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The Spinal Cord & Peripheral Nervous System
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Peripheral Nerves There are 12 pairs of cranial nerves and
31 pairs of spinal nerves.
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Spinal Nerves There are 31 pairs of spinal nerves.
All are mixed nerves (motor & sensory).
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Spinal Nerves There are 31 pairs of spinal nerves.
All are mixed nerves. There are: 8 cervical spinal nerves (C8 comes out between C7 and T1) 12 pairs of thoracic nerves, 5 pairs of lumbar spinal nerves and 5 pairs of sacral nerves.
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Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical enlargement Thoracic nerves T1 – T12 Intercostal nerves Lumbar enlargement Lumbar nerves L1 – L5 Lumbar plexus Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve Co1
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Cranial dura mater Terminus of medulla oblongata of brain Sectioned pedicles of cervical vertebrae Spinal nerve rootlets Dorsal median sulcus of spinal cord (b) Cervical spinal cord.
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Anatomy of the spinal cord.
Epidural space (contains fat) Pia mater Arachnoid mater Spinal meninges Subdural space Dura mater Subarachnoid space (contains CSF) Bone of vertebra Dorsal root ganglion Body of vertebra (a) Cross section of spinal cord and vertebra
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Spinal Nerves The spinal nerve consists of a dorsal root and a ventral root. The ventral root contains efferent, motor neurons (voluntary and involuntary). The dorsal root is, afferent, sensory fibers.
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Spinal Nerves The spinal nerve is only 1-2 cm long coming out from the spine and the branches into a dorsal ramus and a ventral ramus.
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Dorsal horn (interneurons)
Dorsal root (sensory) Dorsal root ganglion Dorsal horn (interneurons) Somatic sensory neuron Visceral sensory neuron Visceral motor neuron Spinal nerve Ventral horn (motor neurons) Ventral root (motor) Somatic motor neuron Interneurons receiving input from somatic sensory neurons Interneurons receiving input from visceral sensory neurons Visceral motor (autonomic) neurons Somatic motor neurons
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Ascending tracts Descending tracts Fasciculus gracilis Ventral white commissure Dorsal white column Fasciculus cuneatus Lateral reticulospinal tract Dorsal spinocerebellar tract Lateral corticospinal tract Rubrospinal tract Ventral spinocerebellar tract Medial reticulospinal tract Lateral spinothalamic tract Ventral corticospinal tract Ventral spinothalamic tract Vestibulospinal tract Tectospinal tract
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Innervations of Specific Body Regions
Except for the spinal nerves T2-T12, all other spinal nerves join into inter lacing networks called nerve plexuses. They are formed only by the ventral rami.
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. Dorsal ramus Ventral ramus Spinal nerve Rami communicantes
Intercostal nerve Sympathetic trunk ganglion Dorsal root ganglion Dorsal root Ventral root Branches of intercostal nerve • Lateral cutaneous • Anterior cutaneous Sternum (b) Cross section of thorax showing the main roots and branches of a spinal nerve.
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Cervical Plexus This is formed from the first 4 cervical spinal nerves.
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Cervical Plexus This is formed from the first 4 cervical spinal nerves. The most important nerve coming from this plexus is the phrenic nerve.
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Cervical Plexus This is formed from the first 4 cervical spinal nerves. The most important nerve coming from this plexus is the phrenic nerve. This is the chief motor nerve for the diaphragm.
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Segmental branches Hypoglossal Ventral nerve (XII) rami:
Ventral rami Segmental branches Hypoglossal nerve (XII) Ventral rami: Lesser occipital nerve C1 Greater auricular nerve C2 Transverse cervical nerve C3 Ansa cervicalis C4 Accessory nerve (XI) Phrenic nerve C5 Supraclavicular nerves
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Cervical Plexus Damage to the spinal cord at C3- C5 could lead to respiratory arrest.
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Brachial Plexus It is formed from C5-C8 and T1 with 5 major roots supplying the muscles of the muscles of the shoulder, thorax and upper limb.
