The Nervous SystemCNS – the brain and spinal cord

Text: Chapter 5Human Biology Stage 3

KeywordsCentral nervous system (CNS)Cerebrospinal fluid (CSF)MeningesNeurons

Cell bodyDentritesAxonSynapse

Myelin sheathSchwann cellsSensory neuronsMotor neurons InterneuronsCerebrum

Cortex

ConvolusionsSulciFissures - Longitudinal fissure

LobesFrontalParietalTemporalOccipital

CerebellumMedulla oblongata

Cardiac centreRespiratory centreVasomotor centre

Spinal cordAscending tractsDescending tracts

The Nervous SystemThe nervous system is the

control centre and communication network of the body

The nervous system can be divided into 2 parts: The central nervous

system (CNS)The peripheral

nervous system (PNS)The CNS includes the

brain and the spinal cord

The PNS is divided into the:Autonomic nervous systemSympathetic nervous

systemParasympathetic

nervous system Somatic nervous systemSkeletal muscle

Both the CNS and the PNS are also involved in maintaining homeostasis in conjunction with the endocrine system

Structure of nerve cellsNerve cells are called

neurons. They are the functional unit of the nervous system.

Neurons have:A cell body that contains

the nucleusDendrites, short extensions

on one side of the cell bodyAn axon, a long extension

on the other side of the cell body

Structure of nerve cells Most axons are covered

in a white, fatty material called the myelin sheath

The myelin sheath is made up of individual cells called Schwann cells

An axon with its covering is called a nerve fibre

There are myelinated fibres and unmyelinated fibres

The myelin sheathFunctions of the

myelin sheath: It acts as an

insulator It protects the axon

from damage It speeds up rate of

impulses It helps with the

repair of injured fibres

The synapseThe axon of one neuron

connects to the dendrites of the next neuron

There is a space in between the axon of the first neuron and the dendrites of the next one

This space is called the synapse

A nerve impulse travels from the dendrites, through the cell body, the axon, across the synapse and into the dendrites of the next neuron

Types of neurons: functionNeurons are classified

according to their function

Sensory neurons carry messages from the receptors to the CNS

Motor neurons carry messages from the CNS to muscles and glands

Interneurons are located in the CNS and are the link between sensory and motor neurons

Types of neurons: structureNeurons can also be classified

according to their structure:Multipolar neurons have

one axon and multiple dendrites extending from the cell body. They are the most common type

Bipolar neurons have one axon and one dendrite, both branching at their ends. They occur in the eye, ear and nose

Unipolar neurons have just one axon extension and the cell body is to one side of the axon

The CNSMost of the neurons in the

CNS are interneurons They have many branches

that are able to send or receive messages

The CNS consists of:grey matter, made up of

neurons with unmyelinated fibres (axons)

white matter, composed of neurons with myelinated fibres (axons)

The brain is the control centre of the body

The spinal cord goes out from the brain and sends a network of nerve fibres to the rest of the body

Protection of the CNSThe brain and spinal

cord are very delicate and important parts of the body and are highly protected

3 structures that support and protect the CNS:BoneMeninges Cerebrospinal fluid

(CSF)

Protection of the CNSThe brain is

protected by the cranium

The spinal cord runs through an opening called the vertebral canal surrounded by bony vertebrae

Protection of the CNSInside the bone covering,

the brain and the spinal cord have 3 layers of connective tissue called the meninges

They cover the entire CNS

Outler layer: tough and fibrous, sticks close to the bone

Middle layer: lose meshInner layer: delicate,

contains blood vessels, close to surface of the brain and spinal cord

Protection of the CNSCSF occupies a space

between the middle and inner layers of the meninges

It circulates through cavities in the brain and spinal cord

It is a clear, watery fluid containing glucose, protein, urea and salts

It acts as a shock absorber It acts as transport, taking

nutrients to the brain and spinal cord and carrying waste away

The brain The brain is divided into 3 main sectionsCerebrumCerebellumBrain stem

The cerebrum carries out the higher order functions

Example: languageThe cerebellum is concerned

with fine motor co-ordinationExample: balanceThe brain stem carries out the

lower order functionsExample: breathing & heart

rate

The cerebrum The cerebrum is the

largest section of the brain It consists of two layers of

grey matter sandwiching a layer of white matter

The outer layer is called the cerebral cortex

The cerebral cortex is greatly folded to increase surface area

Folding produces convolutions which are separated by shallow downfolds called sulci and deep folds called fissures

Grey matter

White matter

Sulci

Fissures

The cerebrum The deepest fissure is

the longitudinal fissure which separates the cerebrum into 2 halves; the left and right hemispheres

Each cerebral hemisphere is divided into 4 lobes: the frontal, temporal, occipital and parietal lobes.

Longitudinal fissure

Cerebrum

Cerebellum

Spinal cord

The lobes Frontal lobeThe boss –

reasoning, planningParietal lobe

Sensory information – dealing with & reacting to the environment

Occipital lobeVision

Temporal lobeLanguage, hearing,

memory

The functional areas 3 functional areas in the

cortex: Sensory area interprets

impulses from receptors Motor area controls

muscular movements Association areas

concerned with emotional and intellectual processes – deciding how to respond

The cerebellumThe cerebellum lies

under the rear of the cerebrum

It controls: PostureBalanceFine motor

coordination (muscle movement)

The hypothalamus The hypothalamus

lies in the middle of the brain between the two hemispheres

It is mostly

concerned with homeostasis

The medulla oblongata The medulla oblongata

lies within the brain stem and contains the: Cardiac centre -

regulates the rate and force of the heart beat

Respiratory centre –controls the depth and rate of breathing

Vasomotor centre -controls the diameter of blood vessels

The brainhttps://www.youtube.com/watch?v=kMKc8nfPATIThe brain 13 mins

The spinal cord The spinal cord is a continuation of the brain stemIt consists of gray matter in the shape of an H

called the central canal which runs the length of the spinal cord and contains CSF

The spinal cord 3 functions of the spinal cord:

It carries sensory impulses towards the brain

It carries motor impulses away from the brain

Reflexes – impulses that bypass the brain

Myelinated nerve fibres of white matter are arranged in bundles called ascending and descending tracts

Ascending tracts are sensory axons that carry impulses upwards towards the brain

Descending tracts contain motor axons that conduct nerve impulses downwards away from the brain