Chapter 4 - the nervous system is highly organised

Central Nervous System

Control centre of the body.

Processes incoming impulses and initiates outgoing impulses.

• Brain

• Spinal cord

Cranium and vertebrae bone

Cranium - protects the brain.

Vertebral canal - spinal cord runs through here.

• Provides a strong rigid structure to protect the structures underneath.

• Hard surface blocks direct contact with the brain / protects the brain from physical impact / physical damage.

Meninges

Three layers of connective tissue that surround the organs.

• Dura mater (outer) - tough and fibrous. Sticks closely to the bones of the skull but not so close in the vertebral canal.

• Arachnoid (middle) - loose mesh of fibres.

• Pia mater (inner) - delicate and contains many blood vessels. Sticks closely to the surface of the brain and spinal cord.

Supports the brain, contains cerebrospinal fluid, protects the brain from physical damage/impact, provides anchor for blood vessels, adheres to and encloses the brain and spinal cord.

Cerebrospinal fluid

Fluid found in between the middle and inner layers of the meninges. Also found in the cavities in the brain (ventricles) and in the central canal in the vertebral column. Clear watery fluid containing a few cells, some glucose, protein, urea and salts. It is formed from blood.

• Acts as a shock absorber

• Provides support

• Transports nutrients and wastes

The brain

Very complex organ in structure and function. Consists of 100 billion neurons mainly interneurons.

• Cerebrum

• Cerebellum

• Hypothalamus

• Medulla oblongata

• Corpus callosum

Cerebrum structure

• Largest part of the brain.

• Consists of outer surface of grey matter (2-4 mm) called the cerebral cortex.

• Deep inside additional grey matter called the basal ganglia.

• In between the two layers of grey matter is white matter.

Cerebral cortex is folded in patterns that greatly increase the surface area. Contains 70% of all the neurons in the CNS/ Folding produces round ridges called convolutions (gyrus) and shallow downfolds called sulcus, and deep downfolds called fissures.

Longitudinal fissure - the deepest fissure, almost separates the cerebrum into two halves (left and right cerebral hemispheres) which are joined by an area of white matter called the corpus callosum.

Patterns of folding of the cerebral cortex vary from person to person, however, certain fissures are fairly constant. Certain prominent sulci divide the cerebral hemispheres into five lobes.

Cerebrum functions

Involved in mental activities such as:

• Thinking

• Reasoning

• Learning

• Memory

• Intelligence

• Sense of responsibility

• Perception of senses

• Initiation and control of voluntary muscle contraction

Lobes of the cerebral cortex

• Frontal lobe - thinking, problem solving, emotions, personality, language, and control of movement.

• Parietal lobe - processing temperature, touch, taste, pain, and movement.

• Temporal lobe - processing memories and linking them with senses; receives auditory information.

• Occipital lobe - vision.

• Insula - recognition of different senses and emotions, addiction and psychiatric disorders.

Functional areas in the cortex

• Sensory areas - receive and process nerve impulses from the sense.

• Motor areas - Send impulses to the muscles, especially for control of voluntary movement.

• Association areas - Intellectual and emotional processes.

Cerebral white matter

Composed of bundles of myelinated nerve fibres called tracts. There are three types of tracts.

1. Connect various areas of the cortex within the same hemisphere.

2. Carry impulses between the left and right hemispheres.

3. Connect cortex to other parts of the brain or the spinal cord.

Hemispheres

They are not identical. Many specialised functions occur in only one hemisphere.

Memory

One of the most important functions of the cerebrum. When memory is stored, new links are made between neurons or existing links are modified.

Corpus callosum

Band of nerve fibres that lies beneath the cerebrum at the base of the longitudinal fissure. Nerve fibres cross from one hemisphere to the other.

Allows the two hemispheres to communicate with each other.

Cerebellum

Structure

• Second largest part of the brain

• Surface folded into parallel ridges

• Outer surface is the grey matter and inner surface is the white matter.

Function

• All functions take place below conscious level.

• Controls posture, balance and fine coordination of voluntary muscle movement.

• Receives information from inner ear (posture and balance) and from stretch receptors in skeletal muscles.

• Impulses do not originate here.

• Without cerebellum we can still move but movements would be spasmodic, jerky and uncontrolled.

Hypothalamus

Small structure that lies in the middle of the brain and cannot be seen from the outside.

Functions:

• Hormone secretion - secretes hormones that control the anterior pituitary and thus regulates growth, metabolism, reproduction, stress response. Produces hormones that are stored in the posterior pituitary that control labour contractions, lactation and water conservation.

• Autonomic effects - influences heart rate, blood pressure, secretion of digestive juices, movements of the alimentary canal and dilation of the pupil.

• Thermoregulation - maintains body temperature.

• Food and water intake - regulates sensations of hunger and satiety, stimulates water drinking and contraction of the bladder.

• Sleep and circadian rhythms - regulates patterns of waking and sleeping.

• Emotional behaviour and sexual response - responses such as fear, anger, aggression, pleasure, contentment.

Medulla oblongata

Structure

• Continuation of the spinal cord, about 3cm long and extends from just above the point where the spinal cord enters the skull.

• Nerve fibres connecting the brain to the spinal cord pass through here.

• Contains cardiac, respiratory and vasomotor centres.

Function

• Cardiac centre - regulates the rate and force of heartbeat.

• Respiratory centre - control rate and depth of breathing.

• Vasomotor centre - regulates the diameter of blood vessels.

• Regulates the reflexes of swallowing, sneezing, coughing and vomiting.