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Roots (ventral rami): C4 Dorsal scapular C5 Nerve to subclavius C6 Suprascapular Upper Posterior divisions C7 Middle Trunks Lateral C8 Lower Cords Posterior T1 Long thoracic Medial Medial pectoral Lateral pectoral Axillary Upper subscapular Musculo- cutaneous Lower subscapular Thoracodorsal Radial Medial cutaneous nerves of the arm and forearm Median Ulnar (a) Roots (rami C5 – T1), trunks, divisions, and cords Anterior divisions Posterior divisions Trunks Roots
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Musculocutaneous nerve Lateral cord Axillary nerve Biceps brachii Posterior cord Coracobrachialis Median nerve Medial cord Radial nerve branches to triceps Radial nerve Ulnar nerve (b) Cadaver photo
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Lumbrosacral Plexus It is formed from L1-L4 and lies within the psoas major muscle. The largest nerve of this plexus is the femoral nerve. The sacral plexus is formed from L4-S4 and lies just below the Lumbar Plexus
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Ventral rami: Iliohypogastric L1 Ilioinguinal Femoral L2
Lateral femoral cutaneous Iliohypogastric Ilioinguinal Obturator L3 Genitofemoral Anterior femoral cutaneous Lateral femoral cutaneous Saphenous L4 Obturator Femoral L5 Lumbosacral trunk (a) Ventral rami and major branches of the lumbar plexus (b) Distribution of the major nerves from the lumbar plexus to the lower limb
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Ventral rami and major branches of the sacral plexus
Superior gluteal L5 Lumbosacral trunk S1 Inferior gluteal Common fibular S2 Tibial S3 Posterior femoral cutaneous S4 Pudendal S5 Sciatic Co1 Ventral rami and major branches of the sacral plexus
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The largest nerve from this is the sciatic nerve.
It is the largest and thickest nerve in the body. It supplies the entire lower limb. Irritation to this nerve gives rise to Sciatica.
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Dermatomes `Dermatomes are areas of the skin innervated by the sensory nerves. These are fairly uniform and follow the nerve segments.
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Anterior view (b) Posterior view C2 C3 C4 C5 T1 T2 T3 T2 T4 T2 T5 T6
L1 T11 C8 L2 S1 L3 C8 S2 T12 L4 S3 L5 C6 L1 L1 C6 S4 C7 S2 C7 S5 C8 S3 C8 L2 L2 S1 S2 S2 S1 L3 L3 L1 L5 L2 L5 L4 L4 L3 L5 L5 L4 S1 S1 Anterior view (b) Posterior view L4 L4 L5 L5 S1
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Reflex Activity Reflexes can be inborn or learned.
An inborn reflex is a predictable, rapid response to a stimulus. Acquired reflexes result from repetition.
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Reflex Arc
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Stretch and Golgi Tendon Reflexes
The muscle spindles found in skeletal muscles provide information on the length of the muscle and the amount of tension on it. A common example of this is the patellar reflex. This reflex causes the muscle to contract in response to stretching.
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The patellar (knee-jerk) reflex – a specific example of a stretch reflex
2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2 – L4) Tapping the patellar ligament excites muscle spindles in the quadriceps muscle. 1 Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + Excitatory synapse – Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b
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Golgi tendon reflexes Golgi tendon reflexes are the opposite and respond by causing muscle relaxation. This helps to prevent the muscle from over contracting.
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contracts. Golgi tendon organs are activated. Afferent fibers synapse
Quadriceps strongly contracts. Golgi tendon organs are activated. 1 Afferent fibers synapse with interneurons in the spinal cord. 2 Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3a 3b Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse – Inhibitory synapse
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Flexor and Crossed-Extensor Reflexes
These are initiated by painful stimuli and cause an automatic withdrawal of the threatened body part. Placing your hand on a hot stove is an example of this type of reflex.
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+ Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers
Afferent fiber Efferent fibers Extensor inhibited Flexor inhibited Arm movements Flexor stimulated Extensor stimulated Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb.
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Superficial Reflexes These are the result of gentle cutaneous stimulation. Common examples are the plantar reflex that tests the integrity of the corticospinal tract.
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Babinski's sign This plantar reflex occurs when the corticospinal tract is damaged. In it the large toe dorsi flexes and the smaller toes fan laterally.
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Spinal Cord Injuries Damage to the spinal cord results in paralysis (loss of motor function) or Paresthesia (sensory loss). These are devastating injuries since the cord will not heal.
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Spinal Cord Injuries
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The Autonomic Nervous System
The motor portion of the peripheral nervous system is divided into the somatic (voluntary) motor system (SMS) and the autonomic motor system (ANS).
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Somatic nervous system
Central nervous system (CNS) Peripheral nervous system (PNS) Figure Place of the ANS in the structural organization of the nervous system. Sensory (afferent) division Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division
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Comparison The SMS stimulates skeletal muscle whereas the ANS innervates cardiac and smooth muscle as well as glands.
