Nervous System

Nervous system is divided into 2

- Peripheral nervous system (PNS) and

- Central nervous system (CNS)

Peripheral nervous system (PNS)consists of cranial nerves and spinal nerves. PNS relays information to and from the central nervous system (CNS).

Nerve cells are called neurons. Axons, dendrites and cell bodies are the main parts of a neuron.

Axons transmit impulses away from the cell body of a neuron

Dendrites are extensions which transmit impulses toward the cell body.

SYNAPSE

The small gap between the axon of one neuron and the dendrites or cell body of the next Neuron is called synapse.

Presence of synapse ensure one-way transmission of impulses.

TYPES OF NEURON

Neurons are classified into 3 main groupsSensory neurons, motor neurons and interneurons.

SENSORY, MOTOR AND INTERNEURON

Motor neuron (efferent neuron) carry impulses to the effectors (muscles and glands).

Motor neuron linked to skeletal muscles are called somatic neuron.

Motor neurons linked to smooth muscle, cardiac muscle and glands are called visceral.

Sensory neurons carry impulses from receptors to the central nervous system (afferent neurons)

Sensory and motor neurons make up the peripheral nervous system .

Interneurons are found entirely within the central nervous system. They are arranged so as to carry only sensory and motor impulses or to integrate these functions. Some interneurons in the brain are concerned with thinking, learning and memory.

Nerve impulses are action potential which carry information along the neuron membrane.

Neurons not carrying an impulse has abundant Na+ ions outside the cell and abundant K+ and negative charged ions inside.

SEQUENCE OF EVENTS IN THE TRANSMISSION OF ACTION POTENTIALPolarisation Neuron membrane has a positive charge outside and negative charge inside Sodium ions are more abundant outside and potassium and negative ions are more

abundant inside Sodium and potassium pump maintain these equilibrium (resting membrane potential)

in the absence of any stimulus.Depolarisation1. Stimulus (chemical, electrical or mechanical) alter the resting membrane potential of

the neuron membrane2. Voltage regulated sodium channels open to permit the influx of sodium ion into the cell

(depolarisation)3. Inside of the membrane becomes more positive compared to the outside (-60mv to

+30mv).4. Voltage regulated sodium channel closes at +30mv.Repolarisation5. Voltage regulated potassium channels are now open and potassium ions flow out of

cell membrane. This results in making the outside more positive than inside.6. Sodium ions are returned outside and potassium ions returned inside the cell by

sodium potassium pump7. The voltage regulated potassium channels begin to close at -70mv.

Nerve plexus is a network of neurons from several segments of the spinal cord that combine to form nerves to specific parts of the body.

MAJOR PERIPHERAL NERVESNerve Spinal nerves that

contributeDistribution

Phrenic C3 – C5 Diaphragm

Radial C5 – C8, T1 Skin and muscles of posterior arm, forearm, and hand, thumb and first 2 fingers

Median C5 – C8, T1 Skin and muscles of anterior arm, forearm and hand

Ulnar C8, T1 Skin and muscles of medial arm, forearm and hand; little finger and ring finger

Intercostal T2 – T12 Intercostal Muscles, abdominal muscles, skin of trunk

Femoral L2- L4 Skin and muscles of anterior thigh, medial leg and foot

Sciatic L4 – S3 Skin and muscles of posterior thigh, leg and foot

BRAIN

Brain is made up of approximately 100 billion neurons and contains trillions of synapses. The major parts of the brain are the medulla, pons, midbrain, (collectively called brain stem), cerebellum, hypothalamus, thalamus and the cerebrum.

The four cavities within the brain are called ventricles . Each ventricle contains a network of capillaries called choroid plexus, which forms the cerebrospinal fluid from blood plasma.

MEDULLA

Medulla extends from the spinal cord to the pons and is anterior to the cerebellum. Medulla contains Cardiac centres that regulate the heart Vasomotor centres that regulate the

diameter of blood vessels and thereby blood pressure

Respiratory centres that regulate breathing

PONS

Pons bulges anteriorly from the upper part of the medulla. Within the pons are 2 respiratory centres that work with those in the medulla to produce a normal breathing rhythm. Cranial nerves V and VIII connect the brain in the pons.

MIDBRAIN

Midbrain extends from the pons to the hypothalamus. It encloses the cerebral aqueduct which is a tunnel that connects the thrid and fourth ventricles. Visual and auditory reflexes are integrated in the midbrain.

CEREBELLUM

Cerebellum is situated inferior to the occipetal lobes of the cerebrum and is separated from the medulla and pons by the fourth ventricle. Some of the functions of cerebellum include regulation of muscle tone, coordination, maintenance of posture and equilibrium.

Cerebellum uses information about gravity and movement provided by receptors in the inner ears to regulate equilibrium.

HYPOTHALAMUS

Hypothalamus is a small area located superior to the pituitory gland and inferior to the thalamus. The main functions of hypothalamus are given below: Production of anti-diuretic hormone and oxytocin which are then

stored in the posterior pituitory gland. Production of releasing factors which stimulate anterior pituitory

gland to secrete hormones. Regulation of body temperature by promoting responses such as

sweating and shivering. Regulation of food intake Integration of the functioning of the autonomic nervous system Simulation of visceral responses during emotional situations. Regulation of body rhythms such as secretion of hormones, sleep

cycles, changes in mood or mental alertness

THALAMUS

The thalamus is situated superior to the hypothalamus and inferior to the cerebrum.

Many of the functions of the thalamus are concerned with sensation. The Thalamus integrates the impulses from the cutaneous receptors and from the cerebellum. Part of the information associated with pain are processed thalamus.

CEREBRUM

Cerebrum is the largest part of the human brain. It consists of two hemispheres which are separated by longitudinal fissure. At the base of the grove this deep groove is the corpus callosum that connects the right and left hemispheres. The surface of the cerebrum is gray matter called the cerebral cortex.

The cerebral cortex is divided into lobes that have the same names as the cranial bones external to them.

CEREBRAL HEMISHPHERES

BASAL GANGLIA

These are paired masses of gray matter within the white matter of cerebral hemispheres. Their functions are certain aspects of voluntary movement and they work with cerebellum. The basal ganglia help regulate muscle tone, coordinate motor movements and posture. It also inhibits unwanted muscular activity.

MENINGES

The connective tissue membrane that covers the brain and spinal cord are called meninges.

Outer layer – Duramater Middle layer – arachnoid mater Innermost layer – Pia mater

CEREBROSPINAL FLUID

Choroid plexus in the ventricles produce a fluid from blood plasma called cerebrospinal fluid (CSF).

Approximately 500ml of CSF is secreted each day, but only approximately 150mls is in the ventricular system at any one time.

Normal CSF pressure is approximately 10mmHg in the lateral recumbent position.