2. The Nervous System - General Structure
The nervous system is divided anatomically into two main divisions; the central nervous system (CNS) and the peripheral nervous system (PNS)
The central nervous system consists of the brain and spinal cord
The peripheral nervous system consists of the cranial and spinal nerves that emerge from the brain and spinal cord respectively
The peripheral nervous system is also functionally and structurally divided into the somatic and autonomic nervous systems
The somatic system is concerned with voluntary activity, and the autonomic system is generally concerned with involuntary or automatic activity

3. The Nervous System

4. The Peripheral Nervous System (Cranial Nerves)
There are 12 pairs of cranial nerves, which serve the head and neck – they include the sensory nerves from the sense organs of the head, the facial muscles and those that move the eyes, tongue and jaw
The Vagus nerves (pair 10) are unusual as they serve abdominal organs They passes down the neck, through the thorax and into the abdomen where they branch to the heart, gut and other organs
Several cranial nerves (including the Vagus nerves) form part of the Autonomic Nervous System

5. The Peripheral Nervous System (Spinal Nerves)
There are 31 pairs of spinal nerves, and they are named and numbered according to where they emerge from the vertebral column
Spinal Nerves
The spinal nerves and either somatic (supplying mainly the skin and voluntary muscles) or visceral (supplying the gut, involuntary muscles and glands)

6. Peripheral Nervous System
Somatic Nervous System
Autonomic Nervous System
The Autonomic Nervous System (ANS) consists mainly of efferent visceral motor neurones (bundled into motor fibres)
Motor neurones of the somatic nervous system transmit impulses away from the CNS to skeletal muscles
Motor neurones of the ANS transmit from the CNS to smooth muscle, cardiac muscle and glands
Sensory neurones of the somatic nervous system transmit impulses to the CNS from generalised sensory receptors, e.g. touch, temperature, visual and taste
The few sensory neurones of the ANS transmit impulses from visceral (internal organs) receptors

8. The Autonomic Nervous System
The autonomic nervous system consists mainly of motor neurones (bundled into fibres) that transmit impulses to cardiac muscle, smooth muscle and glands The few sensory neurones of the ANS transmit impulses from the internal organs into the CNS
The motor portion of the autonomic nervous system is subdivided into sympathetic and parasympathetic systems
Most internal organs are innervated by both sympathetic and parasympathetic motor neurones – dual innervation

9. Ganglia are clusters of neuron cell bodies located outside of the CNS
dorsal root
ventral root
spinal nerve
ganglia
Ganglia are clusters of neuron cell bodies located outside of the CNS
The ganglia form a vertical chain on either side of the vertebral column

10. Vagus nerve

11. Overall the sympathetic system exerts an excitatory effect on the body
Overall the parasympathetic system exerts an inhibitory effect on the body
Sympathetic nerve fibres emerge from the thoracic and lumbar regions of the CNS; parasympathetic nerve fibres emerge from the cranial and sacral regions

12. Control of Heart Rate Cardiac Centre in medulla of brain
Sympathetic and parasympathetic nerves, originating in the cardiac centre, innervate the heart and modify its action
Cardiac Centre
in medulla of brain

13. Control of the Heart Rate
The heart rate is determined by the balance between sympathetic and parasympathetic nerve activity

14. Sympathetic Activity
Sympathetic activity increases both the heart rate and the force of contraction of the heart

15. Parasympathetic Activity
Parasympathetic activity decreases the heart rate

16. Control of Heart Rate
Changes in blood pressure and pH control the rate at which the heart beats through a negative feedback system

17. in the carotid sinuses and aortic arch
The origin of the heartbeat is myogenic, but the rate of the heartbeat can be varied by both nervous and hormonal mechanisms
Cardioacceleratory and cardioinhibitory centres within the medulla of the brain receive information from baroreceptors located
in the carotid sinuses and aortic arch
Baroreceptors detect and respond to changes in blood pressure and transmit impulses to the cardiac centres within the medulla

18. an increase in the force of the contraction of the heart
Baroreceptors transmit impulses to the cardioacceleratory centre when they detect a fall in
blood pressure
The release of noradrenaline from the sympathetic nerve terminals results in an increase in the heart rate and
an increase in the force of the contraction of the heart
Stimulation of the cardioacceleratory centre activates sympathetic nerve fibres that pass from this centre, down the descending tracts of the spinal cord and along spinal nerves to the heart
These sympathetic fibres innervate the SA and AV nodes and the muscular walls of the ventricles

19. These parasympathetic fibres innervate the SA and AV nodes
Baroreceptors transmit impulses to the cardioinhibitory centre when they detect a rise in
blood pressure
Stimulation of the cardioinhibitory centre activates parasympathetic nerve fibres that pass from this centre, down the descending tracts of the spinal cord and along spinal nerves to the heart
These parasympathetic fibres innervate the SA and AV nodes
The release of acetylcholine from the parasympathetic nerve terminals results in a decrease in the heart rate

21. A reflex action is a rapid, automatic response to a stimulus
Simple Reflexes
A reflex action is a rapid, automatic response to a stimulus
Simple reflexes are inborn fixed responses to specific stimuli, and may involve either the brain or the spinal cord
The knee-jerk reflex is very simple as it involves just two neurons, i.e. a direct connection between sensory and motor neurons – this is very unusual

22. Spinal Reflexes
Reflex actions involve receptors, sensory neurones, motor neurones and effectors that form a nervous pathway described as the reflex arc
Reflexes that operate via the spinal cord are called spinal reflexes and include pathways that determine actions such as the withdrawal response, e.g. rapid removal of the hand away from a painful stimulus
Reflexes, such as the withdrawal response, have adaptive value in that they protect the body from harmful stimuli
The withdrawal reflex is an example of a spinal reflex involving three neurones

23. The Spinal Cord The spinal cord is a continuation of the
medulla of the brain
Spinal cord
The spinal cord is a tube of nerve tissue that runs from the brain to the lower back through neural canals
Thoracic vertebra
within the vertebrae

24. The Spinal Cord (transverse section)
White Matter
Grey Matter

25. The Spinal Cord (transverse section)
Butterfly-shaped, grey matter region containing many cell bodies and relay neurones
Dorsal
root
Dorsal
root
ganglion
White matter containing mainly myelinated nerve fibres that carry impulses up and down the cord
Ventral
root
Spinal Nerve

26. Rapid conduction of impulses through the reflex arc
Sensory
neurone
Synapse
Relay
neurone
Response:
arm muscles
contract and
hand withdraws
Pain receptors
stimulated
Motor
neurone
Effector:
muscles of
the arm
Stimulus from
a hot object

27. Reflexes can be modulated (modified) due to various synaptic connections These occur in the reflex arc where the ascending and descending tracts of neurones pass to and from the brain