Spinal cord structure

• Extends from the foramen magnum to the second lumbar vertebra.

• About 44cm in length and about as thick as a little finger.

• Enclosed in the vertebral canal and is protected by the meninges.

• Between the outer meningeal layer and the bone of the vertebral column there is a space called epidural.

• Epidural contains fat, connective tissue and blood vessels. Allows the cord to ben when the spine is bent.

Cross section of the spinal cord

• White matter composed of myelinated nerve fibres is on the outside.

• Consists of Ascending tracts (sensory) and Descending tracts (motor).

• Grey matter consisting of cell bodies and unmyelinated nerve fibres is on the inside.

• Grey matter is roughly H shaped with a central canal in the middle. CSF flows through the central canal.

Spinal cord functions

• Conduction - conduct information up and down the cord.

• Neural integration - receive and integrate information and execute appropriate output.

• Locomotion - involves repetitive, coordinated contraction of muscles for movement.

• Reflex - involuntary, fast, automatic responses to stimulus.

Peripheral Nervous System

Made up of all the nerves that carry messages to and from the CNS and ganglia (knot like swelling in a nerve where the nerve cell bodies are concentrated) which lie outside the brain and spinal cord.

• Cranial nerves - 12 pairs, carry impulses into and away from the brain.

• Spinal nerves - 31 pars, carry impulses away from and into the spinal cord.

Each is joined to the spinal cord by two roots:

• Ventral root - contains axons of motor nerves that have their cell bodies in grey matter of the spinal cord.

• Dorsal root - contains axons of sensory nerves with their cell bodies in the dorsal root ganglion.

Sensory (afferent) division

Carries signals from various receptors (sense organs) to CNS. This pathway informs the CNS of the stimulus within and around the body.

• Somatic sensory division - carries signals from receptors in the skin, muscles, bones, and joints to the CNS.

• Visceral sensory division - carries signals from the viscera of the thoracic and abdominal cavities such as heart, stomach, bladder, lungs.

Motor (efferent) division

Carries signals from the CNS to gland and muscle cells that carry out the body’s responses. Cells and organs that respond to the commands from the nervous system are called effectors.

• Somatic motor division - carries signals to the skeletal muscles. This output produces muscular contractions that are under voluntary and involuntary control, as well as involuntary contractions called somatic reflexes.

• Visceral motor division (autonomic) - carries signals to the glands, cardiac muscle, and smooth muscle. They operate at an unconscious level.

Autonomic nervous system

Functions

• Controls the body’s internal environment and maintains homeostasis.

• Regulates functions such as heart rate, blood pressure, digestion, urination, defecation, air flow to the lungs, body temperature, pupil dilation, release of energy

• Operates without conscious control.

• Regulated by groups of nerve cells in the medulla oblongata, hypothalamus and cerebral cortex.

• Nerve fibres are some cranial nerves and some spinal nerves.

• Carry impulses to the heart, glands and other internal organs.

Somatic pathway

Effector - skeletal muscle (voluntary).

Control - usually voluntary.

Efferent pathway - one nerve fibre from CNS to effector, no ganglia.

Neurotransmitter - Acetylcholine (Ach).

Effect on target cells - excitatory.

Nerves to target organ - one set.

General function - response to external environment.

Autonomic pathway

Effectors - Glands, smooth muscle, cardiac muscle (involuntary).

Control - Usually involuntary.

Efferent pathway - Two nerve fibres (preganglionic and postganglionic) from CNS to effector; synapse at ganglion.

Neurotransmitters - Acetylcholine and noradrenaline.

Effect on target cells - Excitatory or inhibitory.

Nerves to target organ - Two sets - sympathetic and parasympathetic.

General function - Maintain homeostasis.

Sympathetic division

Tends to arouse the body for action (fight or flight). Takes over to increase activities, but inhibits digestion. Exercise, excitement, emergency, and embarrassment.

• Heart - increases rate and strength of contraction.

• Lungs - dilates bronchioles.

• Stomach, intestines - decreases movement.

• Liver - increases breakdown of glycogen and release of glucose.

• Iris - dilates pupil.

• Sweat glands - increases sweat secretion.

• Salivary glands - decreases secretion of saliva.

• Blood vessels - constricts in skin, dilates in skeletal muscle, constricts in internal organs (except in heart and lungs).

• Urinary bladder - relaxes muscles of wall.

• Adrenal medulla - stimulates hormone secretion.

Parasympathetic division

Tends to have a calming effect, but stimulates digestion. Conserves energy. Digestion, defecation, and diuresis.

• Decreases rate and strength of heart contractions.

• Constricts bronchioles in the lungs.

• Increases movement of the stomach and intestines.

• Increases uptake of glucose and synthesis of glycogen in the liver.

• Constricts pupils.

• No effect on sweat glands.

• Increases secretion of saliva in the salivary glands.

• Blood vessels - little effect on skin, no effect on skeletal muscles, little effect on internal organs.

• Constricts muscles of wall of the urinary bladder.

• No effect on the adrenal medulla.

Sympathetic division activation responses

• The rate and force of the contraction of the heart increases, with a consequent increase in blood pressure (pumps more blood to provide more oxygen and nutrients, and remove more accumulated wastes).

• Blood vessels in organs involved in strenuous activity - such as skeletal muscles, heart and liver dilate/vasodilation (to transport more blood, hence more oxygen and nutrients to these organs).

• Blood vessels of organs not involved in activity - such as kidney, stomach, intestines and skin constrict/vasoconstriction (to divert more blood, hence more oxygen and nutrients to the organs involved in strenuous activity).