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

The Nervous System

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

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)

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

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

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

Vagus nerve

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

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

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

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

Parasympathetic Activity Parasympathetic activity decreases the heart rate

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

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

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

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

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

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

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

The Spinal Cord (transverse section) White Matter Grey Matter

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

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

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