathetic

XI- Spinal accessory nerve - neck muscles

XII-Hypoglossal nerve - tongue

• Spinal nervesCervical

Thoracic

Lumbar

Sacral

Brain stem

• This is important part of the nervous system that connects brain with the spinal cord

• Many cranial nerves arise from brain stem

• It also contain vital centresCardiac, respiratory functions,

consciousness

Brain stem

• It has three areas

Midbrain

Pons (or pons Varoli)

Medulla (or medulla oblongata)

Cerebellum

• This is hind brain

• This is connected to the brainstem

• Main function is motor coordination

• Abnormalities result in difficulty in walking called “ataxia”

Sensory functions

• Functions that make us feel the environment

• Nerve signals come from the sensory organ towards the CNSAfferent signals

• There are different types of sensory modalities human nervous system could feel

General and special sensations

• There are two main types of sensations

General sensations

Special sensations

General sensations

• Mechanical sensations TouchPressureVibrationsStretch

• Thermal sensations Hot, cold

• Chemical sensation

• Pain sensation

Somatic and visceral sensations

• Somatic sensations External stimuli sensed by the skin

eg. Feeling of hot environment

• Visceral sensations Internal sensations sensed by the

visceral afferent nerves eg. acidity in stomach

Special sensations• These are specialised sensations

• Vision

Hearing

Taste

Smell

Proprioception

• Joint and position sensation

• Muscles and tendons contain receptors which could feel Muscle lengthJoint anglePosition of bones and joint

• This is very important feedback information for movements of muscles

General plan of the sensory system

• Stimulus

• Receptor

• Nerve pathway

• Central connections

• Brain area

Receptor

Sensory modality

Sensory nerve

Central Connections

Ascending Sensory pathway

Sensory area in the brain

Touch stimulus

AFFERENT

thalamus

sensory cortex

internal capsule

1st order neuron

2nd order neuron

3rd order neuron

Skin receptor

Spinal Cord

Brain stem

Motor functions

• Movements are of two types

Voluntary movements Consciously controlled

movementsWell thought and planned

Involuntary movements Reflex action Control of postureBalance

Voluntary movements

• Planning starts from the brain (frontal lobe)

• Modified by various intermediate structuresBrainstem Cerebellum

• Executed by the brainstem and spinal cord (lower motor neuron)

• Finally contraction of muscles

Brain stem

motor cortex

internal capsule

Uppermotorneuron

Lowermotorneuron

Muscle contraction

Spinal Cord

Motor Nerve

Motor map in the brain

• In the frontal lobe different areas controlling different muscles of the body are distinctly organised

• eg. there is a hand area, thumb area, face area

Representation

•upside down•distorted

concept of homunculus

Map

Cerebellum

• This is very important for motor coordination eg. In walking many muscles

should contract and relax in smooth coordination

• Cerebellum receives proprioception

• Cerebellum is used as an error correction centre

What is a reflex?

• Response to a stimulus

• Stimulus -> Response

• Task:Write down 3 reflexes.

Reflexes

• Response to a stimulus

• This is an involuntary actioneg. Hot object touching the hand

will cause it to withdraw

• This is called withdrawal reflex

What is a reflex?

Stimulus

Effector organ

Response

Centralconnections

Efferent nerve

Afferent nerveReceptor

Central control

Reflexes

• Reflexes involve many systems of the body

• eg.Withdrawal reflex (pain and motor pathways)Pupillary reflex (eye)Salivatory reflex (salivary glands)Swallowing reflex (GIT)Micturition reflex (urinary system) Baroreceptor reflex (blood pressure control)Stretch reflex (main reflex in the motor system)

Autonomic functions

• Controls involuntary functions of the body

• Two main divisions SympatheticParasympathetic

Sympathetic system

• This is involved in fight or flight reaction

• What happens when somebody is excitedBlood pressure increasesHeart rate increasesRespiratory rate increasesIncreased sweating Pupillary dilatation GIT function suppressed

Parasympathetic system

• This is involved when somebody s resting

• Blood pressure decreases

• Heart rate decreases

• Respiratory rate decreases

• Decreased sweating

• Pupillary constriction

• GIT function increased

Higher functions

• Brain is involved in controlling higher functions

• Learning

• Memory

• Intelligence

• They are called cognitive functions

Each side controls opposite of the body

• Right side of the brain controls left side of the body

• There is a cross over of nerve pathways at the brain stem

Basic cell in the nervous system is is a neuron

Nerve signals are electrochemical impulses called “action potentials”