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Comparison In the SMS the motor neuron cell bodies are located in the CNS and their axons extend to the muscles they innervate. It is a single neuron chain.
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Comparison The ANS uses a two neuron chain.
The first is the preganglionic neuron; its cell body resides in the CNS or spinal cord.
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Comparison The ANS uses a two neuron chain.
The first is the preganglionic neuron; its cell body resides in the CNS or spinal cord. The second motor neuron outside of the CNS. This second neuron is called the postganglionic neuron.
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Figure 14.2 Comparison of somatic and autonomic nervous systems.
Neuro- transmitter at effector Cell bodies in central nervous system Effector organs Peripheral nervous system Effect Single neuron from CNS to effector organs SOMATIC NERVOUS SYSTEM ACh + Stimulatory Heavily myelinated axon Skeletal muscle Figure Comparison of somatic and autonomic nervous systems. Two-neuron chain from CNS to effector organs ACh NE Unmyelinated postganglionic axon SYMPATHETIC Ganglion Lightly myelinated preganglionic axons Epinephrine and norepinephrine + ACh Stimulatory or inhibitory, depending on neuro- transmitter and receptors on effector organs AUTONOMIC NERVOUS SYSTEM Adrenal medulla Blood vessel ACh ACh Smooth muscle (e.g., in gut), glands, cardiac muscle PARASYMPATHETIC Lightly myelinated preganglionic axon Unmyelinated postganglionic axon Ganglion Acetylcholine (ACh) Norepinephrine (NE)
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ANS Divisions The ANS is divided up into the parasympathetic and sympathetic divisions. They innervate the same organs and typically have opposite effects.
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Parasympathetic Division
This system is associated with “resting and digesting”. When this system is active, the body’s energy use is low, the body is relaxed and the body can focus on digestive processes.
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Sympathetic Division This is often referred to as the “fight or flight” system. When this system is active, the body is excited and the body is in a high energy state.
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Figure 14.3 Overview of the subdivisions of the ANS.
Parasympathetic Sympathetic Eye Eye Brain stem Salivary glands Skin* Cranial Salivary glands Sympathetic ganglia Heart Cervical Figure Overview of the subdivisions of the ANS. Lungs Lungs T1 Heart Stomach Thoracic Stomach Pancreas Liver and gall- bladder Pancreas L1 Liver and gall- bladder Adrenal gland Lumbar Bladder Bladder Genitals Genitals Sacral
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Parasympathetic (Craniosacral) Division
The cranial outflow includes Cranial nerves III, VII, IX, and X. The sacral outflow innervates the colon and pelvic organs.
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Sympathetic (Thoracolumbar) Division
It supplies the visceral organs but also structures are the body such as the sweat glands and peripheral arteries. All cell bodies for the preganglionic neurons arise in the spinal cord between T1 and L12.
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Visceral reflexes The visceral reflex arc is similar to the somatic reflex arc and contains the same components except the motor neuron is from the ANS.
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Visceral reflexes The visceral pain afferent fibers travel along the same pathways as the somatic pain receptors and gives rise to the so called referred pain.
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Heart Lungs and diaphragm Liver Heart Gallbladder Liver Appendix Stomach Pancreas Small intestine Ovaries Colon Kidneys Urinary bladder Ureters
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ANS Physiology The major neurotransmitters released by the ANS are acetylcholine (Ach) and norepinephrine (NE).
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ANS Physiology Ac is released by:
All ANS preganglionic axons All parasympathetic postganglionic axons at the synapses with their effectors Ach releasing fibers are called cholinergic fibers
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ANS Physiology NE is released by postganglionic sympathetic fibers at the synapses with their effectors. NE releasing fibers are called adrenergic fibers.
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Receptor Type To further complicate the matter, the effects of these neurotransmitters depend on the type of receptor to which they attach.
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Cholinergic Receptors
Two types of receptors bind Ach and they are named for the drugs that can bind to them. These are the nicotinic receptors (preganglionic sympathetic)and the muscarinic receptors (postganglionic parasympathetic).
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Effects of Drugs The existence of various types of receptors has led to the design of specific drugs that can either inhibit or stimulate a selected organ.
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Atropine is an anticholinergic drug that causes the pupil to dilate, dry up respiratory secretions or prevent salivation.
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Disorders POTS Postural Orthostatic Tachycardia Syndrome
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